diff options
Diffstat (limited to 'Documentation/RCU/Design')
17 files changed, 8825 insertions, 3493 deletions
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+ <!-- Arrowhead on XXXpoint 5250 5850 - 5250 4740--> + </g> +</svg> diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html new file mode 100644 index 000000000000..7eb47ac25ad7 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html @@ -0,0 +1,1333 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" + "http://www.w3.org/TR/html4/loose.dtd"> + <html> + <head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title> + <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> + + <p>January 27, 2016</p> + <p>This article was contributed by Paul E. McKenney</p> + +<h3>Introduction</h3> + +This document describes RCU's major data structures and their relationship +to each other. + +<ol> +<li> <a href="#Data-Structure Relationships"> + Data-Structure Relationships</a> +<li> <a href="#The rcu_state Structure"> + The <tt>rcu_state</tt> Structure</a> +<li> <a href="#The rcu_node Structure"> + The <tt>rcu_node</tt> Structure</a> +<li> <a href="#The rcu_data Structure"> + The <tt>rcu_data</tt> Structure</a> +<li> <a href="#The rcu_dynticks Structure"> + The <tt>rcu_dynticks</tt> Structure</a> +<li> <a href="#The rcu_head Structure"> + The <tt>rcu_head</tt> Structure</a> +<li> <a href="#RCU-Specific Fields in the task_struct Structure"> + RCU-Specific Fields in the <tt>task_struct</tt> Structure</a> +<li> <a href="#Accessor Functions"> + Accessor Functions</a> +</ol> + +At the end we have the +<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>. + +<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3> + +<p>RCU is for all intents and purposes a large state machine, and its +data structures maintain the state in such a way as to allow RCU readers +to execute extremely quickly, while also processing the RCU grace periods +requested by updaters in an efficient and extremely scalable fashion. +The efficiency and scalability of RCU updaters is provided primarily +by a combining tree, as shown below: + +</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%"> + +</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure +containing a tree of <tt>rcu_node</tt> structures. +Each leaf node of the <tt>rcu_node</tt> tree has up to 16 +<tt>rcu_data</tt> structures associated with it, so that there +are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures, +one for each possible CPU. +This structure is adjusted at boot time, if needed, to handle the +common case where <tt>nr_cpu_ids</tt> is much less than +<tt>NR_CPUs</tt>. +For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>, +which results in a three-level <tt>rcu_node</tt> tree. +If the actual hardware has only 16 CPUs, RCU will adjust itself +at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node. + +</p><p>The purpose of this combining tree is to allow per-CPU events +such as quiescent states, dyntick-idle transitions, +and CPU hotplug operations to be processed efficiently +and scalably. +Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures, +and other events are recorded by the leaf-level <tt>rcu_node</tt> +structures. +All of these events are combined at each level of the tree until finally +grace periods are completed at the tree's root <tt>rcu_node</tt> +structure. +A grace period can be completed at the root once every CPU +(or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task) +has passed through a quiescent state. +Once a grace period has completed, record of that fact is propagated +back down the tree. + +</p><p>As can be seen from the diagram, on a 64-bit system +a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout +of 64 at the root and a fanout of 16 at the leaves. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why isn't the fanout at the leaves also 64? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Because there are more types of events that affect the leaf-level + <tt>rcu_node</tt> structures than further up the tree. + Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of + 64, the contention on these structures' <tt>->structures</tt> + becomes excessive. + Experimentation on a wide variety of systems has shown that a fanout + of 16 works well for the leaves of the <tt>rcu_node</tt> tree. + </font> + + <p><font color="ffffff">Of course, further experience with + systems having hundreds or thousands of CPUs may demonstrate + that the fanout for the non-leaf <tt>rcu_node</tt> structures + must also be reduced. + Such reduction can be easily carried out when and if it proves + necessary. + In the meantime, if you are using such a system and running into + contention problems on the non-leaf <tt>rcu_node</tt> structures, + you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration + parameter to reduce the non-leaf fanout as needed. + </font> + + <p><font color="ffffff">Kernels built for systems with + strong NUMA characteristics might also need to adjust + <tt>CONFIG_RCU_FANOUT</tt> so that the domains of the + <tt>rcu_node</tt> structures align with hardware boundaries. + However, there has thus far been no need for this. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p>If your system has more than 1,024 CPUs (or more than 512 CPUs on +a 32-bit system), then RCU will automatically add more levels to the +tree. +For example, if you are crazy enough to build a 64-bit system with 65,536 +CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows: + +</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%"> + +</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system +accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for +32-bit systems. +On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be +as small as 2 if you wish, which would permit only 16 CPUs, which +is useful for testing. + +</p><p>This multi-level combining tree allows us to get most of the +performance and scalability +benefits of partitioning, even though RCU grace-period detection is +inherently a global operation. +The trick here is that only the last CPU to report a quiescent state +into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt> +structure at the next level up the tree. +This means that at the leaf-level <tt>rcu_node</tt> structure, only +one access out of sixteen will progress up the tree. +For the internal <tt>rcu_node</tt> structures, the situation is even +more extreme: Only one access out of sixty-four will progress up +the tree. +Because the vast majority of the CPUs do not progress up the tree, +the lock contention remains roughly constant up the tree. +No matter how many CPUs there are in the system, at most 64 quiescent-state +reports per grace period will progress all the way to the root +<tt>rcu_node</tt> structure, thus ensuring that the lock contention +on that root <tt>rcu_node</tt> structure remains acceptably low. + +</p><p>In effect, the combining tree acts like a big shock absorber, +keeping lock contention under control at all tree levels regardless +of the level of loading on the system. + +</p><p>The Linux kernel actually supports multiple flavors of RCU +running concurrently, so RCU builds separate data structures for each +flavor. +For example, for <tt>CONFIG_TREE_RCU=y</tt> kernels, RCU provides +rcu_sched and rcu_bh, as shown below: + +</p><p><img src="BigTreeClassicRCUBH.svg" alt="BigTreeClassicRCUBH.svg" width="33%"> + +</p><p>Energy efficiency is increasingly important, and for that +reason the Linux kernel provides <tt>CONFIG_NO_HZ_IDLE</tt>, which +turns off the scheduling-clock interrupts on idle CPUs, which in +turn allows those CPUs to attain deeper sleep states and to consume +less energy. +CPUs whose scheduling-clock interrupts have been turned off are +said to be in <i>dyntick-idle mode</i>. +RCU must handle dyntick-idle CPUs specially +because RCU would otherwise wake up each CPU on every grace period, +which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>. +RCU uses the <tt>rcu_dynticks</tt> structure to track +which CPUs are in dyntick idle mode, as shown below: + +</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%"> + +</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode +for all flavors of RCU. +Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per +CPU, and all of a given CPU's <tt>rcu_data</tt> structures share +that <tt>rcu_dynticks</tt>, as shown in the figure. + +</p><p>Kernels built with <tt>CONFIG_PREEMPT_RCU</tt> support +rcu_preempt in addition to rcu_sched and rcu_bh, as shown below: + +</p><p><img src="BigTreePreemptRCUBHdyntick.svg" alt="BigTreePreemptRCUBHdyntick.svg" width="35%"> + +</p><p>RCU updaters wait for normal grace periods by registering +RCU callbacks, either directly via <tt>call_rcu()</tt> and +friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>), +there being a separate interface per flavor of RCU) +or indirectly via <tt>synchronize_rcu()</tt> and friends. +RCU callbacks are represented by <tt>rcu_head</tt> structures, +which are queued on <tt>rcu_data</tt> structures while they are +waiting for a grace period to elapse, as shown in the following figure: + +</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%"> + +</p><p>This figure shows how <tt>TREE_RCU</tt>'s and +<tt>PREEMPT_RCU</tt>'s major data structures are related. +Lesser data structures will be introduced with the algorithms that +make use of them. + +</p><p>Note that each of the data structures in the above figure has +its own synchronization: + +<p><ol> +<li> Each <tt>rcu_state</tt> structures has a lock and a mutex, + and some fields are protected by the corresponding root + <tt>rcu_node</tt> structure's lock. +<li> Each <tt>rcu_node</tt> structure has a spinlock. +<li> The fields in <tt>rcu_data</tt> are private to the corresponding + CPU, although a few can be read and written by other CPUs. +<li> Similarly, the fields in <tt>rcu_dynticks</tt> are private + to the corresponding CPU, although a few can be read by + other CPUs. +</ol> + +<p>It is important to note that different data structures can have +very different ideas about the state of RCU at any given time. +For but one example, awareness of the start or end of a given RCU +grace period propagates slowly through the data structures. +This slow propagation is absolutely necessary for RCU to have good +read-side performance. +If this balkanized implementation seems foreign to you, one useful +trick is to consider each instance of these data structures to be +a different person, each having the usual slightly different +view of reality. + +</p><p>The general role of each of these data structures is as +follows: + +</p><ol> +<li> <tt>rcu_state</tt>: + This structure forms the interconnection between the + <tt>rcu_node</tt> and <tt>rcu_data</tt> structures, + tracks grace periods, serves as short-term repository + for callbacks orphaned by CPU-hotplug events, + maintains <tt>rcu_barrier()</tt> state, + tracks expedited grace-period state, + and maintains state used to force quiescent states when + grace periods extend too long, +<li> <tt>rcu_node</tt>: This structure forms the combining + tree that propagates quiescent-state + information from the leaves to the root, and also propagates + grace-period information from the root to the leaves. + It provides local copies of the grace-period state in order + to allow this information to be accessed in a synchronized + manner without suffering the scalability limitations that + would otherwise be imposed by global locking. + In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists + of tasks that have blocked while in their current + RCU read-side critical section. + In <tt>CONFIG_PREEMPT_RCU</tt> with + <tt>CONFIG_RCU_BOOST</tt>, it manages the + per-<tt>rcu_node</tt> priority-boosting + kernel threads (kthreads) and state. + Finally, it records CPU-hotplug state in order to determine + which CPUs should be ignored during a given grace period. +<li> <tt>rcu_data</tt>: This per-CPU structure is the + focus of quiescent-state detection and RCU callback queuing. + It also tracks its relationship to the corresponding leaf + <tt>rcu_node</tt> structure to allow more-efficient + propagation of quiescent states up the <tt>rcu_node</tt> + combining tree. + Like the <tt>rcu_node</tt> structure, it provides a local + copy of the grace-period information to allow for-free + synchronized + access to this information from the corresponding CPU. + Finally, this structure records past dyntick-idle state + for the corresponding CPU and also tracks statistics. +<li> <tt>rcu_dynticks</tt>: + This per-CPU structure tracks the current dyntick-idle + state for the corresponding CPU. + Unlike the other three structures, the <tt>rcu_dynticks</tt> + structure is not replicated per RCU flavor. +<li> <tt>rcu_head</tt>: + This structure represents RCU callbacks, and is the + only structure allocated and managed by RCU users. + The <tt>rcu_head</tt> structure is normally embedded + within the RCU-protected data structure. +</ol> + +<p>If all you wanted from this article was a general notion of how +RCU's data structures are related, you are done. +Otherwise, each of the following sections give more details on +the <tt>rcu_state</tt>, <tt>rcu_node</tt>, <tt>rcu_data</tt>, +and <tt>rcu_dynticks</tt> data structures. + +<h3><a name="The rcu_state Structure"> +The <tt>rcu_state</tt> Structure</a></h3> + +<p>The <tt>rcu_state</tt> structure is the base structure that +represents a flavor of RCU. +This structure forms the interconnection between the +<tt>rcu_node</tt> and <tt>rcu_data</tt> structures, +tracks grace periods, contains the lock used to +synchronize with CPU-hotplug events, +and maintains state used to force quiescent states when +grace periods extend too long, + +</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed, +singly and in groups, in the following sections. +The more specialized fields are covered in the discussion of their +use. + +<h5>Relationship to rcu_node and rcu_data Structures</h5> + +This portion of the <tt>rcu_state</tt> structure is declared +as follows: + +<pre> + 1 struct rcu_node node[NUM_RCU_NODES]; + 2 struct rcu_node *level[NUM_RCU_LVLS + 1]; + 3 struct rcu_data __percpu *rda; +</pre> + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Wait a minute! + You said that the <tt>rcu_node</tt> structures formed a tree, + but they are declared as a flat array! + What gives? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + The tree is laid out in the array. + The first node In the array is the head, the next set of nodes in the + array are children of the head node, and so on until the last set of + nodes in the array are the leaves. + </font> + + <p><font color="ffffff">See the following diagrams to see how + this works. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p>The <tt>rcu_node</tt> tree is embedded into the +<tt>->node[]</tt> array as shown in the following figure: + +</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%"> + +</p><p>One interesting consequence of this mapping is that a +breadth-first traversal of the tree is implemented as a simple +linear scan of the array, which is in fact what the +<tt>rcu_for_each_node_breadth_first()</tt> macro does. +This macro is used at the beginning and ends of grace periods. + +</p><p>Each entry of the <tt>->level</tt> array references +the first <tt>rcu_node</tt> structure on the corresponding level +of the tree, for example, as shown below: + +</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%"> + +</p><p>The zero<sup>th</sup> element of the array references the root +<tt>rcu_node</tt> structure, the first element references the +first child of the root <tt>rcu_node</tt>, and finally the second +element references the first leaf <tt>rcu_node</tt> structure. + +</p><p>For whatever it is worth, if you draw the tree to be tree-shaped +rather than array-shaped, it is easy to draw a planar representation: + +</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%"> + +</p><p>Finally, the <tt>->rda</tt> field references a per-CPU +pointer to the corresponding CPU's <tt>rcu_data</tt> structure. + +</p><p>All of these fields are constant once initialization is complete, +and therefore need no protection. + +<h5>Grace-Period Tracking</h5> + +<p>This portion of the <tt>rcu_state</tt> structure is declared +as follows: + +<pre> + 1 unsigned long gpnum; + 2 unsigned long completed; +</pre> + +<p>RCU grace periods are numbered, and +the <tt>->gpnum</tt> field contains the number of the grace +period that started most recently. +The <tt>->completed</tt> field contains the number of the +grace period that completed most recently. +If the two fields are equal, the RCU grace period that most recently +started has already completed, and therefore the corresponding +flavor of RCU is idle. +If <tt>->gpnum</tt> is one greater than <tt>->completed</tt>, +then <tt>->gpnum</tt> gives the number of the current RCU +grace period, which has not yet completed. +Any other combination of values indicates that something is broken. +These two fields are protected by the root <tt>rcu_node</tt>'s +<tt>->lock</tt> field. + +</p><p>There are <tt>->gpnum</tt> and <tt>->completed</tt> fields +in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures +as well. +The fields in the <tt>rcu_state</tt> structure represent the +most current values, and those of the other structures are compared +in order to detect the start of a new grace period in a distributed +fashion. +The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt> +(down the tree from the root to the leaves) to <tt>rcu_data</tt>. + +<h5>Miscellaneous</h5> + +<p>This portion of the <tt>rcu_state</tt> structure is declared +as follows: + +<pre> + 1 unsigned long gp_max; + 2 char abbr; + 3 char *name; +</pre> + +<p>The <tt>->gp_max</tt> field tracks the duration of the longest +grace period in jiffies. +It is protected by the root <tt>rcu_node</tt>'s <tt>->lock</tt>. + +<p>The <tt>->name</tt> field points to the name of the RCU flavor +(for example, “rcu_sched”), and is constant. +The <tt>->abbr</tt> field contains a one-character abbreviation, +for example, “s” for RCU-sched. + +<h3><a name="The rcu_node Structure"> +The <tt>rcu_node</tt> Structure</a></h3> + +<p>The <tt>rcu_node</tt> structures form the combining +tree that propagates quiescent-state +information from the leaves to the root and also that propagates +grace-period information from the root down to the leaves. +They provides local copies of the grace-period state in order +to allow this information to be accessed in a synchronized +manner without suffering the scalability limitations that +would otherwise be imposed by global locking. +In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists +of tasks that have blocked while in their current +RCU read-side critical section. +In <tt>CONFIG_PREEMPT_RCU</tt> with +<tt>CONFIG_RCU_BOOST</tt>, they manage the +per-<tt>rcu_node</tt> priority-boosting +kernel threads (kthreads) and state. +Finally, they record CPU-hotplug state in order to determine +which CPUs should be ignored during a given grace period. + +</p><p>The <tt>rcu_node</tt> structure's fields are discussed, +singly and in groups, in the following sections. + +<h5>Connection to Combining Tree</h5> + +<p>This portion of the <tt>rcu_node</tt> structure is declared +as follows: + +<pre> + 1 struct rcu_node *parent; + 2 u8 level; + 3 u8 grpnum; + 4 unsigned long grpmask; + 5 int grplo; + 6 int grphi; +</pre> + +<p>The <tt>->parent</tt> pointer references the <tt>rcu_node</tt> +one level up in the tree, and is <tt>NULL</tt> for the root +<tt>rcu_node</tt>. +The RCU implementation makes heavy use of this field to push quiescent +states up the tree. +The <tt>->level</tt> field gives the level in the tree, with +the root being at level zero, its children at level one, and so on. +The <tt>->grpnum</tt> field gives this node's position within +the children of its parent, so this number can range between 0 and 31 +on 32-bit systems and between 0 and 63 on 64-bit systems. +The <tt>->level</tt> and <tt>->grpnum</tt> fields are +used only during initialization and for tracing. +The <tt>->grpmask</tt> field is the bitmask counterpart of +<tt>->grpnum</tt>, and therefore always has exactly one bit set. +This mask is used to clear the bit corresponding to this <tt>rcu_node</tt> +structure in its parent's bitmasks, which are described later. +Finally, the <tt>->grplo</tt> and <tt>->grphi</tt> fields +contain the lowest and highest numbered CPU served by this +<tt>rcu_node</tt> structure, respectively. + +</p><p>All of these fields are constant, and thus do not require any +synchronization. + +<h5>Synchronization</h5> + +<p>This field of the <tt>rcu_node</tt> structure is declared +as follows: + +<pre> + 1 raw_spinlock_t lock; +</pre> + +<p>This field is used to protect the remaining fields in this structure, +unless otherwise stated. +That said, all of the fields in this structure can be accessed without +locking for tracing purposes. +Yes, this can result in confusing traces, but better some tracing confusion +than to be heisenbugged out of existence. + +<h5>Grace-Period Tracking</h5> + +<p>This portion of the <tt>rcu_node</tt> structure is declared +as follows: + +<pre> + 1 unsigned long gpnum; + 2 unsigned long completed; +</pre> + +<p>These fields are the counterparts of the fields of the same name in +the <tt>rcu_state</tt> structure. +They each may lag up to one behind their <tt>rcu_state</tt> +counterparts. +If a given <tt>rcu_node</tt> structure's <tt>->gpnum</tt> and +<tt>->complete</tt> fields are equal, then this <tt>rcu_node</tt> +structure believes that RCU is idle. +Otherwise, as with the <tt>rcu_state</tt> structure, +the <tt>->gpnum</tt> field will be one greater than the +<tt>->complete</tt> fields, with <tt>->gpnum</tt> +indicating which grace period this <tt>rcu_node</tt> believes +is still being waited for. + +</p><p>The <tt>>gpnum</tt> field of each <tt>rcu_node</tt> +structure is updated at the beginning +of each grace period, and the <tt>->completed</tt> fields are +updated at the end of each grace period. + +<h5>Quiescent-State Tracking</h5> + +<p>These fields manage the propagation of quiescent states up the +combining tree. + +</p><p>This portion of the <tt>rcu_node</tt> structure has fields +as follows: + +<pre> + 1 unsigned long qsmask; + 2 unsigned long expmask; + 3 unsigned long qsmaskinit; + 4 unsigned long expmaskinit; +</pre> + +<p>The <tt>->qsmask</tt> field tracks which of this +<tt>rcu_node</tt> structure's children still need to report +quiescent states for the current normal grace period. +Such children will have a value of 1 in their corresponding bit. +Note that the leaf <tt>rcu_node</tt> structures should be +thought of as having <tt>rcu_data</tt> structures as their +children. +Similarly, the <tt>->expmask</tt> field tracks which +of this <tt>rcu_node</tt> structure's children still need to report +quiescent states for the current expedited grace period. +An expedited grace period has +the same conceptual properties as a normal grace period, but the +expedited implementation accepts extreme CPU overhead to obtain +much lower grace-period latency, for example, consuming a few +tens of microseconds worth of CPU time to reduce grace-period +duration from milliseconds to tens of microseconds. +The <tt>->qsmaskinit</tt> field tracks which of this +<tt>rcu_node</tt> structure's children cover for at least +one online CPU. +This mask is used to initialize <tt>->qsmask</tt>, +and <tt>->expmaskinit</tt> is used to initialize +<tt>->expmask</tt> and the beginning of the +normal and expedited grace periods, respectively. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why are these bitmasks protected by locking? + Come on, haven't you heard of atomic instructions??? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Lockless grace-period computation! Such a tantalizing possibility! + </font> + + <p><font color="ffffff">But consider the following sequence of events: + </font> + + <ol> + <li> <font color="ffffff">CPU 0 has been in dyntick-idle + mode for quite some time. + When it wakes up, it notices that the current RCU + grace period needs it to report in, so it sets a + flag where the scheduling clock interrupt will find it. + </font><p> + <li> <font color="ffffff">Meanwhile, CPU 1 is running + <tt>force_quiescent_state()</tt>, + and notices that CPU 0 has been in dyntick idle mode, + which qualifies as an extended quiescent state. + </font><p> + <li> <font color="ffffff">CPU 0's scheduling clock + interrupt fires in the + middle of an RCU read-side critical section, and notices + that the RCU core needs something, so commences RCU softirq + processing. + </font> + <p> + <li> <font color="ffffff">CPU 0's softirq handler + executes and is just about ready + to report its quiescent state up the <tt>rcu_node</tt> + tree. + </font><p> + <li> <font color="ffffff">But CPU 1 beats it to the punch, + completing the current + grace period and starting a new one. + </font><p> + <li> <font color="ffffff">CPU 0 now reports its quiescent + state for the wrong + grace period. + That grace period might now end before the RCU read-side + critical section. + If that happens, disaster will ensue. + </font> + </ol> + + <p><font color="ffffff">So the locking is absolutely required in + order to coordinate + clearing of the bits with the grace-period numbers in + <tt>->gpnum</tt> and <tt>->completed</tt>. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<h5>Blocked-Task Management</h5> + +<p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the +midst of their RCU read-side critical sections, and these tasks +must be tracked explicitly. +The details of exactly why and how they are tracked will be covered +in a separate article on RCU read-side processing. +For now, it is enough to know that the <tt>rcu_node</tt> +structure tracks them. + +<pre> + 1 struct list_head blkd_tasks; + 2 struct list_head *gp_tasks; + 3 struct list_head *exp_tasks; + 4 bool wait_blkd_tasks; +</pre> + +<p>The <tt>->blkd_tasks</tt> field is a list header for +the list of blocked and preempted tasks. +As tasks undergo context switches within RCU read-side critical +sections, their <tt>task_struct</tt> structures are enqueued +(via the <tt>task_struct</tt>'s <tt>->rcu_node_entry</tt> +field) onto the head of the <tt>->blkd_tasks</tt> list for the +leaf <tt>rcu_node</tt> structure corresponding to the CPU +on which the outgoing context switch executed. +As these tasks later exit their RCU read-side critical sections, +they remove themselves from the list. +This list is therefore in reverse time order, so that if one of the tasks +is blocking the current grace period, all subsequent tasks must +also be blocking that same grace period. +Therefore, a single pointer into this list suffices to track +all tasks blocking a given grace period. +That pointer is stored in <tt>->gp_tasks</tt> for normal +grace periods and in <tt>->exp_tasks</tt> for expedited +grace periods. +These last two fields are <tt>NULL</tt> if either there is +no grace period in flight or if there are no blocked tasks +preventing that grace period from completing. +If either of these two pointers is referencing a task that +removes itself from the <tt>->blkd_tasks</tt> list, +then that task must advance the pointer to the next task on +the list, or set the pointer to <tt>NULL</tt> if there +are no subsequent tasks on the list. + +</p><p>For example, suppose that tasks T1, T2, and T3 are +all hard-affinitied to the largest-numbered CPU in the system. +Then if task T1 blocked in an RCU read-side +critical section, then an expedited grace period started, +then task T2 blocked in an RCU read-side critical section, +then a normal grace period started, and finally task 3 blocked +in an RCU read-side critical section, then the state of the +last leaf <tt>rcu_node</tt> structure's blocked-task list +would be as shown below: + +</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%"> + +</p><p>Task T1 is blocking both grace periods, task T2 is +blocking only the normal grace period, and task T3 is blocking +neither grace period. +Note that these tasks will not remove themselves from this list +immediately upon resuming execution. +They will instead remain on the list until they execute the outermost +<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical +section. + +<p> +The <tt>->wait_blkd_tasks</tt> field indicates whether or not +the current grace period is waiting on a blocked task. + +<h5>Sizing the <tt>rcu_node</tt> Array</h5> + +<p>The <tt>rcu_node</tt> array is sized via a series of +C-preprocessor expressions as follows: + +<pre> + 1 #ifdef CONFIG_RCU_FANOUT + 2 #define RCU_FANOUT CONFIG_RCU_FANOUT + 3 #else + 4 # ifdef CONFIG_64BIT + 5 # define RCU_FANOUT 64 + 6 # else + 7 # define RCU_FANOUT 32 + 8 # endif + 9 #endif +10 +11 #ifdef CONFIG_RCU_FANOUT_LEAF +12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF +13 #else +14 # ifdef CONFIG_64BIT +15 # define RCU_FANOUT_LEAF 64 +16 # else +17 # define RCU_FANOUT_LEAF 32 +18 # endif +19 #endif +20 +21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF) +22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT) +23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT) +24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT) +25 +26 #if NR_CPUS <= RCU_FANOUT_1 +27 # define RCU_NUM_LVLS 1 +28 # define NUM_RCU_LVL_0 1 +29 # define NUM_RCU_NODES NUM_RCU_LVL_0 +30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 } +31 # define RCU_NODE_NAME_INIT { "rcu_node_0" } +32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" } +33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" } +34 #elif NR_CPUS <= RCU_FANOUT_2 +35 # define RCU_NUM_LVLS 2 +36 # define NUM_RCU_LVL_0 1 +37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) +38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1) +39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 } +40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" } +41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" } +42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" } +43 #elif NR_CPUS <= RCU_FANOUT_3 +44 # define RCU_NUM_LVLS 3 +45 # define NUM_RCU_LVL_0 1 +46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2) +47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) +48 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2) +49 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 } +50 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" } +51 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" } +52 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" } +53 #elif NR_CPUS <= RCU_FANOUT_4 +54 # define RCU_NUM_LVLS 4 +55 # define NUM_RCU_LVL_0 1 +56 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3) +57 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2) +58 # define NUM_RCU_LVL_3 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) +59 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3) +60 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 } +61 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" } +62 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" } +63 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" } +64 #else +65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS" +66 #endif +</pre> + +<p>The maximum number of levels in the <tt>rcu_node</tt> structure +is currently limited to four, as specified by lines 21-24 +and the structure of the subsequent “if” statement. +For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which +should be sufficient for the next few years at least. +For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which +should see us through the next decade or so. +This four-level tree also allows kernels built with +<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs, +which might be useful in very large systems having eight CPUs per +socket (but please note that no one has yet shown any measurable +performance degradation due to misaligned socket and <tt>rcu_node</tt> +boundaries). +In addition, building kernels with a full four levels of <tt>rcu_node</tt> +tree permits better testing of RCU's combining-tree code. + +</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children +are permitted at each non-leaf level of the <tt>rcu_node</tt> tree. +If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified, +it is set based on the word size of the system, which is also +the Kconfig default. + +</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are +handled by each leaf <tt>rcu_node</tt> structure. +Experience has shown that allowing a given leaf <tt>rcu_node</tt> +structure to handle 64 CPUs, as permitted by the number of bits in +the <tt>->qsmask</tt> field on a 64-bit system, results in +excessive contention for the leaf <tt>rcu_node</tt> structures' +<tt>->lock</tt> fields. +The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore +limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>. +If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value +selected is based on the word size of the system, just as for +<tt>CONFIG_RCU_FANOUT</tt>. +Lines 11-19 perform this computation. + +</p><p>Lines 21-24 compute the maximum number of CPUs supported by +a single-level (which contains a single <tt>rcu_node</tt> structure), +two-level, three-level, and four-level <tt>rcu_node</tt> tree, +respectively, given the fanout specified by <tt>RCU_FANOUT</tt> +and <tt>RCU_FANOUT_LEAF</tt>. +These numbers of CPUs are retained in the +<tt>RCU_FANOUT_1</tt>, +<tt>RCU_FANOUT_2</tt>, +<tt>RCU_FANOUT_3</tt>, and +<tt>RCU_FANOUT_4</tt> +C-preprocessor variables, respectively. + +</p><p>These variables are used to control the C-preprocessor <tt>#if</tt> +statement spanning lines 26-66 that computes the number of +<tt>rcu_node</tt> structures required for each level of the tree, +as well as the number of levels required. +The number of levels is placed in the <tt>NUM_RCU_LVLS</tt> +C-preprocessor variable by lines 27, 35, 44, and 54. +The number of <tt>rcu_node</tt> structures for the topmost level +of the tree is always exactly one, and this value is unconditionally +placed into <tt>NUM_RCU_LVL_0</tt> by lines 28, 36, 45, and 55. +The rest of the levels (if any) of the <tt>rcu_node</tt> tree +are computed by dividing the maximum number of CPUs by the +fanout supported by the number of levels from the current level down, +rounding up. This computation is performed by lines 37, +46-47, and 56-58. +Lines 31-33, 40-42, 50-52, and 62-63 create initializers +for lockdep lock-class names. +Finally, lines 64-66 produce an error if the maximum number of +CPUs is too large for the specified fanout. + +<h3><a name="The rcu_data Structure"> +The <tt>rcu_data</tt> Structure</a></h3> + +<p>The <tt>rcu_data</tt> maintains the per-CPU state for the +corresponding flavor of RCU. +The fields in this structure may be accessed only from the corresponding +CPU (and from tracing) unless otherwise stated. +This structure is the +focus of quiescent-state detection and RCU callback queuing. +It also tracks its relationship to the corresponding leaf +<tt>rcu_node</tt> structure to allow more-efficient +propagation of quiescent states up the <tt>rcu_node</tt> +combining tree. +Like the <tt>rcu_node</tt> structure, it provides a local +copy of the grace-period information to allow for-free +synchronized +access to this information from the corresponding CPU. +Finally, this structure records past dyntick-idle state +for the corresponding CPU and also tracks statistics. + +</p><p>The <tt>rcu_data</tt> structure's fields are discussed, +singly and in groups, in the following sections. + +<h5>Connection to Other Data Structures</h5> + +<p>This portion of the <tt>rcu_data</tt> structure is declared +as follows: + +<pre> + 1 int cpu; + 2 struct rcu_state *rsp; + 3 struct rcu_node *mynode; + 4 struct rcu_dynticks *dynticks; + 5 unsigned long grpmask; + 6 bool beenonline; +</pre> + +<p>The <tt>->cpu</tt> field contains the number of the +corresponding CPU, the <tt>->rsp</tt> pointer references +the corresponding <tt>rcu_state</tt> structure (and is most frequently +used to locate the name of the corresponding flavor of RCU for tracing), +and the <tt>->mynode</tt> field references the corresponding +<tt>rcu_node</tt> structure. +The <tt>->mynode</tt> is used to propagate quiescent states +up the combining tree. +<p>The <tt>->dynticks</tt> pointer references the +<tt>rcu_dynticks</tt> structure corresponding to this +CPU. +Recall that a single per-CPU instance of the <tt>rcu_dynticks</tt> +structure is shared among all flavors of RCU. +These first four fields are constant and therefore require not +synchronization. + +</p><p>The <tt>->grpmask</tt> field indicates the bit in +the <tt>->mynode->qsmask</tt> corresponding to this +<tt>rcu_data</tt> structure, and is also used when propagating +quiescent states. +The <tt>->beenonline</tt> flag is set whenever the corresponding +CPU comes online, which means that the debugfs tracing need not dump +out any <tt>rcu_data</tt> structure for which this flag is not set. + +<h5>Quiescent-State and Grace-Period Tracking</h5> + +<p>This portion of the <tt>rcu_data</tt> structure is declared +as follows: + +<pre> + 1 unsigned long completed; + 2 unsigned long gpnum; + 3 bool cpu_no_qs; + 4 bool core_needs_qs; + 5 bool gpwrap; + 6 unsigned long rcu_qs_ctr_snap; +</pre> + +<p>The <tt>completed</tt> and <tt>gpnum</tt> +fields are the counterparts of the fields of the same name +in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures. +They may each lag up to one behind their <tt>rcu_node</tt> +counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and +<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag +arbitrarily far behind for CPUs in dyntick-idle mode (but these counters +will catch up upon exit from dyntick-idle mode). +If a given <tt>rcu_data</tt> structure's <tt>->gpnum</tt> and +<tt>->complete</tt> fields are equal, then this <tt>rcu_data</tt> +structure believes that RCU is idle. +Otherwise, as with the <tt>rcu_state</tt> and <tt>rcu_node</tt> +structure, +the <tt>->gpnum</tt> field will be one greater than the +<tt>->complete</tt> fields, with <tt>->gpnum</tt> +indicating which grace period this <tt>rcu_data</tt> believes +is still being waited for. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + All this replication of the grace period numbers can only cause + massive confusion. + Why not just keep a global pair of counters and be done with it??? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Because if there was only a single global pair of grace-period + numbers, there would need to be a single global lock to allow + safely accessing and updating them. + And if we are not going to have a single global lock, we need + to carefully manage the numbers on a per-node basis. + Recall from the answer to a previous Quick Quiz that the consequences + of applying a previously sampled quiescent state to the wrong + grace period are quite severe. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p>The <tt>->cpu_no_qs</tt> flag indicates that the +CPU has not yet passed through a quiescent state, +while the <tt>->core_needs_qs</tt> flag indicates that the +RCU core needs a quiescent state from the corresponding CPU. +The <tt>->gpwrap</tt> field indicates that the corresponding +CPU has remained idle for so long that the <tt>completed</tt> +and <tt>gpnum</tt> counters are in danger of overflow, which +will cause the CPU to disregard the values of its counters on +its next exit from idle. +Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect +cases where a given operation has resulted in a quiescent state +for all flavors of RCU, for example, <tt>cond_resched_rcu_qs()</tt>. + +<h5>RCU Callback Handling</h5> + +<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by +the same CPU that registered them. +This is strictly a cache-locality optimization: callbacks can and +do get invoked on CPUs other than the one that registered them. +After all, if the CPU that registered a given callback has gone +offline before the callback can be invoked, there really is no other +choice. + +</p><p>This portion of the <tt>rcu_data</tt> structure is declared +as follows: + +<pre> + 1 struct rcu_head *nxtlist; + 2 struct rcu_head **nxttail[RCU_NEXT_SIZE]; + 3 unsigned long nxtcompleted[RCU_NEXT_SIZE]; + 4 long qlen_lazy; + 5 long qlen; + 6 long qlen_last_fqs_check; + 7 unsigned long n_force_qs_snap; + 8 unsigned long n_cbs_invoked; + 9 unsigned long n_cbs_orphaned; +10 unsigned long n_cbs_adopted; +11 long blimit; +</pre> + +<p>The <tt>->nxtlist</tt> pointer and the +<tt>->nxttail[]</tt> array form a four-segment list with +older callbacks near the head and newer ones near the tail. +Each segment contains callbacks with the corresponding relationship +to the current grace period. +The pointer out of the end of each of the four segments is referenced +by the element of the <tt>->nxttail[]</tt> array indexed by +<tt>RCU_DONE_TAIL</tt> (for callbacks handled by a prior grace period), +<tt>RCU_WAIT_TAIL</tt> (for callbacks waiting on the current grace period), +<tt>RCU_NEXT_READY_TAIL</tt> (for callbacks that will wait on the next +grace period), and +<tt>RCU_NEXT_TAIL</tt> (for callbacks that are not yet associated +with a specific grace period) +respectively, as shown in the following figure. + +</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%"> + +</p><p>In this figure, the <tt>->nxtlist</tt> pointer references the +first +RCU callback in the list. +The <tt>->nxttail[RCU_DONE_TAIL]</tt> array element references +the <tt>->nxtlist</tt> pointer itself, indicating that none +of the callbacks is ready to invoke. +The <tt>->nxttail[RCU_WAIT_TAIL]</tt> array element references callback +CB 2's <tt>->next</tt> pointer, which indicates that +CB 1 and CB 2 are both waiting on the current grace period. +The <tt>->nxttail[RCU_NEXT_READY_TAIL]</tt> array element +references the same RCU callback that <tt>->nxttail[RCU_WAIT_TAIL]</tt> +does, which indicates that there are no callbacks waiting on the next +RCU grace period. +The <tt>->nxttail[RCU_NEXT_TAIL]</tt> array element references +CB 4's <tt>->next</tt> pointer, indicating that all the +remaining RCU callbacks have not yet been assigned to an RCU grace +period. +Note that the <tt>->nxttail[RCU_NEXT_TAIL]</tt> array element +always references the last RCU callback's <tt>->next</tt> pointer +unless the callback list is empty, in which case it references +the <tt>->nxtlist</tt> pointer. + +</p><p>CPUs advance their callbacks from the +<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the +<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments +as grace periods advance. +The CPU advances the callbacks in its <tt>rcu_data</tt> structure +whenever it notices that another RCU grace period has completed. +The CPU detects the completion of an RCU grace period by noticing +that the value of its <tt>rcu_data</tt> structure's +<tt>->completed</tt> field differs from that of its leaf +<tt>rcu_node</tt> structure. +Recall that each <tt>rcu_node</tt> structure's +<tt>->completed</tt> field is updated at the end of each +grace period. + +</p><p>The <tt>->nxtcompleted[]</tt> array records grace-period +numbers corresponding to the list segments. +This allows CPUs that go idle for extended periods to determine +which of their callbacks are ready to be invoked after reawakening. + +</p><p>The <tt>->qlen</tt> counter contains the number of +callbacks in <tt>->nxtlist</tt>, and the +<tt>->qlen_lazy</tt> contains the number of those callbacks that +are known to only free memory, and whose invocation can therefore +be safely deferred. +The <tt>->qlen_last_fqs_check</tt> and +<tt>->n_force_qs_snap</tt> coordinate the forcing of quiescent +states from <tt>call_rcu()</tt> and friends when callback +lists grow excessively long. + +</p><p>The <tt>->n_cbs_invoked</tt>, +<tt>->n_cbs_orphaned</tt>, and <tt>->n_cbs_adopted</tt> +fields count the number of callbacks invoked, +sent to other CPUs when this CPU goes offline, +and received from other CPUs when those other CPUs go offline. +Finally, the <tt>->blimit</tt> counter is the maximum number of +RCU callbacks that may be invoked at a given time. + +<h5>Dyntick-Idle Handling</h5> + +<p>This portion of the <tt>rcu_data</tt> structure is declared +as follows: + +<pre> + 1 int dynticks_snap; + 2 unsigned long dynticks_fqs; +</pre> + +The <tt>->dynticks_snap</tt> field is used to take a snapshot +of the corresponding CPU's dyntick-idle state when forcing +quiescent states, and is therefore accessed from other CPUs. +Finally, the <tt>->dynticks_fqs</tt> field is used to +count the number of times this CPU is determined to be in +dyntick-idle state, and is used for tracing and debugging purposes. + +<h3><a name="The rcu_dynticks Structure"> +The <tt>rcu_dynticks</tt> Structure</a></h3> + +<p>The <tt>rcu_dynticks</tt> maintains the per-CPU dyntick-idle state +for the corresponding CPU. +Unlike the other structures, <tt>rcu_dynticks</tt> is not +replicated over the different flavors of RCU. +The fields in this structure may be accessed only from the corresponding +CPU (and from tracing) unless otherwise stated. +Its fields are as follows: + +<pre> + 1 int dynticks_nesting; + 2 int dynticks_nmi_nesting; + 3 atomic_t dynticks; +</pre> + +<p>The <tt>->dynticks_nesting</tt> field counts the +nesting depth of normal interrupts. +In addition, this counter is incremented when exiting dyntick-idle +mode and decremented when entering it. +This counter can therefore be thought of as counting the number +of reasons why this CPU cannot be permitted to enter dyntick-idle +mode, aside from non-maskable interrupts (NMIs). +NMIs are counted by the <tt>->dynticks_nmi_nesting</tt> +field, except that NMIs that interrupt non-dyntick-idle execution +are not counted. + +</p><p>Finally, the <tt>->dynticks</tt> field counts the corresponding +CPU's transitions to and from dyntick-idle mode, so that this counter +has an even value when the CPU is in dyntick-idle mode and an odd +value otherwise. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why not just count all NMIs? + Wouldn't that be simpler and less error prone? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + It seems simpler only until you think hard about how to go about + updating the <tt>rcu_dynticks</tt> structure's + <tt>->dynticks</tt> field. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p>Additional fields are present for some special-purpose +builds, and are discussed separately. + +<h3><a name="The rcu_head Structure"> +The <tt>rcu_head</tt> Structure</a></h3> + +<p>Each <tt>rcu_head</tt> structure represents an RCU callback. +These structures are normally embedded within RCU-protected data +structures whose algorithms use asynchronous grace periods. +In contrast, when using algorithms that block waiting for RCU grace periods, +RCU users need not provide <tt>rcu_head</tt> structures. + +</p><p>The <tt>rcu_head</tt> structure has fields as follows: + +<pre> + 1 struct rcu_head *next; + 2 void (*func)(struct rcu_head *head); +</pre> + +<p>The <tt>->next</tt> field is used +to link the <tt>rcu_head</tt> structures together in the +lists within the <tt>rcu_data</tt> structures. +The <tt>->func</tt> field is a pointer to the function +to be called when the callback is ready to be invoked, and +this function is passed a pointer to the <tt>rcu_head</tt> +structure. +However, <tt>kfree_rcu()</tt> uses the <tt>->func</tt> +field to record the offset of the <tt>rcu_head</tt> +structure within the enclosing RCU-protected data structure. + +</p><p>Both of these fields are used internally by RCU. +From the viewpoint of RCU users, this structure is an +opaque “cookie”. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Given that the callback function <tt>->func</tt> + is passed a pointer to the <tt>rcu_head</tt> structure, + how is that function supposed to find the beginning of the + enclosing RCU-protected data structure? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In actual practice, there is a separate callback function per + type of RCU-protected data structure. + The callback function can therefore use the <tt>container_of()</tt> + macro in the Linux kernel (or other pointer-manipulation facilities + in other software environments) to find the beginning of the + enclosing structure. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<h3><a name="RCU-Specific Fields in the task_struct Structure"> +RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3> + +<p>The <tt>CONFIG_PREEMPT_RCU</tt> implementation uses some +additional fields in the <tt>task_struct</tt> structure: + +<pre> + 1 #ifdef CONFIG_PREEMPT_RCU + 2 int rcu_read_lock_nesting; + 3 union rcu_special rcu_read_unlock_special; + 4 struct list_head rcu_node_entry; + 5 struct rcu_node *rcu_blocked_node; + 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */ + 7 #ifdef CONFIG_TASKS_RCU + 8 unsigned long rcu_tasks_nvcsw; + 9 bool rcu_tasks_holdout; +10 struct list_head rcu_tasks_holdout_list; +11 int rcu_tasks_idle_cpu; +12 #endif /* #ifdef CONFIG_TASKS_RCU */ +</pre> + +<p>The <tt>->rcu_read_lock_nesting</tt> field records the +nesting level for RCU read-side critical sections, and +the <tt>->rcu_read_unlock_special</tt> field is a bitmask +that records special conditions that require <tt>rcu_read_unlock()</tt> +to do additional work. +The <tt>->rcu_node_entry</tt> field is used to form lists of +tasks that have blocked within preemptible-RCU read-side critical +sections and the <tt>->rcu_blocked_node</tt> field references +the <tt>rcu_node</tt> structure whose list this task is a member of, +or <tt>NULL</tt> if it is not blocked within a preemptible-RCU +read-side critical section. + +<p>The <tt>->rcu_tasks_nvcsw</tt> field tracks the number of +voluntary context switches that this task had undergone at the +beginning of the current tasks-RCU grace period, +<tt>->rcu_tasks_holdout</tt> is set if the current tasks-RCU +grace period is waiting on this task, <tt>->rcu_tasks_holdout_list</tt> +is a list element enqueuing this task on the holdout list, +and <tt>->rcu_tasks_idle_cpu</tt> tracks which CPU this +idle task is running, but only if the task is currently running, +that is, if the CPU is currently idle. + +<h3><a name="Accessor Functions"> +Accessor Functions</a></h3> + +<p>The following listing shows the +<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>, +<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and +<tt>rcu_for_each_leaf_node()</tt> function and macros: + +<pre> + 1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp) + 2 { + 3 return &rsp->node[0]; + 4 } + 5 + 6 #define rcu_for_each_node_breadth_first(rsp, rnp) \ + 7 for ((rnp) = &(rsp)->node[0]; \ + 8 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++) + 9 + 10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \ + 11 for ((rnp) = &(rsp)->node[0]; \ + 12 (rnp) < (rsp)->level[NUM_RCU_LVLS - 1]; (rnp)++) + 13 + 14 #define rcu_for_each_leaf_node(rsp, rnp) \ + 15 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \ + 16 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++) +</pre> + +<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the +first element of the specified <tt>rcu_state</tt> structure's +<tt>->node[]</tt> array, which is the root <tt>rcu_node</tt> +structure. + +</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt> +macro takes advantage of the layout of the <tt>rcu_node</tt> +structures in the <tt>rcu_state</tt> structure's +<tt>->node[]</tt> array, performing a breadth-first traversal by +simply traversing the array in order. +The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates +similarly, but traverses only the first part of the array, thus excluding +the leaf <tt>rcu_node</tt> structures. +Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only +the last part of the array, thus traversing only the leaf +<tt>rcu_node</tt> structures. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and + <tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree + contains only a single node? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In the single-node case, + <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op + and <tt>rcu_for_each_leaf_node()</tt> traverses the single node. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<h3><a name="Summary"> +Summary</a></h3> + +So each flavor of RCU is represented by an <tt>rcu_state</tt> structure, +which contains a combining tree of <tt>rcu_node</tt> and +<tt>rcu_data</tt> structures. +Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle +state is tracked by an <tt>rcu_dynticks</tt> structure. + +If you made it this far, you are well prepared to read the code +walkthroughs in the other articles in this series. + +<h3><a name="Acknowledgments"> +Acknowledgments</a></h3> + +I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul +Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn +for helping me get this document into a more human-readable state. + +<h3><a name="Legal Statement"> +Legal Statement</a></h3> + +<p>This work represents the view of the author and does not necessarily +represent the view of IBM. + +</p><p>Linux is a registered trademark of Linus Torvalds. + +</p><p>Other company, product, and service names may be trademarks or +service marks of others. + +</body></html> diff --git a/Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg b/Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg new file mode 100644 index 000000000000..2bf12b468206 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg @@ -0,0 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font-weight="normal" - font-size="324" - id="text14" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3017">Read-Mostly, Stale &</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="1350" - font-style="normal" - font-weight="normal" - font-size="324" - id="text16" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3019">Inconsistent Data OK</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="1800" - font-style="normal" - font-weight="normal" - font-size="324" - id="text18" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3021">(RCU Works Great!!!)</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="3825" - font-style="normal" - font-weight="normal" - font-size="324" - id="text20" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3023">(RCU Works Well)</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="3375" - font-style="normal" - font-weight="normal" - font-size="324" - id="text22" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3025">Read-Mostly, Need Consistent Data</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="5175" - font-style="normal" - font-weight="normal" - font-size="324" - id="text24" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3027">Read-Write, Need Consistent Data</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="6975" - font-style="normal" - font-weight="normal" - font-size="324" - id="text26" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - sodipodi:linespacing="125%">Update-Mostly, Need Consistent Data</text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="5625" - font-style="normal" - font-weight="normal" - font-size="324" - id="text28" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3029">(RCU Might Be OK...)</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="7875" - font-style="normal" - font-weight="normal" - font-size="324" - id="text30" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - sodipodi:linespacing="125%">(1) Provide Existence Guarantees For Update-Friendly Mechanisms</text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="8325" - font-style="normal" - font-weight="normal" - font-size="324" - id="text32" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - sodipodi:linespacing="125%">(2) Provide Wait-Free Read-Side Primitives for Real-Time Use)</text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="7425" - font-style="normal" - font-weight="normal" - font-size="324" - id="text34" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - sodipodi:linespacing="125%">(RCU is Very Unlikely to be the Right Tool For The Job, But it Can:</text> - </g> -</svg> diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index a725f9900ec8..e7e24b3e86e2 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -1,5 +1,3 @@ -<!-- DO NOT HAND EDIT. --> -<!-- Instead, edit Documentation/RCU/Design/Requirements/Requirements.htmlx and run 'sh htmlqqz.sh Documentation/RCU/Design/Requirements/Requirements' --> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> @@ -65,8 +63,8 @@ All that aside, here are the categories of currently known RCU requirements: <p> This is followed by a <a href="#Summary">summary</a>, -which is in turn followed by the inevitable -<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>. +however, the answers to each quick quiz immediately follows the quiz. +Select the big white space with your mouse to see the answer. <h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2> @@ -153,13 +151,27 @@ Therefore, the outcome: </blockquote> cannot happen. -<p><a name="Quick Quiz 1"><b>Quick Quiz 1</b>:</a> -Wait a minute! -You said that updaters can make useful forward progress concurrently -with readers, but pre-existing readers will block -<tt>synchronize_rcu()</tt>!!! -Just who are you trying to fool??? -<br><a href="#qq1answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Wait a minute! + You said that updaters can make useful forward progress concurrently + with readers, but pre-existing readers will block + <tt>synchronize_rcu()</tt>!!! + Just who are you trying to fool??? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + First, if updaters do not wish to be blocked by readers, they can use + <tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will + be discussed later. + Second, even when using <tt>synchronize_rcu()</tt>, the other + update-side code does run concurrently with readers, whether + pre-existing or not. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> This scenario resembles one of the first uses of RCU in @@ -210,9 +222,20 @@ to guarantee that <tt>do_something()</tt> never runs concurrently with <tt>recovery()</tt>, but with little or no synchronization overhead in <tt>do_something_dlm()</tt>. -<p><a name="Quick Quiz 2"><b>Quick Quiz 2</b>:</a> -Why is the <tt>synchronize_rcu()</tt> on line 28 needed? -<br><a href="#qq2answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why is the <tt>synchronize_rcu()</tt> on line 28 needed? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Without that extra grace period, memory reordering could result in + <tt>do_something_dlm()</tt> executing <tt>do_something()</tt> + concurrently with the last bits of <tt>recovery()</tt>. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> In order to avoid fatal problems such as deadlocks, @@ -332,12 +355,27 @@ It also prevents any number of “interesting” compiler optimizations, for example, the use of <tt>gp</tt> as a scratch location immediately preceding the assignment. -<p><a name="Quick Quiz 3"><b>Quick Quiz 3</b>:</a> -But <tt>rcu_assign_pointer()</tt> does nothing to prevent the -two assignments to <tt>p->a</tt> and <tt>p->b</tt> -from being reordered. -Can't that also cause problems? -<br><a href="#qq3answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + But <tt>rcu_assign_pointer()</tt> does nothing to prevent the + two assignments to <tt>p->a</tt> and <tt>p->b</tt> + from being reordered. + Can't that also cause problems? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + No, it cannot. + The readers cannot see either of these two fields until + the assignment to <tt>gp</tt>, by which time both fields are + fully initialized. + So reordering the assignments + to <tt>p->a</tt> and <tt>p->b</tt> cannot possibly + cause any problems. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> It is tempting to assume that the reader need not do anything special @@ -494,11 +532,42 @@ The <tt>rcu_access_pointer()</tt> on line 6 is similar to code protected by the corresponding update-side lock. </ol> -<p><a name="Quick Quiz 4"><b>Quick Quiz 4</b>:</a> -Without the <tt>rcu_dereference()</tt> or the -<tt>rcu_access_pointer()</tt>, what destructive optimizations -might the compiler make use of? -<br><a href="#qq4answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Without the <tt>rcu_dereference()</tt> or the + <tt>rcu_access_pointer()</tt>, what destructive optimizations + might the compiler make use of? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Let's start with what happens to <tt>do_something_gp()</tt> + if it fails to use <tt>rcu_dereference()</tt>. + It could reuse a value formerly fetched from this same pointer. + It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time + manner, resulting in <i>load tearing</i>, in turn resulting a bytewise + mash-up of two distince pointer values. + It might even use value-speculation optimizations, where it makes + a wrong guess, but by the time it gets around to checking the + value, an update has changed the pointer to match the wrong guess. + Too bad about any dereferences that returned pre-initialization garbage + in the meantime! + </font> + + <p><font color="ffffff"> + For <tt>remove_gp_synchronous()</tt>, as long as all modifications + to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>, + the above optimizations are harmless. + However, + with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>, + <tt>sparse</tt> will complain if you + define <tt>gp</tt> with <tt>__rcu</tt> and then + access it without using + either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> In short, RCU's publish-subscribe guarantee is provided by the combination @@ -571,17 +640,156 @@ systems with more than one CPU: <tt>synchronize_rcu()</tt> migrates in the meantime. </ol> -<p><a name="Quick Quiz 5"><b>Quick Quiz 5</b>:</a> -Given that multiple CPUs can start RCU read-side critical sections -at any time without any ordering whatsoever, how can RCU possibly tell whether -or not a given RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>? -<br><a href="#qq5answer">Answer</a> - -<p><a name="Quick Quiz 6"><b>Quick Quiz 6</b>:</a> -The first and second guarantees require unbelievably strict ordering! -Are all these memory barriers <i> really</i> required? -<br><a href="#qq6answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Given that multiple CPUs can start RCU read-side critical sections + at any time without any ordering whatsoever, how can RCU possibly + tell whether or not a given RCU read-side critical section starts + before a given instance of <tt>synchronize_rcu()</tt>? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + If RCU cannot tell whether or not a given + RCU read-side critical section starts before a + given instance of <tt>synchronize_rcu()</tt>, + then it must assume that the RCU read-side critical section + started first. + In other words, a given instance of <tt>synchronize_rcu()</tt> + can avoid waiting on a given RCU read-side critical section only + if it can prove that <tt>synchronize_rcu()</tt> started first. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + The first and second guarantees require unbelievably strict ordering! + Are all these memory barriers <i> really</i> required? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Yes, they really are required. + To see why the first guarantee is required, consider the following + sequence of events: + </font> + + <ol> + <li> <font color="ffffff"> + CPU 1: <tt>rcu_read_lock()</tt> + </font> + <li> <font color="ffffff"> + CPU 1: <tt>q = rcu_dereference(gp); + /* Very likely to return p. */</tt> + </font> + <li> <font color="ffffff"> + CPU 0: <tt>list_del_rcu(p);</tt> + </font> + <li> <font color="ffffff"> + CPU 0: <tt>synchronize_rcu()</tt> starts. + </font> + <li> <font color="ffffff"> + CPU 1: <tt>do_something_with(q->a); + /* No smp_mb(), so might happen after kfree(). */</tt> + </font> + <li> <font color="ffffff"> + CPU 1: <tt>rcu_read_unlock()</tt> + </font> + <li> <font color="ffffff"> + CPU 0: <tt>synchronize_rcu()</tt> returns. + </font> + <li> <font color="ffffff"> + CPU 0: <tt>kfree(p);</tt> + </font> + </ol> + + <p><font color="ffffff"> + Therefore, there absolutely must be a full memory barrier between the + end of the RCU read-side critical section and the end of the + grace period. + </font> + + <p><font color="ffffff"> + The sequence of events demonstrating the necessity of the second rule + is roughly similar: + </font> + + <ol> + <li> <font color="ffffff">CPU 0: <tt>list_del_rcu(p);</tt> + </font> + <li> <font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> starts. + </font> + <li> <font color="ffffff">CPU 1: <tt>rcu_read_lock()</tt> + </font> + <li> <font color="ffffff">CPU 1: <tt>q = rcu_dereference(gp); + /* Might return p if no memory barrier. */</tt> + </font> + <li> <font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> returns. + </font> + <li> <font color="ffffff">CPU 0: <tt>kfree(p);</tt> + </font> + <li> <font color="ffffff"> + CPU 1: <tt>do_something_with(q->a); /* Boom!!! */</tt> + </font> + <li> <font color="ffffff">CPU 1: <tt>rcu_read_unlock()</tt> + </font> + </ol> + + <p><font color="ffffff"> + And similarly, without a memory barrier between the beginning of the + grace period and the beginning of the RCU read-side critical section, + CPU 1 might end up accessing the freelist. + </font> + + <p><font color="ffffff"> + The “as if” rule of course applies, so that any + implementation that acts as if the appropriate memory barriers + were in place is a correct implementation. + That said, it is much easier to fool yourself into believing + that you have adhered to the as-if rule than it is to actually + adhere to it! +</font></td></tr> +<tr><td> </td></tr> +</table> + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + You claim that <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt> + generate absolutely no code in some kernel builds. + This means that the compiler might arbitrarily rearrange consecutive + RCU read-side critical sections. + Given such rearrangement, if a given RCU read-side critical section + is done, how can you be sure that all prior RCU read-side critical + sections are done? + Won't the compiler rearrangements make that impossible to determine? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In cases where <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt> + generate absolutely no code, RCU infers quiescent states only at + special locations, for example, within the scheduler. + Because calls to <tt>schedule()</tt> had better prevent calling-code + accesses to shared variables from being rearranged across the call to + <tt>schedule()</tt>, if RCU detects the end of a given RCU read-side + critical section, it will necessarily detect the end of all prior + RCU read-side critical sections, no matter how aggressively the + compiler scrambles the code. + </font> + + <p><font color="ffffff"> + Again, this all assumes that the compiler cannot scramble code across + calls to the scheduler, out of interrupt handlers, into the idle loop, + into user-mode code, and so on. + But if your kernel build allows that sort of scrambling, you have broken + far more than just RCU! +</font></td></tr> +<tr><td> </td></tr> +</table> <p> Note that these memory-barrier requirements do not replace the fundamental @@ -626,9 +834,19 @@ inconvenience can be avoided through use of the <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members described later in this document. -<p><a name="Quick Quiz 7"><b>Quick Quiz 7</b>:</a> -But how does the upgrade-to-write operation exclude other readers? -<br><a href="#qq7answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + But how does the upgrade-to-write operation exclude other readers? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + It doesn't, just like normal RCU updates, which also do not exclude + RCU readers. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> This guarantee allows lookup code to be shared between read-side @@ -714,9 +932,20 @@ to do significant reordering. This is by design: Any significant ordering constraints would slow down these fast-path APIs. -<p><a name="Quick Quiz 8"><b>Quick Quiz 8</b>:</a> -Can't the compiler also reorder this code? -<br><a href="#qq8answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Can't the compiler also reorder this code? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + No, the volatile casts in <tt>READ_ONCE()</tt> and + <tt>WRITE_ONCE()</tt> prevent the compiler from reordering in + this particular case. +</font></td></tr> +<tr><td> </td></tr> +</table> <h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3> @@ -769,10 +998,28 @@ new readers can start immediately after <tt>synchronize_rcu()</tt> starts, and <tt>synchronize_rcu()</tt> is under no obligation to wait for these new readers. -<p><a name="Quick Quiz 9"><b>Quick Quiz 9</b>:</a> -Suppose that synchronize_rcu() did wait until all readers had completed. -Would the updater be able to rely on this? -<br><a href="#qq9answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Suppose that synchronize_rcu() did wait until <i>all</i> + readers had completed instead of waiting only on + pre-existing readers. + For how long would the updater be able to rely on there + being no readers? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + For no time at all. + Even if <tt>synchronize_rcu()</tt> were to wait until + all readers had completed, a new reader might start immediately after + <tt>synchronize_rcu()</tt> completed. + Therefore, the code following + <tt>synchronize_rcu()</tt> can <i>never</i> rely on there being + no readers. +</font></td></tr> +<tr><td> </td></tr> +</table> <h3><a name="Grace Periods Don't Partition Read-Side Critical Sections"> Grace Periods Don't Partition Read-Side Critical Sections</a></h3> @@ -969,11 +1216,24 @@ grace period. As a result, an RCU read-side critical section cannot partition a pair of RCU grace periods. -<p><a name="Quick Quiz 10"><b>Quick Quiz 10</b>:</a> -How long a sequence of grace periods, each separated by an RCU read-side -critical section, would be required to partition the RCU read-side -critical sections at the beginning and end of the chain? -<br><a href="#qq10answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + How long a sequence of grace periods, each separated by an RCU + read-side critical section, would be required to partition the RCU + read-side critical sections at the beginning and end of the chain? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In theory, an infinite number. + In practice, an unknown number that is sensitive to both implementation + details and timing considerations. + Therefore, even in practice, RCU users must abide by the + theoretical rather than the practical answer. +</font></td></tr> +<tr><td> </td></tr> +</table> <h3><a name="Disabling Preemption Does Not Block Grace Periods"> Disabling Preemption Does Not Block Grace Periods</a></h3> @@ -1109,12 +1369,27 @@ These classes is covered in the following sections. <h3><a name="Specialization">Specialization</a></h3> <p> -RCU is and always has been intended primarily for read-mostly situations, as -illustrated by the following figure. -This means that RCU's read-side primitives are optimized, often at the +RCU is and always has been intended primarily for read-mostly situations, +which means that RCU's read-side primitives are optimized, often at the expense of its update-side primitives. +Experience thus far is captured by the following list of situations: -<p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p> +<ol> +<li> Read-mostly data, where stale and inconsistent data is not + a problem: RCU works great! +<li> Read-mostly data, where data must be consistent: + RCU works well. +<li> Read-write data, where data must be consistent: + RCU <i>might</i> work OK. + Or not. +<li> Write-mostly data, where data must be consistent: + RCU is very unlikely to be the right tool for the job, + with the following exceptions, where RCU can provide: + <ol type=a> + <li> Existence guarantees for update-friendly mechanisms. + <li> Wait-free read-side primitives for real-time use. + </ol> +</ol> <p> This focus on read-mostly situations means that RCU must interoperate @@ -1127,9 +1402,43 @@ synchronization primitives be legal within RCU read-side critical sections, including spinlocks, sequence locks, atomic operations, reference counters, and memory barriers. -<p><a name="Quick Quiz 11"><b>Quick Quiz 11</b>:</a> -What about sleeping locks? -<br><a href="#qq11answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + What about sleeping locks? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + These are forbidden within Linux-kernel RCU read-side critical + sections because it is not legal to place a quiescent state + (in this case, voluntary context switch) within an RCU read-side + critical section. + However, sleeping locks may be used within userspace RCU read-side + critical sections, and also within Linux-kernel sleepable RCU + <a href="#Sleepable RCU"><font color="ffffff">(SRCU)</font></a> + read-side critical sections. + In addition, the -rt patchset turns spinlocks into a + sleeping locks so that the corresponding critical sections + can be preempted, which also means that these sleeplockified + spinlocks (but not other sleeping locks!) may be acquire within + -rt-Linux-kernel RCU read-side critical sections. + </font> + + <p><font color="ffffff"> + Note that it <i>is</i> legal for a normal RCU read-side + critical section to conditionally acquire a sleeping locks + (as in <tt>mutex_trylock()</tt>), but only as long as it does + not loop indefinitely attempting to conditionally acquire that + sleeping locks. + The key point is that things like <tt>mutex_trylock()</tt> + either return with the mutex held, or return an error indication if + the mutex was not immediately available. + Either way, <tt>mutex_trylock()</tt> returns immediately without + sleeping. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> It often comes as a surprise that many algorithms do not require a @@ -1160,10 +1469,7 @@ some period of time, so the exact wait period is a judgment call. One of our pair of veternarians might wait 30 seconds before pronouncing the cat dead, while the other might insist on waiting a full minute. The two veternarians would then disagree on the state of the cat during -the final 30 seconds of the minute following the last heartbeat, as -fancifully illustrated below: - -<p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p> +the final 30 seconds of the minute following the last heartbeat. <p> Interestingly enough, this same situation applies to hardware. @@ -1343,7 +1649,8 @@ situations where neither <tt>synchronize_rcu()</tt> nor <tt>synchronize_rcu_expedited()</tt> would be legal, including within preempt-disable code, <tt>local_bh_disable()</tt> code, interrupt-disable code, and interrupt handlers. -However, even <tt>call_rcu()</tt> is illegal within NMI handlers. +However, even <tt>call_rcu()</tt> is illegal within NMI handlers +and from idle and offline CPUs. The callback function (<tt>remove_gp_cb()</tt> in this case) will be executed within softirq (software interrupt) environment within the Linux kernel, @@ -1354,12 +1661,27 @@ write an RCU callback function that takes too long. Long-running operations should be relegated to separate threads or (in the Linux kernel) workqueues. -<p><a name="Quick Quiz 12"><b>Quick Quiz 12</b>:</a> -Why does line 19 use <tt>rcu_access_pointer()</tt>? -After all, <tt>call_rcu()</tt> on line 25 stores into the -structure, which would interact badly with concurrent insertions. -Doesn't this mean that <tt>rcu_dereference()</tt> is required? -<br><a href="#qq12answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why does line 19 use <tt>rcu_access_pointer()</tt>? + After all, <tt>call_rcu()</tt> on line 25 stores into the + structure, which would interact badly with concurrent insertions. + Doesn't this mean that <tt>rcu_dereference()</tt> is required? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Presumably the <tt>->gp_lock</tt> acquired on line 18 excludes + any changes, including any insertions that <tt>rcu_dereference()</tt> + would protect against. + Therefore, any insertions will be delayed until after + <tt>->gp_lock</tt> + is released on line 25, which in turn means that + <tt>rcu_access_pointer()</tt> suffices. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> However, all that <tt>remove_gp_cb()</tt> is doing is @@ -1406,14 +1728,31 @@ This was due to the fact that RCU was not heavily used within DYNIX/ptx, so the very few places that needed something like <tt>synchronize_rcu()</tt> simply open-coded it. -<p><a name="Quick Quiz 13"><b>Quick Quiz 13</b>:</a> -Earlier it was claimed that <tt>call_rcu()</tt> and -<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked -by readers. -But how can that be correct, given that the invocation of the callback -and the freeing of the memory (respectively) must still wait for -a grace period to elapse? -<br><a href="#qq13answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Earlier it was claimed that <tt>call_rcu()</tt> and + <tt>kfree_rcu()</tt> allowed updaters to avoid being blocked + by readers. + But how can that be correct, given that the invocation of the callback + and the freeing of the memory (respectively) must still wait for + a grace period to elapse? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + We could define things this way, but keep in mind that this sort of + definition would say that updates in garbage-collected languages + cannot complete until the next time the garbage collector runs, + which does not seem at all reasonable. + The key point is that in most cases, an updater using either + <tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the + next update as soon as it has invoked <tt>call_rcu()</tt> or + <tt>kfree_rcu()</tt>, without having to wait for a subsequent + grace period. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> But what if the updater must wait for the completion of code to be @@ -1838,11 +2177,26 @@ kthreads to be spawned. Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler initialization can result in deadlock. -<p><a name="Quick Quiz 14"><b>Quick Quiz 14</b>:</a> -So what happens with <tt>synchronize_rcu()</tt> during -scheduler initialization for <tt>CONFIG_PREEMPT=n</tt> -kernels? -<br><a href="#qq14answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + So what happens with <tt>synchronize_rcu()</tt> during + scheduler initialization for <tt>CONFIG_PREEMPT=n</tt> + kernels? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt> + maps directly to <tt>synchronize_sched()</tt>. + Therefore, <tt>synchronize_rcu()</tt> works normally throughout + boot in <tt>CONFIG_PREEMPT=n</tt> kernels. + However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels, + so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt> + during scheduler initialization. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> I learned of these boot-time requirements as a result of a series of @@ -2171,6 +2525,14 @@ This real-time requirement motivated the grace-period kthread, which also simplified handling of a number of race conditions. <p> +RCU must avoid degrading real-time response for CPU-bound threads, whether +executing in usermode (which is one use case for +<tt>CONFIG_NO_HZ_FULL=y</tt>) or in the kernel. +That said, CPU-bound loops in the kernel must execute +<tt>cond_resched_rcu_qs()</tt> at least once per few tens of milliseconds +in order to avoid receiving an IPI from RCU. + +<p> Finally, RCU's status as a synchronization primitive means that any RCU failure can result in arbitrary memory corruption that can be extremely difficult to debug. @@ -2223,6 +2585,8 @@ described in a separate section. <li> <a href="#Sched Flavor">Sched Flavor</a> <li> <a href="#Sleepable RCU">Sleepable RCU</a> <li> <a href="#Tasks RCU">Tasks RCU</a> +<li> <a href="#Waiting for Multiple Grace Periods"> + Waiting for Multiple Grace Periods</a> </ol> <h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3> @@ -2472,6 +2836,94 @@ The tasks-RCU API is quite compact, consisting only of <tt>synchronize_rcu_tasks()</tt>, and <tt>rcu_barrier_tasks()</tt>. +<h3><a name="Waiting for Multiple Grace Periods"> +Waiting for Multiple Grace Periods</a></h3> + +<p> +Perhaps you have an RCU protected data structure that is accessed from +RCU read-side critical sections, from softirq handlers, and from +hardware interrupt handlers. +That is three flavors of RCU, the normal flavor, the bottom-half flavor, +and the sched flavor. +How to wait for a compound grace period? + +<p> +The best approach is usually to “just say no!” and +insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt> +around each RCU read-side critical section, regardless of what +environment it happens to be in. +But suppose that some of the RCU read-side critical sections are +on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt> +is not a viable option, so that <tt>rcu_read_lock()</tt> and +<tt>rcu_read_unlock()</tt> are not free. +What then? + +<p> +You <i>could</i> wait on all three grace periods in succession, as follows: + +<blockquote> +<pre> + 1 synchronize_rcu(); + 2 synchronize_rcu_bh(); + 3 synchronize_sched(); +</pre> +</blockquote> + +<p> +This works, but triples the update-side latency penalty. +In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt> +may be used to wait on all three flavors of grace period concurrently: + +<blockquote> +<pre> + 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched); +</pre> +</blockquote> + +<p> +But what if it is necessary to also wait on SRCU? +This can be done as follows: + +<blockquote> +<pre> + 1 static void call_my_srcu(struct rcu_head *head, + 2 void (*func)(struct rcu_head *head)) + 3 { + 4 call_srcu(&my_srcu, head, func); + 5 } + 6 + 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu); +</pre> +</blockquote> + +<p> +If you needed to wait on multiple different flavors of SRCU +(but why???), you would need to create a wrapper function resembling +<tt>call_my_srcu()</tt> for each SRCU flavor. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + But what if I need to wait for multiple RCU flavors, but I also need + the grace periods to be expedited? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + If you are using expedited grace periods, there should be less penalty + for waiting on them in succession. + But if that is nevertheless a problem, you can use workqueues + or multiple kthreads to wait on the various expedited grace + periods concurrently. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p> +Again, it is usually better to adjust the RCU read-side critical sections +to use a single flavor of RCU, but when this is not feasible, you can use +<tt>synchronize_rcu_mult()</tt>. + <h2><a name="Possible Future Changes">Possible Future Changes</a></h2> <p> @@ -2569,329 +3021,4 @@ and is provided under the terms of the Creative Commons Attribution-Share Alike 3.0 United States license. -<h3><a name="Answers to Quick Quizzes"> -Answers to Quick Quizzes</a></h3> - -<a name="qq1answer"></a> -<p><b>Quick Quiz 1</b>: -Wait a minute! -You said that updaters can make useful forward progress concurrently -with readers, but pre-existing readers will block -<tt>synchronize_rcu()</tt>!!! -Just who are you trying to fool??? - - -</p><p><b>Answer</b>: -First, if updaters do not wish to be blocked by readers, they can use -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will -be discussed later. -Second, even when using <tt>synchronize_rcu()</tt>, the other -update-side code does run concurrently with readers, whether pre-existing -or not. - - -</p><p><a href="#Quick%20Quiz%201"><b>Back to Quick Quiz 1</b>.</a> - -<a name="qq2answer"></a> -<p><b>Quick Quiz 2</b>: -Why is the <tt>synchronize_rcu()</tt> on line 28 needed? - - -</p><p><b>Answer</b>: -Without that extra grace period, memory reordering could result in -<tt>do_something_dlm()</tt> executing <tt>do_something()</tt> -concurrently with the last bits of <tt>recovery()</tt>. - - -</p><p><a href="#Quick%20Quiz%202"><b>Back to Quick Quiz 2</b>.</a> - -<a name="qq3answer"></a> -<p><b>Quick Quiz 3</b>: -But <tt>rcu_assign_pointer()</tt> does nothing to prevent the -two assignments to <tt>p->a</tt> and <tt>p->b</tt> -from being reordered. -Can't that also cause problems? - - -</p><p><b>Answer</b>: -No, it cannot. -The readers cannot see either of these two fields until -the assignment to <tt>gp</tt>, by which time both fields are -fully initialized. -So reordering the assignments -to <tt>p->a</tt> and <tt>p->b</tt> cannot possibly -cause any problems. - - -</p><p><a href="#Quick%20Quiz%203"><b>Back to Quick Quiz 3</b>.</a> - -<a name="qq4answer"></a> -<p><b>Quick Quiz 4</b>: -Without the <tt>rcu_dereference()</tt> or the -<tt>rcu_access_pointer()</tt>, what destructive optimizations -might the compiler make use of? - - -</p><p><b>Answer</b>: -Let's start with what happens to <tt>do_something_gp()</tt> -if it fails to use <tt>rcu_dereference()</tt>. -It could reuse a value formerly fetched from this same pointer. -It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time -manner, resulting in <i>load tearing</i>, in turn resulting a bytewise -mash-up of two distince pointer values. -It might even use value-speculation optimizations, where it makes a wrong -guess, but by the time it gets around to checking the value, an update -has changed the pointer to match the wrong guess. -Too bad about any dereferences that returned pre-initialization garbage -in the meantime! - -<p> -For <tt>remove_gp_synchronous()</tt>, as long as all modifications -to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>, -the above optimizations are harmless. -However, -with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>, -<tt>sparse</tt> will complain if you -define <tt>gp</tt> with <tt>__rcu</tt> and then -access it without using -either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>. - - -</p><p><a href="#Quick%20Quiz%204"><b>Back to Quick Quiz 4</b>.</a> - -<a name="qq5answer"></a> -<p><b>Quick Quiz 5</b>: -Given that multiple CPUs can start RCU read-side critical sections -at any time without any ordering whatsoever, how can RCU possibly tell whether -or not a given RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>? - - -</p><p><b>Answer</b>: -If RCU cannot tell whether or not a given -RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>, -then it must assume that the RCU read-side critical section -started first. -In other words, a given instance of <tt>synchronize_rcu()</tt> -can avoid waiting on a given RCU read-side critical section only -if it can prove that <tt>synchronize_rcu()</tt> started first. - - -</p><p><a href="#Quick%20Quiz%205"><b>Back to Quick Quiz 5</b>.</a> - -<a name="qq6answer"></a> -<p><b>Quick Quiz 6</b>: -The first and second guarantees require unbelievably strict ordering! -Are all these memory barriers <i> really</i> required? - - -</p><p><b>Answer</b>: -Yes, they really are required. -To see why the first guarantee is required, consider the following -sequence of events: - -<ol> -<li> CPU 1: <tt>rcu_read_lock()</tt> -<li> CPU 1: <tt>q = rcu_dereference(gp); - /* Very likely to return p. */</tt> -<li> CPU 0: <tt>list_del_rcu(p);</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> starts. -<li> CPU 1: <tt>do_something_with(q->a); - /* No smp_mb(), so might happen after kfree(). */</tt> -<li> CPU 1: <tt>rcu_read_unlock()</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> returns. -<li> CPU 0: <tt>kfree(p);</tt> -</ol> - -<p> -Therefore, there absolutely must be a full memory barrier between the -end of the RCU read-side critical section and the end of the -grace period. - -<p> -The sequence of events demonstrating the necessity of the second rule -is roughly similar: - -<ol> -<li> CPU 0: <tt>list_del_rcu(p);</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> starts. -<li> CPU 1: <tt>rcu_read_lock()</tt> -<li> CPU 1: <tt>q = rcu_dereference(gp); - /* Might return p if no memory barrier. */</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> returns. -<li> CPU 0: <tt>kfree(p);</tt> -<li> CPU 1: <tt>do_something_with(q->a); /* Boom!!! */</tt> -<li> CPU 1: <tt>rcu_read_unlock()</tt> -</ol> - -<p> -And similarly, without a memory barrier between the beginning of the -grace period and the beginning of the RCU read-side critical section, -CPU 1 might end up accessing the freelist. - -<p> -The “as if” rule of course applies, so that any implementation -that acts as if the appropriate memory barriers were in place is a -correct implementation. -That said, it is much easier to fool yourself into believing that you have -adhered to the as-if rule than it is to actually adhere to it! - - -</p><p><a href="#Quick%20Quiz%206"><b>Back to Quick Quiz 6</b>.</a> - -<a name="qq7answer"></a> -<p><b>Quick Quiz 7</b>: -But how does the upgrade-to-write operation exclude other readers? - - -</p><p><b>Answer</b>: -It doesn't, just like normal RCU updates, which also do not exclude -RCU readers. - - -</p><p><a href="#Quick%20Quiz%207"><b>Back to Quick Quiz 7</b>.</a> - -<a name="qq8answer"></a> -<p><b>Quick Quiz 8</b>: -Can't the compiler also reorder this code? - - -</p><p><b>Answer</b>: -No, the volatile casts in <tt>READ_ONCE()</tt> and -<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in -this particular case. - - -</p><p><a href="#Quick%20Quiz%208"><b>Back to Quick Quiz 8</b>.</a> - -<a name="qq9answer"></a> -<p><b>Quick Quiz 9</b>: -Suppose that synchronize_rcu() did wait until all readers had completed. -Would the updater be able to rely on this? - - -</p><p><b>Answer</b>: -No. -Even if <tt>synchronize_rcu()</tt> were to wait until -all readers had completed, a new reader might start immediately after -<tt>synchronize_rcu()</tt> completed. -Therefore, the code following -<tt>synchronize_rcu()</tt> cannot rely on there being no readers -in any case. - - -</p><p><a href="#Quick%20Quiz%209"><b>Back to Quick Quiz 9</b>.</a> - -<a name="qq10answer"></a> -<p><b>Quick Quiz 10</b>: -How long a sequence of grace periods, each separated by an RCU read-side -critical section, would be required to partition the RCU read-side -critical sections at the beginning and end of the chain? - - -</p><p><b>Answer</b>: -In theory, an infinite number. -In practice, an unknown number that is sensitive to both implementation -details and timing considerations. -Therefore, even in practice, RCU users must abide by the theoretical rather -than the practical answer. - - -</p><p><a href="#Quick%20Quiz%2010"><b>Back to Quick Quiz 10</b>.</a> - -<a name="qq11answer"></a> -<p><b>Quick Quiz 11</b>: -What about sleeping locks? - - -</p><p><b>Answer</b>: -These are forbidden within Linux-kernel RCU read-side critical sections -because it is not legal to place a quiescent state (in this case, -voluntary context switch) within an RCU read-side critical section. -However, sleeping locks may be used within userspace RCU read-side critical -sections, and also within Linux-kernel sleepable RCU -<a href="#Sleepable RCU">(SRCU)</a> -read-side critical sections. -In addition, the -rt patchset turns spinlocks into a sleeping locks so -that the corresponding critical sections can be preempted, which -also means that these sleeplockified spinlocks (but not other sleeping locks!) -may be acquire within -rt-Linux-kernel RCU read-side critical sections. - -<p> -Note that it <i>is</i> legal for a normal RCU read-side critical section -to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>), -but only as long as it does not loop indefinitely attempting to -conditionally acquire that sleeping locks. -The key point is that things like <tt>mutex_trylock()</tt> -either return with the mutex held, or return an error indication if -the mutex was not immediately available. -Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping. - - -</p><p><a href="#Quick%20Quiz%2011"><b>Back to Quick Quiz 11</b>.</a> - -<a name="qq12answer"></a> -<p><b>Quick Quiz 12</b>: -Why does line 19 use <tt>rcu_access_pointer()</tt>? -After all, <tt>call_rcu()</tt> on line 25 stores into the -structure, which would interact badly with concurrent insertions. -Doesn't this mean that <tt>rcu_dereference()</tt> is required? - - -</p><p><b>Answer</b>: -Presumably the <tt>->gp_lock</tt> acquired on line 18 excludes -any changes, including any insertions that <tt>rcu_dereference()</tt> -would protect against. -Therefore, any insertions will be delayed until after <tt>->gp_lock</tt> -is released on line 25, which in turn means that -<tt>rcu_access_pointer()</tt> suffices. - - -</p><p><a href="#Quick%20Quiz%2012"><b>Back to Quick Quiz 12</b>.</a> - -<a name="qq13answer"></a> -<p><b>Quick Quiz 13</b>: -Earlier it was claimed that <tt>call_rcu()</tt> and -<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked -by readers. -But how can that be correct, given that the invocation of the callback -and the freeing of the memory (respectively) must still wait for -a grace period to elapse? - - -</p><p><b>Answer</b>: -We could define things this way, but keep in mind that this sort of -definition would say that updates in garbage-collected languages -cannot complete until the next time the garbage collector runs, -which does not seem at all reasonable. -The key point is that in most cases, an updater using either -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the -next update as soon as it has invoked <tt>call_rcu()</tt> or -<tt>kfree_rcu()</tt>, without having to wait for a subsequent -grace period. - - -</p><p><a href="#Quick%20Quiz%2013"><b>Back to Quick Quiz 13</b>.</a> - -<a name="qq14answer"></a> -<p><b>Quick Quiz 14</b>: -So what happens with <tt>synchronize_rcu()</tt> during -scheduler initialization for <tt>CONFIG_PREEMPT=n</tt> -kernels? - - -</p><p><b>Answer</b>: -In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt> -maps directly to <tt>synchronize_sched()</tt>. -Therefore, <tt>synchronize_rcu()</tt> works normally throughout -boot in <tt>CONFIG_PREEMPT=n</tt> kernels. -However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels, -so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt> -during scheduler initialization. - - -</p><p><a href="#Quick%20Quiz%2014"><b>Back to Quick Quiz 14</b>.</a> - - </body></html> diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx deleted file mode 100644 index 3a97ba490c42..000000000000 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ /dev/null @@ -1,2741 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" - "http://www.w3.org/TR/html4/loose.dtd"> - <html> - <head><title>A Tour Through RCU's Requirements [LWN.net]</title> - <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8"> - -<h1>A Tour Through RCU's Requirements</h1> - -<p>Copyright IBM Corporation, 2015</p> -<p>Author: Paul E. McKenney</p> -<p><i>The initial version of this document appeared in the -<a href="https://lwn.net/">LWN</a> articles -<a href="https://lwn.net/Articles/652156/">here</a>, -<a href="https://lwn.net/Articles/652677/">here</a>, and -<a href="https://lwn.net/Articles/653326/">here</a>.</i></p> - -<h2>Introduction</h2> - -<p> -Read-copy update (RCU) is a synchronization mechanism that is often -used as a replacement for reader-writer locking. -RCU is unusual in that updaters do not block readers, -which means that RCU's read-side primitives can be exceedingly fast -and scalable. -In addition, updaters can make useful forward progress concurrently -with readers. -However, all this concurrency between RCU readers and updaters does raise -the question of exactly what RCU readers are doing, which in turn -raises the question of exactly what RCU's requirements are. - -<p> -This document therefore summarizes RCU's requirements, and can be thought -of as an informal, high-level specification for RCU. -It is important to understand that RCU's specification is primarily -empirical in nature; -in fact, I learned about many of these requirements the hard way. -This situation might cause some consternation, however, not only -has this learning process been a lot of fun, but it has also been -a great privilege to work with so many people willing to apply -technologies in interesting new ways. - -<p> -All that aside, here are the categories of currently known RCU requirements: -</p> - -<ol> -<li> <a href="#Fundamental Requirements"> - Fundamental Requirements</a> -<li> <a href="#Fundamental Non-Requirements">Fundamental Non-Requirements</a> -<li> <a href="#Parallelism Facts of Life"> - Parallelism Facts of Life</a> -<li> <a href="#Quality-of-Implementation Requirements"> - Quality-of-Implementation Requirements</a> -<li> <a href="#Linux Kernel Complications"> - Linux Kernel Complications</a> -<li> <a href="#Software-Engineering Requirements"> - Software-Engineering Requirements</a> -<li> <a href="#Other RCU Flavors"> - Other RCU Flavors</a> -<li> <a href="#Possible Future Changes"> - Possible Future Changes</a> -</ol> - -<p> -This is followed by a <a href="#Summary">summary</a>, -which is in turn followed by the inevitable -<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>. - -<h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2> - -<p> -RCU's fundamental requirements are the closest thing RCU has to hard -mathematical requirements. -These are: - -<ol> -<li> <a href="#Grace-Period Guarantee"> - Grace-Period Guarantee</a> -<li> <a href="#Publish-Subscribe Guarantee"> - Publish-Subscribe Guarantee</a> -<li> <a href="#Memory-Barrier Guarantees"> - Memory-Barrier Guarantees</a> -<li> <a href="#RCU Primitives Guaranteed to Execute Unconditionally"> - RCU Primitives Guaranteed to Execute Unconditionally</a> -<li> <a href="#Guaranteed Read-to-Write Upgrade"> - Guaranteed Read-to-Write Upgrade</a> -</ol> - -<h3><a name="Grace-Period Guarantee">Grace-Period Guarantee</a></h3> - -<p> -RCU's grace-period guarantee is unusual in being premeditated: -Jack Slingwine and I had this guarantee firmly in mind when we started -work on RCU (then called “rclock”) in the early 1990s. -That said, the past two decades of experience with RCU have produced -a much more detailed understanding of this guarantee. - -<p> -RCU's grace-period guarantee allows updaters to wait for the completion -of all pre-existing RCU read-side critical sections. -An RCU read-side critical section -begins with the marker <tt>rcu_read_lock()</tt> and ends with -the marker <tt>rcu_read_unlock()</tt>. -These markers may be nested, and RCU treats a nested set as one -big RCU read-side critical section. -Production-quality implementations of <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> are extremely lightweight, and in -fact have exactly zero overhead in Linux kernels built for production -use with <tt>CONFIG_PREEMPT=n</tt>. - -<p> -This guarantee allows ordering to be enforced with extremely low -overhead to readers, for example: - -<blockquote> -<pre> - 1 int x, y; - 2 - 3 void thread0(void) - 4 { - 5 rcu_read_lock(); - 6 r1 = READ_ONCE(x); - 7 r2 = READ_ONCE(y); - 8 rcu_read_unlock(); - 9 } -10 -11 void thread1(void) -12 { -13 WRITE_ONCE(x, 1); -14 synchronize_rcu(); -15 WRITE_ONCE(y, 1); -16 } -</pre> -</blockquote> - -<p> -Because the <tt>synchronize_rcu()</tt> on line 14 waits for -all pre-existing readers, any instance of <tt>thread0()</tt> that -loads a value of zero from <tt>x</tt> must complete before -<tt>thread1()</tt> stores to <tt>y</tt>, so that instance must -also load a value of zero from <tt>y</tt>. -Similarly, any instance of <tt>thread0()</tt> that loads a value of -one from <tt>y</tt> must have started after the -<tt>synchronize_rcu()</tt> started, and must therefore also load -a value of one from <tt>x</tt>. -Therefore, the outcome: -<blockquote> -<pre> -(r1 == 0 && r2 == 1) -</pre> -</blockquote> -cannot happen. - -<p>@@QQ@@ -Wait a minute! -You said that updaters can make useful forward progress concurrently -with readers, but pre-existing readers will block -<tt>synchronize_rcu()</tt>!!! -Just who are you trying to fool??? -<p>@@QQA@@ -First, if updaters do not wish to be blocked by readers, they can use -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will -be discussed later. -Second, even when using <tt>synchronize_rcu()</tt>, the other -update-side code does run concurrently with readers, whether pre-existing -or not. -<p>@@QQE@@ - -<p> -This scenario resembles one of the first uses of RCU in -<a href="https://en.wikipedia.org/wiki/DYNIX">DYNIX/ptx</a>, -which managed a distributed lock manager's transition into -a state suitable for handling recovery from node failure, -more or less as follows: - -<blockquote> -<pre> - 1 #define STATE_NORMAL 0 - 2 #define STATE_WANT_RECOVERY 1 - 3 #define STATE_RECOVERING 2 - 4 #define STATE_WANT_NORMAL 3 - 5 - 6 int state = STATE_NORMAL; - 7 - 8 void do_something_dlm(void) - 9 { -10 int state_snap; -11 -12 rcu_read_lock(); -13 state_snap = READ_ONCE(state); -14 if (state_snap == STATE_NORMAL) -15 do_something(); -16 else -17 do_something_carefully(); -18 rcu_read_unlock(); -19 } -20 -21 void start_recovery(void) -22 { -23 WRITE_ONCE(state, STATE_WANT_RECOVERY); -24 synchronize_rcu(); -25 WRITE_ONCE(state, STATE_RECOVERING); -26 recovery(); -27 WRITE_ONCE(state, STATE_WANT_NORMAL); -28 synchronize_rcu(); -29 WRITE_ONCE(state, STATE_NORMAL); -30 } -</pre> -</blockquote> - -<p> -The RCU read-side critical section in <tt>do_something_dlm()</tt> -works with the <tt>synchronize_rcu()</tt> in <tt>start_recovery()</tt> -to guarantee that <tt>do_something()</tt> never runs concurrently -with <tt>recovery()</tt>, but with little or no synchronization -overhead in <tt>do_something_dlm()</tt>. - -<p>@@QQ@@ -Why is the <tt>synchronize_rcu()</tt> on line 28 needed? -<p>@@QQA@@ -Without that extra grace period, memory reordering could result in -<tt>do_something_dlm()</tt> executing <tt>do_something()</tt> -concurrently with the last bits of <tt>recovery()</tt>. -<p>@@QQE@@ - -<p> -In order to avoid fatal problems such as deadlocks, -an RCU read-side critical section must not contain calls to -<tt>synchronize_rcu()</tt>. -Similarly, an RCU read-side critical section must not -contain anything that waits, directly or indirectly, on completion of -an invocation of <tt>synchronize_rcu()</tt>. - -<p> -Although RCU's grace-period guarantee is useful in and of itself, with -<a href="https://lwn.net/Articles/573497/">quite a few use cases</a>, -it would be good to be able to use RCU to coordinate read-side -access to linked data structures. -For this, the grace-period guarantee is not sufficient, as can -be seen in function <tt>add_gp_buggy()</tt> below. -We will look at the reader's code later, but in the meantime, just think of -the reader as locklessly picking up the <tt>gp</tt> pointer, -and, if the value loaded is non-<tt>NULL</tt>, locklessly accessing the -<tt>->a</tt> and <tt>->b</tt> fields. - -<blockquote> -<pre> - 1 bool add_gp_buggy(int a, int b) - 2 { - 3 p = kmalloc(sizeof(*p), GFP_KERNEL); - 4 if (!p) - 5 return -ENOMEM; - 6 spin_lock(&gp_lock); - 7 if (rcu_access_pointer(gp)) { - 8 spin_unlock(&gp_lock); - 9 return false; -10 } -11 p->a = a; -12 p->b = a; -13 gp = p; /* ORDERING BUG */ -14 spin_unlock(&gp_lock); -15 return true; -16 } -</pre> -</blockquote> - -<p> -The problem is that both the compiler and weakly ordered CPUs are within -their rights to reorder this code as follows: - -<blockquote> -<pre> - 1 bool add_gp_buggy_optimized(int a, int b) - 2 { - 3 p = kmalloc(sizeof(*p), GFP_KERNEL); - 4 if (!p) - 5 return -ENOMEM; - 6 spin_lock(&gp_lock); - 7 if (rcu_access_pointer(gp)) { - 8 spin_unlock(&gp_lock); - 9 return false; -10 } -<b>11 gp = p; /* ORDERING BUG */ -12 p->a = a; -13 p->b = a;</b> -14 spin_unlock(&gp_lock); -15 return true; -16 } -</pre> -</blockquote> - -<p> -If an RCU reader fetches <tt>gp</tt> just after -<tt>add_gp_buggy_optimized</tt> executes line 11, -it will see garbage in the <tt>->a</tt> and <tt>->b</tt> -fields. -And this is but one of many ways in which compiler and hardware optimizations -could cause trouble. -Therefore, we clearly need some way to prevent the compiler and the CPU from -reordering in this manner, which brings us to the publish-subscribe -guarantee discussed in the next section. - -<h3><a name="Publish-Subscribe Guarantee">Publish/Subscribe Guarantee</a></h3> - -<p> -RCU's publish-subscribe guarantee allows data to be inserted -into a linked data structure without disrupting RCU readers. -The updater uses <tt>rcu_assign_pointer()</tt> to insert the -new data, and readers use <tt>rcu_dereference()</tt> to -access data, whether new or old. -The following shows an example of insertion: - -<blockquote> -<pre> - 1 bool add_gp(int a, int b) - 2 { - 3 p = kmalloc(sizeof(*p), GFP_KERNEL); - 4 if (!p) - 5 return -ENOMEM; - 6 spin_lock(&gp_lock); - 7 if (rcu_access_pointer(gp)) { - 8 spin_unlock(&gp_lock); - 9 return false; -10 } -11 p->a = a; -12 p->b = a; -13 rcu_assign_pointer(gp, p); -14 spin_unlock(&gp_lock); -15 return true; -16 } -</pre> -</blockquote> - -<p> -The <tt>rcu_assign_pointer()</tt> on line 13 is conceptually -equivalent to a simple assignment statement, but also guarantees -that its assignment will -happen after the two assignments in lines 11 and 12, -similar to the C11 <tt>memory_order_release</tt> store operation. -It also prevents any number of “interesting” compiler -optimizations, for example, the use of <tt>gp</tt> as a scratch -location immediately preceding the assignment. - -<p>@@QQ@@ -But <tt>rcu_assign_pointer()</tt> does nothing to prevent the -two assignments to <tt>p->a</tt> and <tt>p->b</tt> -from being reordered. -Can't that also cause problems? -<p>@@QQA@@ -No, it cannot. -The readers cannot see either of these two fields until -the assignment to <tt>gp</tt>, by which time both fields are -fully initialized. -So reordering the assignments -to <tt>p->a</tt> and <tt>p->b</tt> cannot possibly -cause any problems. -<p>@@QQE@@ - -<p> -It is tempting to assume that the reader need not do anything special -to control its accesses to the RCU-protected data, -as shown in <tt>do_something_gp_buggy()</tt> below: - -<blockquote> -<pre> - 1 bool do_something_gp_buggy(void) - 2 { - 3 rcu_read_lock(); - 4 p = gp; /* OPTIMIZATIONS GALORE!!! */ - 5 if (p) { - 6 do_something(p->a, p->b); - 7 rcu_read_unlock(); - 8 return true; - 9 } -10 rcu_read_unlock(); -11 return false; -12 } -</pre> -</blockquote> - -<p> -However, this temptation must be resisted because there are a -surprisingly large number of ways that the compiler -(to say nothing of -<a href="https://h71000.www7.hp.com/wizard/wiz_2637.html">DEC Alpha CPUs</a>) -can trip this code up. -For but one example, if the compiler were short of registers, it -might choose to refetch from <tt>gp</tt> rather than keeping -a separate copy in <tt>p</tt> as follows: - -<blockquote> -<pre> - 1 bool do_something_gp_buggy_optimized(void) - 2 { - 3 rcu_read_lock(); - 4 if (gp) { /* OPTIMIZATIONS GALORE!!! */ -<b> 5 do_something(gp->a, gp->b);</b> - 6 rcu_read_unlock(); - 7 return true; - 8 } - 9 rcu_read_unlock(); -10 return false; -11 } -</pre> -</blockquote> - -<p> -If this function ran concurrently with a series of updates that -replaced the current structure with a new one, -the fetches of <tt>gp->a</tt> -and <tt>gp->b</tt> might well come from two different structures, -which could cause serious confusion. -To prevent this (and much else besides), <tt>do_something_gp()</tt> uses -<tt>rcu_dereference()</tt> to fetch from <tt>gp</tt>: - -<blockquote> -<pre> - 1 bool do_something_gp(void) - 2 { - 3 rcu_read_lock(); - 4 p = rcu_dereference(gp); - 5 if (p) { - 6 do_something(p->a, p->b); - 7 rcu_read_unlock(); - 8 return true; - 9 } -10 rcu_read_unlock(); -11 return false; -12 } -</pre> -</blockquote> - -<p> -The <tt>rcu_dereference()</tt> uses volatile casts and (for DEC Alpha) -memory barriers in the Linux kernel. -Should a -<a href="http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf">high-quality implementation of C11 <tt>memory_order_consume</tt> [PDF]</a> -ever appear, then <tt>rcu_dereference()</tt> could be implemented -as a <tt>memory_order_consume</tt> load. -Regardless of the exact implementation, a pointer fetched by -<tt>rcu_dereference()</tt> may not be used outside of the -outermost RCU read-side critical section containing that -<tt>rcu_dereference()</tt>, unless protection of -the corresponding data element has been passed from RCU to some -other synchronization mechanism, most commonly locking or -<a href="https://www.kernel.org/doc/Documentation/RCU/rcuref.txt">reference counting</a>. - -<p> -In short, updaters use <tt>rcu_assign_pointer()</tt> and readers -use <tt>rcu_dereference()</tt>, and these two RCU API elements -work together to ensure that readers have a consistent view of -newly added data elements. - -<p> -Of course, it is also necessary to remove elements from RCU-protected -data structures, for example, using the following process: - -<ol> -<li> Remove the data element from the enclosing structure. -<li> Wait for all pre-existing RCU read-side critical sections - to complete (because only pre-existing readers can possibly have - a reference to the newly removed data element). -<li> At this point, only the updater has a reference to the - newly removed data element, so it can safely reclaim - the data element, for example, by passing it to <tt>kfree()</tt>. -</ol> - -This process is implemented by <tt>remove_gp_synchronous()</tt>: - -<blockquote> -<pre> - 1 bool remove_gp_synchronous(void) - 2 { - 3 struct foo *p; - 4 - 5 spin_lock(&gp_lock); - 6 p = rcu_access_pointer(gp); - 7 if (!p) { - 8 spin_unlock(&gp_lock); - 9 return false; -10 } -11 rcu_assign_pointer(gp, NULL); -12 spin_unlock(&gp_lock); -13 synchronize_rcu(); -14 kfree(p); -15 return true; -16 } -</pre> -</blockquote> - -<p> -This function is straightforward, with line 13 waiting for a grace -period before line 14 frees the old data element. -This waiting ensures that readers will reach line 7 of -<tt>do_something_gp()</tt> before the data element referenced by -<tt>p</tt> is freed. -The <tt>rcu_access_pointer()</tt> on line 6 is similar to -<tt>rcu_dereference()</tt>, except that: - -<ol> -<li> The value returned by <tt>rcu_access_pointer()</tt> - cannot be dereferenced. - If you want to access the value pointed to as well as - the pointer itself, use <tt>rcu_dereference()</tt> - instead of <tt>rcu_access_pointer()</tt>. -<li> The call to <tt>rcu_access_pointer()</tt> need not be - protected. - In contrast, <tt>rcu_dereference()</tt> must either be - within an RCU read-side critical section or in a code - segment where the pointer cannot change, for example, in - code protected by the corresponding update-side lock. -</ol> - -<p>@@QQ@@ -Without the <tt>rcu_dereference()</tt> or the -<tt>rcu_access_pointer()</tt>, what destructive optimizations -might the compiler make use of? -<p>@@QQA@@ -Let's start with what happens to <tt>do_something_gp()</tt> -if it fails to use <tt>rcu_dereference()</tt>. -It could reuse a value formerly fetched from this same pointer. -It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time -manner, resulting in <i>load tearing</i>, in turn resulting a bytewise -mash-up of two distince pointer values. -It might even use value-speculation optimizations, where it makes a wrong -guess, but by the time it gets around to checking the value, an update -has changed the pointer to match the wrong guess. -Too bad about any dereferences that returned pre-initialization garbage -in the meantime! - -<p> -For <tt>remove_gp_synchronous()</tt>, as long as all modifications -to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>, -the above optimizations are harmless. -However, -with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>, -<tt>sparse</tt> will complain if you -define <tt>gp</tt> with <tt>__rcu</tt> and then -access it without using -either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>. -<p>@@QQE@@ - -<p> -In short, RCU's publish-subscribe guarantee is provided by the combination -of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>. -This guarantee allows data elements to be safely added to RCU-protected -linked data structures without disrupting RCU readers. -This guarantee can be used in combination with the grace-period -guarantee to also allow data elements to be removed from RCU-protected -linked data structures, again without disrupting RCU readers. - -<p> -This guarantee was only partially premeditated. -DYNIX/ptx used an explicit memory barrier for publication, but had nothing -resembling <tt>rcu_dereference()</tt> for subscription, nor did it -have anything resembling the <tt>smp_read_barrier_depends()</tt> -that was later subsumed into <tt>rcu_dereference()</tt>. -The need for these operations made itself known quite suddenly at a -late-1990s meeting with the DEC Alpha architects, back in the days when -DEC was still a free-standing company. -It took the Alpha architects a good hour to convince me that any sort -of barrier would ever be needed, and it then took me a good <i>two</i> hours -to convince them that their documentation did not make this point clear. -More recent work with the C and C++ standards committees have provided -much education on tricks and traps from the compiler. -In short, compilers were much less tricky in the early 1990s, but in -2015, don't even think about omitting <tt>rcu_dereference()</tt>! - -<h3><a name="Memory-Barrier Guarantees">Memory-Barrier Guarantees</a></h3> - -<p> -The previous section's simple linked-data-structure scenario clearly -demonstrates the need for RCU's stringent memory-ordering guarantees on -systems with more than one CPU: - -<ol> -<li> Each CPU that has an RCU read-side critical section that - begins before <tt>synchronize_rcu()</tt> starts is - guaranteed to execute a full memory barrier between the time - that the RCU read-side critical section ends and the time that - <tt>synchronize_rcu()</tt> returns. - Without this guarantee, a pre-existing RCU read-side critical section - might hold a reference to the newly removed <tt>struct foo</tt> - after the <tt>kfree()</tt> on line 14 of - <tt>remove_gp_synchronous()</tt>. -<li> Each CPU that has an RCU read-side critical section that ends - after <tt>synchronize_rcu()</tt> returns is guaranteed - to execute a full memory barrier between the time that - <tt>synchronize_rcu()</tt> begins and the time that the RCU - read-side critical section begins. - Without this guarantee, a later RCU read-side critical section - running after the <tt>kfree()</tt> on line 14 of - <tt>remove_gp_synchronous()</tt> might - later run <tt>do_something_gp()</tt> and find the - newly deleted <tt>struct foo</tt>. -<li> If the task invoking <tt>synchronize_rcu()</tt> remains - on a given CPU, then that CPU is guaranteed to execute a full - memory barrier sometime during the execution of - <tt>synchronize_rcu()</tt>. - This guarantee ensures that the <tt>kfree()</tt> on - line 14 of <tt>remove_gp_synchronous()</tt> really does - execute after the removal on line 11. -<li> If the task invoking <tt>synchronize_rcu()</tt> migrates - among a group of CPUs during that invocation, then each of the - CPUs in that group is guaranteed to execute a full memory barrier - sometime during the execution of <tt>synchronize_rcu()</tt>. - This guarantee also ensures that the <tt>kfree()</tt> on - line 14 of <tt>remove_gp_synchronous()</tt> really does - execute after the removal on - line 11, but also in the case where the thread executing the - <tt>synchronize_rcu()</tt> migrates in the meantime. -</ol> - -<p>@@QQ@@ -Given that multiple CPUs can start RCU read-side critical sections -at any time without any ordering whatsoever, how can RCU possibly tell whether -or not a given RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>? -<p>@@QQA@@ -If RCU cannot tell whether or not a given -RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>, -then it must assume that the RCU read-side critical section -started first. -In other words, a given instance of <tt>synchronize_rcu()</tt> -can avoid waiting on a given RCU read-side critical section only -if it can prove that <tt>synchronize_rcu()</tt> started first. -<p>@@QQE@@ - -<p>@@QQ@@ -The first and second guarantees require unbelievably strict ordering! -Are all these memory barriers <i> really</i> required? -<p>@@QQA@@ -Yes, they really are required. -To see why the first guarantee is required, consider the following -sequence of events: - -<ol> -<li> CPU 1: <tt>rcu_read_lock()</tt> -<li> CPU 1: <tt>q = rcu_dereference(gp); - /* Very likely to return p. */</tt> -<li> CPU 0: <tt>list_del_rcu(p);</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> starts. -<li> CPU 1: <tt>do_something_with(q->a); - /* No smp_mb(), so might happen after kfree(). */</tt> -<li> CPU 1: <tt>rcu_read_unlock()</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> returns. -<li> CPU 0: <tt>kfree(p);</tt> -</ol> - -<p> -Therefore, there absolutely must be a full memory barrier between the -end of the RCU read-side critical section and the end of the -grace period. - -<p> -The sequence of events demonstrating the necessity of the second rule -is roughly similar: - -<ol> -<li> CPU 0: <tt>list_del_rcu(p);</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> starts. -<li> CPU 1: <tt>rcu_read_lock()</tt> -<li> CPU 1: <tt>q = rcu_dereference(gp); - /* Might return p if no memory barrier. */</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> returns. -<li> CPU 0: <tt>kfree(p);</tt> -<li> CPU 1: <tt>do_something_with(q->a); /* Boom!!! */</tt> -<li> CPU 1: <tt>rcu_read_unlock()</tt> -</ol> - -<p> -And similarly, without a memory barrier between the beginning of the -grace period and the beginning of the RCU read-side critical section, -CPU 1 might end up accessing the freelist. - -<p> -The “as if” rule of course applies, so that any implementation -that acts as if the appropriate memory barriers were in place is a -correct implementation. -That said, it is much easier to fool yourself into believing that you have -adhered to the as-if rule than it is to actually adhere to it! -<p>@@QQE@@ - -<p> -Note that these memory-barrier requirements do not replace the fundamental -RCU requirement that a grace period wait for all pre-existing readers. -On the contrary, the memory barriers called out in this section must operate in -such a way as to <i>enforce</i> this fundamental requirement. -Of course, different implementations enforce this requirement in different -ways, but enforce it they must. - -<h3><a name="RCU Primitives Guaranteed to Execute Unconditionally">RCU Primitives Guaranteed to Execute Unconditionally</a></h3> - -<p> -The common-case RCU primitives are unconditional. -They are invoked, they do their job, and they return, with no possibility -of error, and no need to retry. -This is a key RCU design philosophy. - -<p> -However, this philosophy is pragmatic rather than pigheaded. -If someone comes up with a good justification for a particular conditional -RCU primitive, it might well be implemented and added. -After all, this guarantee was reverse-engineered, not premeditated. -The unconditional nature of the RCU primitives was initially an -accident of implementation, and later experience with synchronization -primitives with conditional primitives caused me to elevate this -accident to a guarantee. -Therefore, the justification for adding a conditional primitive to -RCU would need to be based on detailed and compelling use cases. - -<h3><a name="Guaranteed Read-to-Write Upgrade">Guaranteed Read-to-Write Upgrade</a></h3> - -<p> -As far as RCU is concerned, it is always possible to carry out an -update within an RCU read-side critical section. -For example, that RCU read-side critical section might search for -a given data element, and then might acquire the update-side -spinlock in order to update that element, all while remaining -in that RCU read-side critical section. -Of course, it is necessary to exit the RCU read-side critical section -before invoking <tt>synchronize_rcu()</tt>, however, this -inconvenience can be avoided through use of the -<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members -described later in this document. - -<p>@@QQ@@ -But how does the upgrade-to-write operation exclude other readers? -<p>@@QQA@@ -It doesn't, just like normal RCU updates, which also do not exclude -RCU readers. -<p>@@QQE@@ - -<p> -This guarantee allows lookup code to be shared between read-side -and update-side code, and was premeditated, appearing in the earliest -DYNIX/ptx RCU documentation. - -<h2><a name="Fundamental Non-Requirements">Fundamental Non-Requirements</a></h2> - -<p> -RCU provides extremely lightweight readers, and its read-side guarantees, -though quite useful, are correspondingly lightweight. -It is therefore all too easy to assume that RCU is guaranteeing more -than it really is. -Of course, the list of things that RCU does not guarantee is infinitely -long, however, the following sections list a few non-guarantees that -have caused confusion. -Except where otherwise noted, these non-guarantees were premeditated. - -<ol> -<li> <a href="#Readers Impose Minimal Ordering"> - Readers Impose Minimal Ordering</a> -<li> <a href="#Readers Do Not Exclude Updaters"> - Readers Do Not Exclude Updaters</a> -<li> <a href="#Updaters Only Wait For Old Readers"> - Updaters Only Wait For Old Readers</a> -<li> <a href="#Grace Periods Don't Partition Read-Side Critical Sections"> - Grace Periods Don't Partition Read-Side Critical Sections</a> -<li> <a href="#Read-Side Critical Sections Don't Partition Grace Periods"> - Read-Side Critical Sections Don't Partition Grace Periods</a> -<li> <a href="#Disabling Preemption Does Not Block Grace Periods"> - Disabling Preemption Does Not Block Grace Periods</a> -</ol> - -<h3><a name="Readers Impose Minimal Ordering">Readers Impose Minimal Ordering</a></h3> - -<p> -Reader-side markers such as <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> provide absolutely no ordering guarantees -except through their interaction with the grace-period APIs such as -<tt>synchronize_rcu()</tt>. -To see this, consider the following pair of threads: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 WRITE_ONCE(x, 1); - 5 rcu_read_unlock(); - 6 rcu_read_lock(); - 7 WRITE_ONCE(y, 1); - 8 rcu_read_unlock(); - 9 } -10 -11 void thread1(void) -12 { -13 rcu_read_lock(); -14 r1 = READ_ONCE(y); -15 rcu_read_unlock(); -16 rcu_read_lock(); -17 r2 = READ_ONCE(x); -18 rcu_read_unlock(); -19 } -</pre> -</blockquote> - -<p> -After <tt>thread0()</tt> and <tt>thread1()</tt> execute -concurrently, it is quite possible to have - -<blockquote> -<pre> -(r1 == 1 && r2 == 0) -</pre> -</blockquote> - -(that is, <tt>y</tt> appears to have been assigned before <tt>x</tt>), -which would not be possible if <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> had much in the way of ordering -properties. -But they do not, so the CPU is within its rights -to do significant reordering. -This is by design: Any significant ordering constraints would slow down -these fast-path APIs. - -<p>@@QQ@@ -Can't the compiler also reorder this code? -<p>@@QQA@@ -No, the volatile casts in <tt>READ_ONCE()</tt> and -<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in -this particular case. -<p>@@QQE@@ - -<h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3> - -<p> -Neither <tt>rcu_read_lock()</tt> nor <tt>rcu_read_unlock()</tt> -exclude updates. -All they do is to prevent grace periods from ending. -The following example illustrates this: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 r1 = READ_ONCE(y); - 5 if (r1) { - 6 do_something_with_nonzero_x(); - 7 r2 = READ_ONCE(x); - 8 WARN_ON(!r2); /* BUG!!! */ - 9 } -10 rcu_read_unlock(); -11 } -12 -13 void thread1(void) -14 { -15 spin_lock(&my_lock); -16 WRITE_ONCE(x, 1); -17 WRITE_ONCE(y, 1); -18 spin_unlock(&my_lock); -19 } -</pre> -</blockquote> - -<p> -If the <tt>thread0()</tt> function's <tt>rcu_read_lock()</tt> -excluded the <tt>thread1()</tt> function's update, -the <tt>WARN_ON()</tt> could never fire. -But the fact is that <tt>rcu_read_lock()</tt> does not exclude -much of anything aside from subsequent grace periods, of which -<tt>thread1()</tt> has none, so the -<tt>WARN_ON()</tt> can and does fire. - -<h3><a name="Updaters Only Wait For Old Readers">Updaters Only Wait For Old Readers</a></h3> - -<p> -It might be tempting to assume that after <tt>synchronize_rcu()</tt> -completes, there are no readers executing. -This temptation must be avoided because -new readers can start immediately after <tt>synchronize_rcu()</tt> -starts, and <tt>synchronize_rcu()</tt> is under no -obligation to wait for these new readers. - -<p>@@QQ@@ -Suppose that synchronize_rcu() did wait until all readers had completed. -Would the updater be able to rely on this? -<p>@@QQA@@ -No. -Even if <tt>synchronize_rcu()</tt> were to wait until -all readers had completed, a new reader might start immediately after -<tt>synchronize_rcu()</tt> completed. -Therefore, the code following -<tt>synchronize_rcu()</tt> cannot rely on there being no readers -in any case. -<p>@@QQE@@ - -<h3><a name="Grace Periods Don't Partition Read-Side Critical Sections"> -Grace Periods Don't Partition Read-Side Critical Sections</a></h3> - -<p> -It is tempting to assume that if any part of one RCU read-side critical -section precedes a given grace period, and if any part of another RCU -read-side critical section follows that same grace period, then all of -the first RCU read-side critical section must precede all of the second. -However, this just isn't the case: A single grace period does not -partition the set of RCU read-side critical sections. -An example of this situation can be illustrated as follows, where -<tt>x</tt>, <tt>y</tt>, and <tt>z</tt> are initially all zero: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 WRITE_ONCE(a, 1); - 5 WRITE_ONCE(b, 1); - 6 rcu_read_unlock(); - 7 } - 8 - 9 void thread1(void) -10 { -11 r1 = READ_ONCE(a); -12 synchronize_rcu(); -13 WRITE_ONCE(c, 1); -14 } -15 -16 void thread2(void) -17 { -18 rcu_read_lock(); -19 r2 = READ_ONCE(b); -20 r3 = READ_ONCE(c); -21 rcu_read_unlock(); -22 } -</pre> -</blockquote> - -<p> -It turns out that the outcome: - -<blockquote> -<pre> -(r1 == 1 && r2 == 0 && r3 == 1) -</pre> -</blockquote> - -is entirely possible. -The following figure show how this can happen, with each circled -<tt>QS</tt> indicating the point at which RCU recorded a -<i>quiescent state</i> for each thread, that is, a state in which -RCU knows that the thread cannot be in the midst of an RCU read-side -critical section that started before the current grace period: - -<p><img src="GPpartitionReaders1.svg" alt="GPpartitionReaders1.svg" width="60%"></p> - -<p> -If it is necessary to partition RCU read-side critical sections in this -manner, it is necessary to use two grace periods, where the first -grace period is known to end before the second grace period starts: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 WRITE_ONCE(a, 1); - 5 WRITE_ONCE(b, 1); - 6 rcu_read_unlock(); - 7 } - 8 - 9 void thread1(void) -10 { -11 r1 = READ_ONCE(a); -12 synchronize_rcu(); -13 WRITE_ONCE(c, 1); -14 } -15 -16 void thread2(void) -17 { -18 r2 = READ_ONCE(c); -19 synchronize_rcu(); -20 WRITE_ONCE(d, 1); -21 } -22 -23 void thread3(void) -24 { -25 rcu_read_lock(); -26 r3 = READ_ONCE(b); -27 r4 = READ_ONCE(d); -28 rcu_read_unlock(); -29 } -</pre> -</blockquote> - -<p> -Here, if <tt>(r1 == 1)</tt>, then -<tt>thread0()</tt>'s write to <tt>b</tt> must happen -before the end of <tt>thread1()</tt>'s grace period. -If in addition <tt>(r4 == 1)</tt>, then -<tt>thread3()</tt>'s read from <tt>b</tt> must happen -after the beginning of <tt>thread2()</tt>'s grace period. -If it is also the case that <tt>(r2 == 1)</tt>, then the -end of <tt>thread1()</tt>'s grace period must precede the -beginning of <tt>thread2()</tt>'s grace period. -This mean that the two RCU read-side critical sections cannot overlap, -guaranteeing that <tt>(r3 == 1)</tt>. -As a result, the outcome: - -<blockquote> -<pre> -(r1 == 1 && r2 == 1 && r3 == 0 && r4 == 1) -</pre> -</blockquote> - -cannot happen. - -<p> -This non-requirement was also non-premeditated, but became apparent -when studying RCU's interaction with memory ordering. - -<h3><a name="Read-Side Critical Sections Don't Partition Grace Periods"> -Read-Side Critical Sections Don't Partition Grace Periods</a></h3> - -<p> -It is also tempting to assume that if an RCU read-side critical section -happens between a pair of grace periods, then those grace periods cannot -overlap. -However, this temptation leads nowhere good, as can be illustrated by -the following, with all variables initially zero: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 WRITE_ONCE(a, 1); - 5 WRITE_ONCE(b, 1); - 6 rcu_read_unlock(); - 7 } - 8 - 9 void thread1(void) -10 { -11 r1 = READ_ONCE(a); -12 synchronize_rcu(); -13 WRITE_ONCE(c, 1); -14 } -15 -16 void thread2(void) -17 { -18 rcu_read_lock(); -19 WRITE_ONCE(d, 1); -20 r2 = READ_ONCE(c); -21 rcu_read_unlock(); -22 } -23 -24 void thread3(void) -25 { -26 r3 = READ_ONCE(d); -27 synchronize_rcu(); -28 WRITE_ONCE(e, 1); -29 } -30 -31 void thread4(void) -32 { -33 rcu_read_lock(); -34 r4 = READ_ONCE(b); -35 r5 = READ_ONCE(e); -36 rcu_read_unlock(); -37 } -</pre> -</blockquote> - -<p> -In this case, the outcome: - -<blockquote> -<pre> -(r1 == 1 && r2 == 1 && r3 == 1 && r4 == 0 && r5 == 1) -</pre> -</blockquote> - -is entirely possible, as illustrated below: - -<p><img src="ReadersPartitionGP1.svg" alt="ReadersPartitionGP1.svg" width="100%"></p> - -<p> -Again, an RCU read-side critical section can overlap almost all of a -given grace period, just so long as it does not overlap the entire -grace period. -As a result, an RCU read-side critical section cannot partition a pair -of RCU grace periods. - -<p>@@QQ@@ -How long a sequence of grace periods, each separated by an RCU read-side -critical section, would be required to partition the RCU read-side -critical sections at the beginning and end of the chain? -<p>@@QQA@@ -In theory, an infinite number. -In practice, an unknown number that is sensitive to both implementation -details and timing considerations. -Therefore, even in practice, RCU users must abide by the theoretical rather -than the practical answer. -<p>@@QQE@@ - -<h3><a name="Disabling Preemption Does Not Block Grace Periods"> -Disabling Preemption Does Not Block Grace Periods</a></h3> - -<p> -There was a time when disabling preemption on any given CPU would block -subsequent grace periods. -However, this was an accident of implementation and is not a requirement. -And in the current Linux-kernel implementation, disabling preemption -on a given CPU in fact does not block grace periods, as Oleg Nesterov -<a href="https://lkml.kernel.org/g/20150614193825.GA19582@redhat.com">demonstrated</a>. - -<p> -If you need a preempt-disable region to block grace periods, you need to add -<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>, for example -as follows: - -<blockquote> -<pre> - 1 preempt_disable(); - 2 rcu_read_lock(); - 3 do_something(); - 4 rcu_read_unlock(); - 5 preempt_enable(); - 6 - 7 /* Spinlocks implicitly disable preemption. */ - 8 spin_lock(&mylock); - 9 rcu_read_lock(); -10 do_something(); -11 rcu_read_unlock(); -12 spin_unlock(&mylock); -</pre> -</blockquote> - -<p> -In theory, you could enter the RCU read-side critical section first, -but it is more efficient to keep the entire RCU read-side critical -section contained in the preempt-disable region as shown above. -Of course, RCU read-side critical sections that extend outside of -preempt-disable regions will work correctly, but such critical sections -can be preempted, which forces <tt>rcu_read_unlock()</tt> to do -more work. -And no, this is <i>not</i> an invitation to enclose all of your RCU -read-side critical sections within preempt-disable regions, because -doing so would degrade real-time response. - -<p> -This non-requirement appeared with preemptible RCU. -If you need a grace period that waits on non-preemptible code regions, use -<a href="#Sched Flavor">RCU-sched</a>. - -<h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2> - -<p> -These parallelism facts of life are by no means specific to RCU, but -the RCU implementation must abide by them. -They therefore bear repeating: - -<ol> -<li> Any CPU or task may be delayed at any time, - and any attempts to avoid these delays by disabling - preemption, interrupts, or whatever are completely futile. - This is most obvious in preemptible user-level - environments and in virtualized environments (where - a given guest OS's VCPUs can be preempted at any time by - the underlying hypervisor), but can also happen in bare-metal - environments due to ECC errors, NMIs, and other hardware - events. - Although a delay of more than about 20 seconds can result - in splats, the RCU implementation is obligated to use - algorithms that can tolerate extremely long delays, but where - “extremely long” is not long enough to allow - wrap-around when incrementing a 64-bit counter. -<li> Both the compiler and the CPU can reorder memory accesses. - Where it matters, RCU must use compiler directives and - memory-barrier instructions to preserve ordering. -<li> Conflicting writes to memory locations in any given cache line - will result in expensive cache misses. - Greater numbers of concurrent writes and more-frequent - concurrent writes will result in more dramatic slowdowns. - RCU is therefore obligated to use algorithms that have - sufficient locality to avoid significant performance and - scalability problems. -<li> As a rough rule of thumb, only one CPU's worth of processing - may be carried out under the protection of any given exclusive - lock. - RCU must therefore use scalable locking designs. -<li> Counters are finite, especially on 32-bit systems. - RCU's use of counters must therefore tolerate counter wrap, - or be designed such that counter wrap would take way more - time than a single system is likely to run. - An uptime of ten years is quite possible, a runtime - of a century much less so. - As an example of the latter, RCU's dyntick-idle nesting counter - allows 54 bits for interrupt nesting level (this counter - is 64 bits even on a 32-bit system). - Overflowing this counter requires 2<sup>54</sup> - half-interrupts on a given CPU without that CPU ever going idle. - If a half-interrupt happened every microsecond, it would take - 570 years of runtime to overflow this counter, which is currently - believed to be an acceptably long time. -<li> Linux systems can have thousands of CPUs running a single - Linux kernel in a single shared-memory environment. - RCU must therefore pay close attention to high-end scalability. -</ol> - -<p> -This last parallelism fact of life means that RCU must pay special -attention to the preceding facts of life. -The idea that Linux might scale to systems with thousands of CPUs would -have been met with some skepticism in the 1990s, but these requirements -would have otherwise have been unsurprising, even in the early 1990s. - -<h2><a name="Quality-of-Implementation Requirements">Quality-of-Implementation Requirements</a></h2> - -<p> -These sections list quality-of-implementation requirements. -Although an RCU implementation that ignores these requirements could -still be used, it would likely be subject to limitations that would -make it inappropriate for industrial-strength production use. -Classes of quality-of-implementation requirements are as follows: - -<ol> -<li> <a href="#Specialization">Specialization</a> -<li> <a href="#Performance and Scalability">Performance and Scalability</a> -<li> <a href="#Composability">Composability</a> -<li> <a href="#Corner Cases">Corner Cases</a> -</ol> - -<p> -These classes is covered in the following sections. - -<h3><a name="Specialization">Specialization</a></h3> - -<p> -RCU is and always has been intended primarily for read-mostly situations, as -illustrated by the following figure. -This means that RCU's read-side primitives are optimized, often at the -expense of its update-side primitives. - -<p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p> - -<p> -This focus on read-mostly situations means that RCU must interoperate -with other synchronization primitives. -For example, the <tt>add_gp()</tt> and <tt>remove_gp_synchronous()</tt> -examples discussed earlier use RCU to protect readers and locking to -coordinate updaters. -However, the need extends much farther, requiring that a variety of -synchronization primitives be legal within RCU read-side critical sections, -including spinlocks, sequence locks, atomic operations, reference -counters, and memory barriers. - -<p>@@QQ@@ -What about sleeping locks? -<p>@@QQA@@ -These are forbidden within Linux-kernel RCU read-side critical sections -because it is not legal to place a quiescent state (in this case, -voluntary context switch) within an RCU read-side critical section. -However, sleeping locks may be used within userspace RCU read-side critical -sections, and also within Linux-kernel sleepable RCU -<a href="#Sleepable RCU">(SRCU)</a> -read-side critical sections. -In addition, the -rt patchset turns spinlocks into a sleeping locks so -that the corresponding critical sections can be preempted, which -also means that these sleeplockified spinlocks (but not other sleeping locks!) -may be acquire within -rt-Linux-kernel RCU read-side critical sections. - -<p> -Note that it <i>is</i> legal for a normal RCU read-side critical section -to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>), -but only as long as it does not loop indefinitely attempting to -conditionally acquire that sleeping locks. -The key point is that things like <tt>mutex_trylock()</tt> -either return with the mutex held, or return an error indication if -the mutex was not immediately available. -Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping. -<p>@@QQE@@ - -<p> -It often comes as a surprise that many algorithms do not require a -consistent view of data, but many can function in that mode, -with network routing being the poster child. -Internet routing algorithms take significant time to propagate -updates, so that by the time an update arrives at a given system, -that system has been sending network traffic the wrong way for -a considerable length of time. -Having a few threads continue to send traffic the wrong way for a -few more milliseconds is clearly not a problem: In the worst case, -TCP retransmissions will eventually get the data where it needs to go. -In general, when tracking the state of the universe outside of the -computer, some level of inconsistency must be tolerated due to -speed-of-light delays if nothing else. - -<p> -Furthermore, uncertainty about external state is inherent in many cases. -For example, a pair of veternarians might use heartbeat to determine -whether or not a given cat was alive. -But how long should they wait after the last heartbeat to decide that -the cat is in fact dead? -Waiting less than 400 milliseconds makes no sense because this would -mean that a relaxed cat would be considered to cycle between death -and life more than 100 times per minute. -Moreover, just as with human beings, a cat's heart might stop for -some period of time, so the exact wait period is a judgment call. -One of our pair of veternarians might wait 30 seconds before pronouncing -the cat dead, while the other might insist on waiting a full minute. -The two veternarians would then disagree on the state of the cat during -the final 30 seconds of the minute following the last heartbeat, as -fancifully illustrated below: - -<p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p> - -<p> -Interestingly enough, this same situation applies to hardware. -When push comes to shove, how do we tell whether or not some -external server has failed? -We send messages to it periodically, and declare it failed if we -don't receive a response within a given period of time. -Policy decisions can usually tolerate short -periods of inconsistency. -The policy was decided some time ago, and is only now being put into -effect, so a few milliseconds of delay is normally inconsequential. - -<p> -However, there are algorithms that absolutely must see consistent data. -For example, the translation between a user-level SystemV semaphore -ID to the corresponding in-kernel data structure is protected by RCU, -but it is absolutely forbidden to update a semaphore that has just been -removed. -In the Linux kernel, this need for consistency is accommodated by acquiring -spinlocks located in the in-kernel data structure from within -the RCU read-side critical section, and this is indicated by the -green box in the figure above. -Many other techniques may be used, and are in fact used within the -Linux kernel. - -<p> -In short, RCU is not required to maintain consistency, and other -mechanisms may be used in concert with RCU when consistency is required. -RCU's specialization allows it to do its job extremely well, and its -ability to interoperate with other synchronization mechanisms allows -the right mix of synchronization tools to be used for a given job. - -<h3><a name="Performance and Scalability">Performance and Scalability</a></h3> - -<p> -Energy efficiency is a critical component of performance today, -and Linux-kernel RCU implementations must therefore avoid unnecessarily -awakening idle CPUs. -I cannot claim that this requirement was premeditated. -In fact, I learned of it during a telephone conversation in which I -was given “frank and open” feedback on the importance -of energy efficiency in battery-powered systems and on specific -energy-efficiency shortcomings of the Linux-kernel RCU implementation. -In my experience, the battery-powered embedded community will consider -any unnecessary wakeups to be extremely unfriendly acts. -So much so that mere Linux-kernel-mailing-list posts are -insufficient to vent their ire. - -<p> -Memory consumption is not particularly important for in most -situations, and has become decreasingly -so as memory sizes have expanded and memory -costs have plummeted. -However, as I learned from Matt Mackall's -<a href="http://elinux.org/Linux_Tiny-FAQ">bloatwatch</a> -efforts, memory footprint is critically important on single-CPU systems with -non-preemptible (<tt>CONFIG_PREEMPT=n</tt>) kernels, and thus -<a href="https://lkml.kernel.org/g/20090113221724.GA15307@linux.vnet.ibm.com">tiny RCU</a> -was born. -Josh Triplett has since taken over the small-memory banner with his -<a href="https://tiny.wiki.kernel.org/">Linux kernel tinification</a> -project, which resulted in -<a href="#Sleepable RCU">SRCU</a> -becoming optional for those kernels not needing it. - -<p> -The remaining performance requirements are, for the most part, -unsurprising. -For example, in keeping with RCU's read-side specialization, -<tt>rcu_dereference()</tt> should have negligible overhead (for -example, suppression of a few minor compiler optimizations). -Similarly, in non-preemptible environments, <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> should have exactly zero overhead. - -<p> -In preemptible environments, in the case where the RCU read-side -critical section was not preempted (as will be the case for the -highest-priority real-time process), <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> should have minimal overhead. -In particular, they should not contain atomic read-modify-write -operations, memory-barrier instructions, preemption disabling, -interrupt disabling, or backwards branches. -However, in the case where the RCU read-side critical section was preempted, -<tt>rcu_read_unlock()</tt> may acquire spinlocks and disable interrupts. -This is why it is better to nest an RCU read-side critical section -within a preempt-disable region than vice versa, at least in cases -where that critical section is short enough to avoid unduly degrading -real-time latencies. - -<p> -The <tt>synchronize_rcu()</tt> grace-period-wait primitive is -optimized for throughput. -It may therefore incur several milliseconds of latency in addition to -the duration of the longest RCU read-side critical section. -On the other hand, multiple concurrent invocations of -<tt>synchronize_rcu()</tt> are required to use batching optimizations -so that they can be satisfied by a single underlying grace-period-wait -operation. -For example, in the Linux kernel, it is not unusual for a single -grace-period-wait operation to serve more than -<a href="https://www.usenix.org/conference/2004-usenix-annual-technical-conference/making-rcu-safe-deep-sub-millisecond-response">1,000 separate invocations</a> -of <tt>synchronize_rcu()</tt>, thus amortizing the per-invocation -overhead down to nearly zero. -However, the grace-period optimization is also required to avoid -measurable degradation of real-time scheduling and interrupt latencies. - -<p> -In some cases, the multi-millisecond <tt>synchronize_rcu()</tt> -latencies are unacceptable. -In these cases, <tt>synchronize_rcu_expedited()</tt> may be used -instead, reducing the grace-period latency down to a few tens of -microseconds on small systems, at least in cases where the RCU read-side -critical sections are short. -There are currently no special latency requirements for -<tt>synchronize_rcu_expedited()</tt> on large systems, but, -consistent with the empirical nature of the RCU specification, -that is subject to change. -However, there most definitely are scalability requirements: -A storm of <tt>synchronize_rcu_expedited()</tt> invocations on 4096 -CPUs should at least make reasonable forward progress. -In return for its shorter latencies, <tt>synchronize_rcu_expedited()</tt> -is permitted to impose modest degradation of real-time latency -on non-idle online CPUs. -That said, it will likely be necessary to take further steps to reduce this -degradation, hopefully to roughly that of a scheduling-clock interrupt. - -<p> -There are a number of situations where even -<tt>synchronize_rcu_expedited()</tt>'s reduced grace-period -latency is unacceptable. -In these situations, the asynchronous <tt>call_rcu()</tt> can be -used in place of <tt>synchronize_rcu()</tt> as follows: - -<blockquote> -<pre> - 1 struct foo { - 2 int a; - 3 int b; - 4 struct rcu_head rh; - 5 }; - 6 - 7 static void remove_gp_cb(struct rcu_head *rhp) - 8 { - 9 struct foo *p = container_of(rhp, struct foo, rh); -10 -11 kfree(p); -12 } -13 -14 bool remove_gp_asynchronous(void) -15 { -16 struct foo *p; -17 -18 spin_lock(&gp_lock); -19 p = rcu_dereference(gp); -20 if (!p) { -21 spin_unlock(&gp_lock); -22 return false; -23 } -24 rcu_assign_pointer(gp, NULL); -25 call_rcu(&p->rh, remove_gp_cb); -26 spin_unlock(&gp_lock); -27 return true; -28 } -</pre> -</blockquote> - -<p> -A definition of <tt>struct foo</tt> is finally needed, and appears -on lines 1-5. -The function <tt>remove_gp_cb()</tt> is passed to <tt>call_rcu()</tt> -on line 25, and will be invoked after the end of a subsequent -grace period. -This gets the same effect as <tt>remove_gp_synchronous()</tt>, -but without forcing the updater to wait for a grace period to elapse. -The <tt>call_rcu()</tt> function may be used in a number of -situations where neither <tt>synchronize_rcu()</tt> nor -<tt>synchronize_rcu_expedited()</tt> would be legal, -including within preempt-disable code, <tt>local_bh_disable()</tt> code, -interrupt-disable code, and interrupt handlers. -However, even <tt>call_rcu()</tt> is illegal within NMI handlers. -The callback function (<tt>remove_gp_cb()</tt> in this case) will be -executed within softirq (software interrupt) environment within the -Linux kernel, -either within a real softirq handler or under the protection -of <tt>local_bh_disable()</tt>. -In both the Linux kernel and in userspace, it is bad practice to -write an RCU callback function that takes too long. -Long-running operations should be relegated to separate threads or -(in the Linux kernel) workqueues. - -<p>@@QQ@@ -Why does line 19 use <tt>rcu_access_pointer()</tt>? -After all, <tt>call_rcu()</tt> on line 25 stores into the -structure, which would interact badly with concurrent insertions. -Doesn't this mean that <tt>rcu_dereference()</tt> is required? -<p>@@QQA@@ -Presumably the <tt>->gp_lock</tt> acquired on line 18 excludes -any changes, including any insertions that <tt>rcu_dereference()</tt> -would protect against. -Therefore, any insertions will be delayed until after <tt>->gp_lock</tt> -is released on line 25, which in turn means that -<tt>rcu_access_pointer()</tt> suffices. -<p>@@QQE@@ - -<p> -However, all that <tt>remove_gp_cb()</tt> is doing is -invoking <tt>kfree()</tt> on the data element. -This is a common idiom, and is supported by <tt>kfree_rcu()</tt>, -which allows “fire and forget” operation as shown below: - -<blockquote> -<pre> - 1 struct foo { - 2 int a; - 3 int b; - 4 struct rcu_head rh; - 5 }; - 6 - 7 bool remove_gp_faf(void) - 8 { - 9 struct foo *p; -10 -11 spin_lock(&gp_lock); -12 p = rcu_dereference(gp); -13 if (!p) { -14 spin_unlock(&gp_lock); -15 return false; -16 } -17 rcu_assign_pointer(gp, NULL); -18 kfree_rcu(p, rh); -19 spin_unlock(&gp_lock); -20 return true; -21 } -</pre> -</blockquote> - -<p> -Note that <tt>remove_gp_faf()</tt> simply invokes -<tt>kfree_rcu()</tt> and proceeds, without any need to pay any -further attention to the subsequent grace period and <tt>kfree()</tt>. -It is permissible to invoke <tt>kfree_rcu()</tt> from the same -environments as for <tt>call_rcu()</tt>. -Interestingly enough, DYNIX/ptx had the equivalents of -<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>, but not -<tt>synchronize_rcu()</tt>. -This was due to the fact that RCU was not heavily used within DYNIX/ptx, -so the very few places that needed something like -<tt>synchronize_rcu()</tt> simply open-coded it. - -<p>@@QQ@@ -Earlier it was claimed that <tt>call_rcu()</tt> and -<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked -by readers. -But how can that be correct, given that the invocation of the callback -and the freeing of the memory (respectively) must still wait for -a grace period to elapse? -<p>@@QQA@@ -We could define things this way, but keep in mind that this sort of -definition would say that updates in garbage-collected languages -cannot complete until the next time the garbage collector runs, -which does not seem at all reasonable. -The key point is that in most cases, an updater using either -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the -next update as soon as it has invoked <tt>call_rcu()</tt> or -<tt>kfree_rcu()</tt>, without having to wait for a subsequent -grace period. -<p>@@QQE@@ - -<p> -But what if the updater must wait for the completion of code to be -executed after the end of the grace period, but has other tasks -that can be carried out in the meantime? -The polling-style <tt>get_state_synchronize_rcu()</tt> and -<tt>cond_synchronize_rcu()</tt> functions may be used for this -purpose, as shown below: - -<blockquote> -<pre> - 1 bool remove_gp_poll(void) - 2 { - 3 struct foo *p; - 4 unsigned long s; - 5 - 6 spin_lock(&gp_lock); - 7 p = rcu_access_pointer(gp); - 8 if (!p) { - 9 spin_unlock(&gp_lock); -10 return false; -11 } -12 rcu_assign_pointer(gp, NULL); -13 spin_unlock(&gp_lock); -14 s = get_state_synchronize_rcu(); -15 do_something_while_waiting(); -16 cond_synchronize_rcu(s); -17 kfree(p); -18 return true; -19 } -</pre> -</blockquote> - -<p> -On line 14, <tt>get_state_synchronize_rcu()</tt> obtains a -“cookie” from RCU, -then line 15 carries out other tasks, -and finally, line 16 returns immediately if a grace period has -elapsed in the meantime, but otherwise waits as required. -The need for <tt>get_state_synchronize_rcu</tt> and -<tt>cond_synchronize_rcu()</tt> has appeared quite recently, -so it is too early to tell whether they will stand the test of time. - -<p> -RCU thus provides a range of tools to allow updaters to strike the -required tradeoff between latency, flexibility and CPU overhead. - -<h3><a name="Composability">Composability</a></h3> - -<p> -Composability has received much attention in recent years, perhaps in part -due to the collision of multicore hardware with object-oriented techniques -designed in single-threaded environments for single-threaded use. -And in theory, RCU read-side critical sections may be composed, and in -fact may be nested arbitrarily deeply. -In practice, as with all real-world implementations of composable -constructs, there are limitations. - -<p> -Implementations of RCU for which <tt>rcu_read_lock()</tt> -and <tt>rcu_read_unlock()</tt> generate no code, such as -Linux-kernel RCU when <tt>CONFIG_PREEMPT=n</tt>, can be -nested arbitrarily deeply. -After all, there is no overhead. -Except that if all these instances of <tt>rcu_read_lock()</tt> -and <tt>rcu_read_unlock()</tt> are visible to the compiler, -compilation will eventually fail due to exhausting memory, -mass storage, or user patience, whichever comes first. -If the nesting is not visible to the compiler, as is the case with -mutually recursive functions each in its own translation unit, -stack overflow will result. -If the nesting takes the form of loops, either the control variable -will overflow or (in the Linux kernel) you will get an RCU CPU stall warning. -Nevertheless, this class of RCU implementations is one -of the most composable constructs in existence. - -<p> -RCU implementations that explicitly track nesting depth -are limited by the nesting-depth counter. -For example, the Linux kernel's preemptible RCU limits nesting to -<tt>INT_MAX</tt>. -This should suffice for almost all practical purposes. -That said, a consecutive pair of RCU read-side critical sections -between which there is an operation that waits for a grace period -cannot be enclosed in another RCU read-side critical section. -This is because it is not legal to wait for a grace period within -an RCU read-side critical section: To do so would result either -in deadlock or -in RCU implicitly splitting the enclosing RCU read-side critical -section, neither of which is conducive to a long-lived and prosperous -kernel. - -<p> -It is worth noting that RCU is not alone in limiting composability. -For example, many transactional-memory implementations prohibit -composing a pair of transactions separated by an irrevocable -operation (for example, a network receive operation). -For another example, lock-based critical sections can be composed -surprisingly freely, but only if deadlock is avoided. - -<p> -In short, although RCU read-side critical sections are highly composable, -care is required in some situations, just as is the case for any other -composable synchronization mechanism. - -<h3><a name="Corner Cases">Corner Cases</a></h3> - -<p> -A given RCU workload might have an endless and intense stream of -RCU read-side critical sections, perhaps even so intense that there -was never a point in time during which there was not at least one -RCU read-side critical section in flight. -RCU cannot allow this situation to block grace periods: As long as -all the RCU read-side critical sections are finite, grace periods -must also be finite. - -<p> -That said, preemptible RCU implementations could potentially result -in RCU read-side critical sections being preempted for long durations, -which has the effect of creating a long-duration RCU read-side -critical section. -This situation can arise only in heavily loaded systems, but systems using -real-time priorities are of course more vulnerable. -Therefore, RCU priority boosting is provided to help deal with this -case. -That said, the exact requirements on RCU priority boosting will likely -evolve as more experience accumulates. - -<p> -Other workloads might have very high update rates. -Although one can argue that such workloads should instead use -something other than RCU, the fact remains that RCU must -handle such workloads gracefully. -This requirement is another factor driving batching of grace periods, -but it is also the driving force behind the checks for large numbers -of queued RCU callbacks in the <tt>call_rcu()</tt> code path. -Finally, high update rates should not delay RCU read-side critical -sections, although some read-side delays can occur when using -<tt>synchronize_rcu_expedited()</tt>, courtesy of this function's use -of <tt>try_stop_cpus()</tt>. -(In the future, <tt>synchronize_rcu_expedited()</tt> will be -converted to use lighter-weight inter-processor interrupts (IPIs), -but this will still disturb readers, though to a much smaller degree.) - -<p> -Although all three of these corner cases were understood in the early -1990s, a simple user-level test consisting of <tt>close(open(path))</tt> -in a tight loop -in the early 2000s suddenly provided a much deeper appreciation of the -high-update-rate corner case. -This test also motivated addition of some RCU code to react to high update -rates, for example, if a given CPU finds itself with more than 10,000 -RCU callbacks queued, it will cause RCU to take evasive action by -more aggressively starting grace periods and more aggressively forcing -completion of grace-period processing. -This evasive action causes the grace period to complete more quickly, -but at the cost of restricting RCU's batching optimizations, thus -increasing the CPU overhead incurred by that grace period. - -<h2><a name="Software-Engineering Requirements"> -Software-Engineering Requirements</a></h2> - -<p> -Between Murphy's Law and “To err is human”, it is necessary to -guard against mishaps and misuse: - -<ol> -<li> It is all too easy to forget to use <tt>rcu_read_lock()</tt> - everywhere that it is needed, so kernels built with - <tt>CONFIG_PROVE_RCU=y</tt> will spat if - <tt>rcu_dereference()</tt> is used outside of an - RCU read-side critical section. - Update-side code can use <tt>rcu_dereference_protected()</tt>, - which takes a - <a href="https://lwn.net/Articles/371986/">lockdep expression</a> - to indicate what is providing the protection. - If the indicated protection is not provided, a lockdep splat - is emitted. - - <p> - Code shared between readers and updaters can use - <tt>rcu_dereference_check()</tt>, which also takes a - lockdep expression, and emits a lockdep splat if neither - <tt>rcu_read_lock()</tt> nor the indicated protection - is in place. - In addition, <tt>rcu_dereference_raw()</tt> is used in those - (hopefully rare) cases where the required protection cannot - be easily described. - Finally, <tt>rcu_read_lock_held()</tt> is provided to - allow a function to verify that it has been invoked within - an RCU read-side critical section. - I was made aware of this set of requirements shortly after Thomas - Gleixner audited a number of RCU uses. -<li> A given function might wish to check for RCU-related preconditions - upon entry, before using any other RCU API. - The <tt>rcu_lockdep_assert()</tt> does this job, - asserting the expression in kernels having lockdep enabled - and doing nothing otherwise. -<li> It is also easy to forget to use <tt>rcu_assign_pointer()</tt> - and <tt>rcu_dereference()</tt>, perhaps (incorrectly) - substituting a simple assignment. - To catch this sort of error, a given RCU-protected pointer may be - tagged with <tt>__rcu</tt>, after which running sparse - with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt> will complain - about simple-assignment accesses to that pointer. - Arnd Bergmann made me aware of this requirement, and also - supplied the needed - <a href="https://lwn.net/Articles/376011/">patch series</a>. -<li> Kernels built with <tt>CONFIG_DEBUG_OBJECTS_RCU_HEAD=y</tt> - will splat if a data element is passed to <tt>call_rcu()</tt> - twice in a row, without a grace period in between. - (This error is similar to a double free.) - The corresponding <tt>rcu_head</tt> structures that are - dynamically allocated are automatically tracked, but - <tt>rcu_head</tt> structures allocated on the stack - must be initialized with <tt>init_rcu_head_on_stack()</tt> - and cleaned up with <tt>destroy_rcu_head_on_stack()</tt>. - Similarly, statically allocated non-stack <tt>rcu_head</tt> - structures must be initialized with <tt>init_rcu_head()</tt> - and cleaned up with <tt>destroy_rcu_head()</tt>. - Mathieu Desnoyers made me aware of this requirement, and also - supplied the needed - <a href="https://lkml.kernel.org/g/20100319013024.GA28456@Krystal">patch</a>. -<li> An infinite loop in an RCU read-side critical section will - eventually trigger an RCU CPU stall warning splat, with - the duration of “eventually” being controlled by the - <tt>RCU_CPU_STALL_TIMEOUT</tt> <tt>Kconfig</tt> option, or, - alternatively, by the - <tt>rcupdate.rcu_cpu_stall_timeout</tt> boot/sysfs - parameter. - However, RCU is not obligated to produce this splat - unless there is a grace period waiting on that particular - RCU read-side critical section. - <p> - Some extreme workloads might intentionally delay - RCU grace periods, and systems running those workloads can - be booted with <tt>rcupdate.rcu_cpu_stall_suppress</tt> - to suppress the splats. - This kernel parameter may also be set via <tt>sysfs</tt>. - Furthermore, RCU CPU stall warnings are counter-productive - during sysrq dumps and during panics. - RCU therefore supplies the <tt>rcu_sysrq_start()</tt> and - <tt>rcu_sysrq_end()</tt> API members to be called before - and after long sysrq dumps. - RCU also supplies the <tt>rcu_panic()</tt> notifier that is - automatically invoked at the beginning of a panic to suppress - further RCU CPU stall warnings. - - <p> - This requirement made itself known in the early 1990s, pretty - much the first time that it was necessary to debug a CPU stall. - That said, the initial implementation in DYNIX/ptx was quite - generic in comparison with that of Linux. -<li> Although it would be very good to detect pointers leaking out - of RCU read-side critical sections, there is currently no - good way of doing this. - One complication is the need to distinguish between pointers - leaking and pointers that have been handed off from RCU to - some other synchronization mechanism, for example, reference - counting. -<li> In kernels built with <tt>CONFIG_RCU_TRACE=y</tt>, RCU-related - information is provided via both debugfs and event tracing. -<li> Open-coded use of <tt>rcu_assign_pointer()</tt> and - <tt>rcu_dereference()</tt> to create typical linked - data structures can be surprisingly error-prone. - Therefore, RCU-protected - <a href="https://lwn.net/Articles/609973/#RCU List APIs">linked lists</a> - and, more recently, RCU-protected - <a href="https://lwn.net/Articles/612100/">hash tables</a> - are available. - Many other special-purpose RCU-protected data structures are - available in the Linux kernel and the userspace RCU library. -<li> Some linked structures are created at compile time, but still - require <tt>__rcu</tt> checking. - The <tt>RCU_POINTER_INITIALIZER()</tt> macro serves this - purpose. -<li> It is not necessary to use <tt>rcu_assign_pointer()</tt> - when creating linked structures that are to be published via - a single external pointer. - The <tt>RCU_INIT_POINTER()</tt> macro is provided for - this task and also for assigning <tt>NULL</tt> pointers - at runtime. -</ol> - -<p> -This not a hard-and-fast list: RCU's diagnostic capabilities will -continue to be guided by the number and type of usage bugs found -in real-world RCU usage. - -<h2><a name="Linux Kernel Complications">Linux Kernel Complications</a></h2> - -<p> -The Linux kernel provides an interesting environment for all kinds of -software, including RCU. -Some of the relevant points of interest are as follows: - -<ol> -<li> <a href="#Configuration">Configuration</a>. -<li> <a href="#Firmware Interface">Firmware Interface</a>. -<li> <a href="#Early Boot">Early Boot</a>. -<li> <a href="#Interrupts and NMIs"> - Interrupts and non-maskable interrupts (NMIs)</a>. -<li> <a href="#Loadable Modules">Loadable Modules</a>. -<li> <a href="#Hotplug CPU">Hotplug CPU</a>. -<li> <a href="#Scheduler and RCU">Scheduler and RCU</a>. -<li> <a href="#Tracing and RCU">Tracing and RCU</a>. -<li> <a href="#Energy Efficiency">Energy Efficiency</a>. -<li> <a href="#Memory Efficiency">Memory Efficiency</a>. -<li> <a href="#Performance, Scalability, Response Time, and Reliability"> - Performance, Scalability, Response Time, and Reliability</a>. -</ol> - -<p> -This list is probably incomplete, but it does give a feel for the -most notable Linux-kernel complications. -Each of the following sections covers one of the above topics. - -<h3><a name="Configuration">Configuration</a></h3> - -<p> -RCU's goal is automatic configuration, so that almost nobody -needs to worry about RCU's <tt>Kconfig</tt> options. -And for almost all users, RCU does in fact work well -“out of the box.” - -<p> -However, there are specialized use cases that are handled by -kernel boot parameters and <tt>Kconfig</tt> options. -Unfortunately, the <tt>Kconfig</tt> system will explicitly ask users -about new <tt>Kconfig</tt> options, which requires almost all of them -be hidden behind a <tt>CONFIG_RCU_EXPERT</tt> <tt>Kconfig</tt> option. - -<p> -This all should be quite obvious, but the fact remains that -Linus Torvalds recently had to -<a href="https://lkml.kernel.org/g/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com">remind</a> -me of this requirement. - -<h3><a name="Firmware Interface">Firmware Interface</a></h3> - -<p> -In many cases, kernel obtains information about the system from the -firmware, and sometimes things are lost in translation. -Or the translation is accurate, but the original message is bogus. - -<p> -For example, some systems' firmware overreports the number of CPUs, -sometimes by a large factor. -If RCU naively believed the firmware, as it used to do, -it would create too many per-CPU kthreads. -Although the resulting system will still run correctly, the extra -kthreads needlessly consume memory and can cause confusion -when they show up in <tt>ps</tt> listings. - -<p> -RCU must therefore wait for a given CPU to actually come online before -it can allow itself to believe that the CPU actually exists. -The resulting “ghost CPUs” (which are never going to -come online) cause a number of -<a href="https://paulmck.livejournal.com/37494.html">interesting complications</a>. - -<h3><a name="Early Boot">Early Boot</a></h3> - -<p> -The Linux kernel's boot sequence is an interesting process, -and RCU is used early, even before <tt>rcu_init()</tt> -is invoked. -In fact, a number of RCU's primitives can be used as soon as the -initial task's <tt>task_struct</tt> is available and the -boot CPU's per-CPU variables are set up. -The read-side primitives (<tt>rcu_read_lock()</tt>, -<tt>rcu_read_unlock()</tt>, <tt>rcu_dereference()</tt>, -and <tt>rcu_access_pointer()</tt>) will operate normally very early on, -as will <tt>rcu_assign_pointer()</tt>. - -<p> -Although <tt>call_rcu()</tt> may be invoked at any -time during boot, callbacks are not guaranteed to be invoked until after -the scheduler is fully up and running. -This delay in callback invocation is due to the fact that RCU does not -invoke callbacks until it is fully initialized, and this full initialization -cannot occur until after the scheduler has initialized itself to the -point where RCU can spawn and run its kthreads. -In theory, it would be possible to invoke callbacks earlier, -however, this is not a panacea because there would be severe restrictions -on what operations those callbacks could invoke. - -<p> -Perhaps surprisingly, <tt>synchronize_rcu()</tt>, -<a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a> -(<a href="#Bottom-Half Flavor">discussed below</a>), -and -<a href="#Sched Flavor"><tt>synchronize_sched()</tt></a> -will all operate normally -during very early boot, the reason being that there is only one CPU -and preemption is disabled. -This means that the call <tt>synchronize_rcu()</tt> (or friends) -itself is a quiescent -state and thus a grace period, so the early-boot implementation can -be a no-op. - -<p> -Both <tt>synchronize_rcu_bh()</tt> and <tt>synchronize_sched()</tt> -continue to operate normally through the remainder of boot, courtesy -of the fact that preemption is disabled across their RCU read-side -critical sections and also courtesy of the fact that there is still -only one CPU. -However, once the scheduler starts initializing, preemption is enabled. -There is still only a single CPU, but the fact that preemption is enabled -means that the no-op implementation of <tt>synchronize_rcu()</tt> no -longer works in <tt>CONFIG_PREEMPT=y</tt> kernels. -Therefore, as soon as the scheduler starts initializing, the early-boot -fastpath is disabled. -This means that <tt>synchronize_rcu()</tt> switches to its runtime -mode of operation where it posts callbacks, which in turn means that -any call to <tt>synchronize_rcu()</tt> will block until the corresponding -callback is invoked. -Unfortunately, the callback cannot be invoked until RCU's runtime -grace-period machinery is up and running, which cannot happen until -the scheduler has initialized itself sufficiently to allow RCU's -kthreads to be spawned. -Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler -initialization can result in deadlock. - -<p>@@QQ@@ -So what happens with <tt>synchronize_rcu()</tt> during -scheduler initialization for <tt>CONFIG_PREEMPT=n</tt> -kernels? -<p>@@QQA@@ -In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt> -maps directly to <tt>synchronize_sched()</tt>. -Therefore, <tt>synchronize_rcu()</tt> works normally throughout -boot in <tt>CONFIG_PREEMPT=n</tt> kernels. -However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels, -so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt> -during scheduler initialization. -<p>@@QQE@@ - -<p> -I learned of these boot-time requirements as a result of a series of -system hangs. - -<h3><a name="Interrupts and NMIs">Interrupts and NMIs</a></h3> - -<p> -The Linux kernel has interrupts, and RCU read-side critical sections are -legal within interrupt handlers and within interrupt-disabled regions -of code, as are invocations of <tt>call_rcu()</tt>. - -<p> -Some Linux-kernel architectures can enter an interrupt handler from -non-idle process context, and then just never leave it, instead stealthily -transitioning back to process context. -This trick is sometimes used to invoke system calls from inside the kernel. -These “half-interrupts” mean that RCU has to be very careful -about how it counts interrupt nesting levels. -I learned of this requirement the hard way during a rewrite -of RCU's dyntick-idle code. - -<p> -The Linux kernel has non-maskable interrupts (NMIs), and -RCU read-side critical sections are legal within NMI handlers. -Thankfully, RCU update-side primitives, including -<tt>call_rcu()</tt>, are prohibited within NMI handlers. - -<p> -The name notwithstanding, some Linux-kernel architectures -can have nested NMIs, which RCU must handle correctly. -Andy Lutomirski -<a href="https://lkml.kernel.org/g/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com">surprised me</a> -with this requirement; -he also kindly surprised me with -<a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a> -that meets this requirement. - -<h3><a name="Loadable Modules">Loadable Modules</a></h3> - -<p> -The Linux kernel has loadable modules, and these modules can -also be unloaded. -After a given module has been unloaded, any attempt to call -one of its functions results in a segmentation fault. -The module-unload functions must therefore cancel any -delayed calls to loadable-module functions, for example, -any outstanding <tt>mod_timer()</tt> must be dealt with -via <tt>del_timer_sync()</tt> or similar. - -<p> -Unfortunately, there is no way to cancel an RCU callback; -once you invoke <tt>call_rcu()</tt>, the callback function is -going to eventually be invoked, unless the system goes down first. -Because it is normally considered socially irresponsible to crash the system -in response to a module unload request, we need some other way -to deal with in-flight RCU callbacks. - -<p> -RCU therefore provides -<tt><a href="https://lwn.net/Articles/217484/">rcu_barrier()</a></tt>, -which waits until all in-flight RCU callbacks have been invoked. -If a module uses <tt>call_rcu()</tt>, its exit function should therefore -prevent any future invocation of <tt>call_rcu()</tt>, then invoke -<tt>rcu_barrier()</tt>. -In theory, the underlying module-unload code could invoke -<tt>rcu_barrier()</tt> unconditionally, but in practice this would -incur unacceptable latencies. - -<p> -Nikita Danilov noted this requirement for an analogous filesystem-unmount -situation, and Dipankar Sarma incorporated <tt>rcu_barrier()</tt> into RCU. -The need for <tt>rcu_barrier()</tt> for module unloading became -apparent later. - -<h3><a name="Hotplug CPU">Hotplug CPU</a></h3> - -<p> -The Linux kernel supports CPU hotplug, which means that CPUs -can come and go. -It is of course illegal to use any RCU API member from an offline CPU. -This requirement was present from day one in DYNIX/ptx, but -on the other hand, the Linux kernel's CPU-hotplug implementation -is “interesting.” - -<p> -The Linux-kernel CPU-hotplug implementation has notifiers that -are used to allow the various kernel subsystems (including RCU) -to respond appropriately to a given CPU-hotplug operation. -Most RCU operations may be invoked from CPU-hotplug notifiers, -including even normal synchronous grace-period operations -such as <tt>synchronize_rcu()</tt>. -However, expedited grace-period operations such as -<tt>synchronize_rcu_expedited()</tt> are not supported, -due to the fact that current implementations block CPU-hotplug -operations, which could result in deadlock. - -<p> -In addition, all-callback-wait operations such as -<tt>rcu_barrier()</tt> are also not supported, due to the -fact that there are phases of CPU-hotplug operations where -the outgoing CPU's callbacks will not be invoked until after -the CPU-hotplug operation ends, which could also result in deadlock. - -<h3><a name="Scheduler and RCU">Scheduler and RCU</a></h3> - -<p> -RCU depends on the scheduler, and the scheduler uses RCU to -protect some of its data structures. -This means the scheduler is forbidden from acquiring -the runqueue locks and the priority-inheritance locks -in the middle of an outermost RCU read-side critical section unless either -(1) it releases them before exiting that same -RCU read-side critical section, or -(2) interrupts are disabled across -that entire RCU read-side critical section. -This same prohibition also applies (recursively!) to any lock that is acquired -while holding any lock to which this prohibition applies. -Adhering to this rule prevents preemptible RCU from invoking -<tt>rcu_read_unlock_special()</tt> while either runqueue or -priority-inheritance locks are held, thus avoiding deadlock. - -<p> -Prior to v4.4, it was only necessary to disable preemption across -RCU read-side critical sections that acquired scheduler locks. -In v4.4, expedited grace periods started using IPIs, and these -IPIs could force a <tt>rcu_read_unlock()</tt> to take the slowpath. -Therefore, this expedited-grace-period change required disabling of -interrupts, not just preemption. - -<p> -For RCU's part, the preemptible-RCU <tt>rcu_read_unlock()</tt> -implementation must be written carefully to avoid similar deadlocks. -In particular, <tt>rcu_read_unlock()</tt> must tolerate an -interrupt where the interrupt handler invokes both -<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>. -This possibility requires <tt>rcu_read_unlock()</tt> to use -negative nesting levels to avoid destructive recursion via -interrupt handler's use of RCU. - -<p> -This pair of mutual scheduler-RCU requirements came as a -<a href="https://lwn.net/Articles/453002/">complete surprise</a>. - -<p> -As noted above, RCU makes use of kthreads, and it is necessary to -avoid excessive CPU-time accumulation by these kthreads. -This requirement was no surprise, but RCU's violation of it -when running context-switch-heavy workloads when built with -<tt>CONFIG_NO_HZ_FULL=y</tt> -<a href="http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf">did come as a surprise [PDF]</a>. -RCU has made good progress towards meeting this requirement, even -for context-switch-have <tt>CONFIG_NO_HZ_FULL=y</tt> workloads, -but there is room for further improvement. - -<h3><a name="Tracing and RCU">Tracing and RCU</a></h3> - -<p> -It is possible to use tracing on RCU code, but tracing itself -uses RCU. -For this reason, <tt>rcu_dereference_raw_notrace()</tt> -is provided for use by tracing, which avoids the destructive -recursion that could otherwise ensue. -This API is also used by virtualization in some architectures, -where RCU readers execute in environments in which tracing -cannot be used. -The tracing folks both located the requirement and provided the -needed fix, so this surprise requirement was relatively painless. - -<h3><a name="Energy Efficiency">Energy Efficiency</a></h3> - -<p> -Interrupting idle CPUs is considered socially unacceptable, -especially by people with battery-powered embedded systems. -RCU therefore conserves energy by detecting which CPUs are -idle, including tracking CPUs that have been interrupted from idle. -This is a large part of the energy-efficiency requirement, -so I learned of this via an irate phone call. - -<p> -Because RCU avoids interrupting idle CPUs, it is illegal to -execute an RCU read-side critical section on an idle CPU. -(Kernels built with <tt>CONFIG_PROVE_RCU=y</tt> will splat -if you try it.) -The <tt>RCU_NONIDLE()</tt> macro and <tt>_rcuidle</tt> -event tracing is provided to work around this restriction. -In addition, <tt>rcu_is_watching()</tt> may be used to -test whether or not it is currently legal to run RCU read-side -critical sections on this CPU. -I learned of the need for diagnostics on the one hand -and <tt>RCU_NONIDLE()</tt> on the other while inspecting -idle-loop code. -Steven Rostedt supplied <tt>_rcuidle</tt> event tracing, -which is used quite heavily in the idle loop. - -<p> -It is similarly socially unacceptable to interrupt an -<tt>nohz_full</tt> CPU running in userspace. -RCU must therefore track <tt>nohz_full</tt> userspace -execution. -And in -<a href="https://lwn.net/Articles/558284/"><tt>CONFIG_NO_HZ_FULL_SYSIDLE=y</tt></a> -kernels, RCU must separately track idle CPUs on the one hand and -CPUs that are either idle or executing in userspace on the other. -In both cases, RCU must be able to sample state at two points in -time, and be able to determine whether or not some other CPU spent -any time idle and/or executing in userspace. - -<p> -These energy-efficiency requirements have proven quite difficult to -understand and to meet, for example, there have been more than five -clean-sheet rewrites of RCU's energy-efficiency code, the last of -which was finally able to demonstrate -<a href="http://www.rdrop.com/users/paulmck/realtime/paper/AMPenergy.2013.04.19a.pdf">real energy savings running on real hardware [PDF]</a>. -As noted earlier, -I learned of many of these requirements via angry phone calls: -Flaming me on the Linux-kernel mailing list was apparently not -sufficient to fully vent their ire at RCU's energy-efficiency bugs! - -<h3><a name="Memory Efficiency">Memory Efficiency</a></h3> - -<p> -Although small-memory non-realtime systems can simply use Tiny RCU, -code size is only one aspect of memory efficiency. -Another aspect is the size of the <tt>rcu_head</tt> structure -used by <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>. -Although this structure contains nothing more than a pair of pointers, -it does appear in many RCU-protected data structures, including -some that are size critical. -The <tt>page</tt> structure is a case in point, as evidenced by -the many occurrences of the <tt>union</tt> keyword within that structure. - -<p> -This need for memory efficiency is one reason that RCU uses hand-crafted -singly linked lists to track the <tt>rcu_head</tt> structures that -are waiting for a grace period to elapse. -It is also the reason why <tt>rcu_head</tt> structures do not contain -debug information, such as fields tracking the file and line of the -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> that posted them. -Although this information might appear in debug-only kernel builds at some -point, in the meantime, the <tt>->func</tt> field will often provide -the needed debug information. - -<p> -However, in some cases, the need for memory efficiency leads to even -more extreme measures. -Returning to the <tt>page</tt> structure, the <tt>rcu_head</tt> field -shares storage with a great many other structures that are used at -various points in the corresponding page's lifetime. -In order to correctly resolve certain -<a href="https://lkml.kernel.org/g/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com">race conditions</a>, -the Linux kernel's memory-management subsystem needs a particular bit -to remain zero during all phases of grace-period processing, -and that bit happens to map to the bottom bit of the -<tt>rcu_head</tt> structure's <tt>->next</tt> field. -RCU makes this guarantee as long as <tt>call_rcu()</tt> -is used to post the callback, as opposed to <tt>kfree_rcu()</tt> -or some future “lazy” -variant of <tt>call_rcu()</tt> that might one day be created for -energy-efficiency purposes. - -<h3><a name="Performance, Scalability, Response Time, and Reliability"> -Performance, Scalability, Response Time, and Reliability</a></h3> - -<p> -Expanding on the -<a href="#Performance and Scalability">earlier discussion</a>, -RCU is used heavily by hot code paths in performance-critical -portions of the Linux kernel's networking, security, virtualization, -and scheduling code paths. -RCU must therefore use efficient implementations, especially in its -read-side primitives. -To that end, it would be good if preemptible RCU's implementation -of <tt>rcu_read_lock()</tt> could be inlined, however, doing -this requires resolving <tt>#include</tt> issues with the -<tt>task_struct</tt> structure. - -<p> -The Linux kernel supports hardware configurations with up to -4096 CPUs, which means that RCU must be extremely scalable. -Algorithms that involve frequent acquisitions of global locks or -frequent atomic operations on global variables simply cannot be -tolerated within the RCU implementation. -RCU therefore makes heavy use of a combining tree based on the -<tt>rcu_node</tt> structure. -RCU is required to tolerate all CPUs continuously invoking any -combination of RCU's runtime primitives with minimal per-operation -overhead. -In fact, in many cases, increasing load must <i>decrease</i> the -per-operation overhead, witness the batching optimizations for -<tt>synchronize_rcu()</tt>, <tt>call_rcu()</tt>, -<tt>synchronize_rcu_expedited()</tt>, and <tt>rcu_barrier()</tt>. -As a general rule, RCU must cheerfully accept whatever the -rest of the Linux kernel decides to throw at it. - -<p> -The Linux kernel is used for real-time workloads, especially -in conjunction with the -<a href="https://rt.wiki.kernel.org/index.php/Main_Page">-rt patchset</a>. -The real-time-latency response requirements are such that the -traditional approach of disabling preemption across RCU -read-side critical sections is inappropriate. -Kernels built with <tt>CONFIG_PREEMPT=y</tt> therefore -use an RCU implementation that allows RCU read-side critical -sections to be preempted. -This requirement made its presence known after users made it -clear that an earlier -<a href="https://lwn.net/Articles/107930/">real-time patch</a> -did not meet their needs, in conjunction with some -<a href="https://lkml.kernel.org/g/20050318002026.GA2693@us.ibm.com">RCU issues</a> -encountered by a very early version of the -rt patchset. - -<p> -In addition, RCU must make do with a sub-100-microsecond real-time latency -budget. -In fact, on smaller systems with the -rt patchset, the Linux kernel -provides sub-20-microsecond real-time latencies for the whole kernel, -including RCU. -RCU's scalability and latency must therefore be sufficient for -these sorts of configurations. -To my surprise, the sub-100-microsecond real-time latency budget -<a href="http://www.rdrop.com/users/paulmck/realtime/paper/bigrt.2013.01.31a.LCA.pdf"> -applies to even the largest systems [PDF]</a>, -up to and including systems with 4096 CPUs. -This real-time requirement motivated the grace-period kthread, which -also simplified handling of a number of race conditions. - -<p> -Finally, RCU's status as a synchronization primitive means that -any RCU failure can result in arbitrary memory corruption that can be -extremely difficult to debug. -This means that RCU must be extremely reliable, which in -practice also means that RCU must have an aggressive stress-test -suite. -This stress-test suite is called <tt>rcutorture</tt>. - -<p> -Although the need for <tt>rcutorture</tt> was no surprise, -the current immense popularity of the Linux kernel is posing -interesting—and perhaps unprecedented—validation -challenges. -To see this, keep in mind that there are well over one billion -instances of the Linux kernel running today, given Android -smartphones, Linux-powered televisions, and servers. -This number can be expected to increase sharply with the advent of -the celebrated Internet of Things. - -<p> -Suppose that RCU contains a race condition that manifests on average -once per million years of runtime. -This bug will be occurring about three times per <i>day</i> across -the installed base. -RCU could simply hide behind hardware error rates, given that no one -should really expect their smartphone to last for a million years. -However, anyone taking too much comfort from this thought should -consider the fact that in most jurisdictions, a successful multi-year -test of a given mechanism, which might include a Linux kernel, -suffices for a number of types of safety-critical certifications. -In fact, rumor has it that the Linux kernel is already being used -in production for safety-critical applications. -I don't know about you, but I would feel quite bad if a bug in RCU -killed someone. -Which might explain my recent focus on validation and verification. - -<h2><a name="Other RCU Flavors">Other RCU Flavors</a></h2> - -<p> -One of the more surprising things about RCU is that there are now -no fewer than five <i>flavors</i>, or API families. -In addition, the primary flavor that has been the sole focus up to -this point has two different implementations, non-preemptible and -preemptible. -The other four flavors are listed below, with requirements for each -described in a separate section. - -<ol> -<li> <a href="#Bottom-Half Flavor">Bottom-Half Flavor</a> -<li> <a href="#Sched Flavor">Sched Flavor</a> -<li> <a href="#Sleepable RCU">Sleepable RCU</a> -<li> <a href="#Tasks RCU">Tasks RCU</a> -</ol> - -<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3> - -<p> -The softirq-disable (AKA “bottom-half”, -hence the “_bh” abbreviations) -flavor of RCU, or <i>RCU-bh</i>, was developed by -Dipankar Sarma to provide a flavor of RCU that could withstand the -network-based denial-of-service attacks researched by Robert -Olsson. -These attacks placed so much networking load on the system -that some of the CPUs never exited softirq execution, -which in turn prevented those CPUs from ever executing a context switch, -which, in the RCU implementation of that time, prevented grace periods -from ever ending. -The result was an out-of-memory condition and a system hang. - -<p> -The solution was the creation of RCU-bh, which does -<tt>local_bh_disable()</tt> -across its read-side critical sections, and which uses the transition -from one type of softirq processing to another as a quiescent state -in addition to context switch, idle, user mode, and offline. -This means that RCU-bh grace periods can complete even when some of -the CPUs execute in softirq indefinitely, thus allowing algorithms -based on RCU-bh to withstand network-based denial-of-service attacks. - -<p> -Because -<tt>rcu_read_lock_bh()</tt> and <tt>rcu_read_unlock_bh()</tt> -disable and re-enable softirq handlers, any attempt to start a softirq -handlers during the -RCU-bh read-side critical section will be deferred. -In this case, <tt>rcu_read_unlock_bh()</tt> -will invoke softirq processing, which can take considerable time. -One can of course argue that this softirq overhead should be associated -with the code following the RCU-bh read-side critical section rather -than <tt>rcu_read_unlock_bh()</tt>, but the fact -is that most profiling tools cannot be expected to make this sort -of fine distinction. -For example, suppose that a three-millisecond-long RCU-bh read-side -critical section executes during a time of heavy networking load. -There will very likely be an attempt to invoke at least one softirq -handler during that three milliseconds, but any such invocation will -be delayed until the time of the <tt>rcu_read_unlock_bh()</tt>. -This can of course make it appear at first glance as if -<tt>rcu_read_unlock_bh()</tt> was executing very slowly. - -<p> -The -<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-bh API</a> -includes -<tt>rcu_read_lock_bh()</tt>, -<tt>rcu_read_unlock_bh()</tt>, -<tt>rcu_dereference_bh()</tt>, -<tt>rcu_dereference_bh_check()</tt>, -<tt>synchronize_rcu_bh()</tt>, -<tt>synchronize_rcu_bh_expedited()</tt>, -<tt>call_rcu_bh()</tt>, -<tt>rcu_barrier_bh()</tt>, and -<tt>rcu_read_lock_bh_held()</tt>. - -<h3><a name="Sched Flavor">Sched Flavor</a></h3> - -<p> -Before preemptible RCU, waiting for an RCU grace period had the -side effect of also waiting for all pre-existing interrupt -and NMI handlers. -However, there are legitimate preemptible-RCU implementations that -do not have this property, given that any point in the code outside -of an RCU read-side critical section can be a quiescent state. -Therefore, <i>RCU-sched</i> was created, which follows “classic” -RCU in that an RCU-sched grace period waits for for pre-existing -interrupt and NMI handlers. -In kernels built with <tt>CONFIG_PREEMPT=n</tt>, the RCU and RCU-sched -APIs have identical implementations, while kernels built with -<tt>CONFIG_PREEMPT=y</tt> provide a separate implementation for each. - -<p> -Note well that in <tt>CONFIG_PREEMPT=y</tt> kernels, -<tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt> -disable and re-enable preemption, respectively. -This means that if there was a preemption attempt during the -RCU-sched read-side critical section, <tt>rcu_read_unlock_sched()</tt> -will enter the scheduler, with all the latency and overhead entailed. -Just as with <tt>rcu_read_unlock_bh()</tt>, this can make it look -as if <tt>rcu_read_unlock_sched()</tt> was executing very slowly. -However, the highest-priority task won't be preempted, so that task -will enjoy low-overhead <tt>rcu_read_unlock_sched()</tt> invocations. - -<p> -The -<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-sched API</a> -includes -<tt>rcu_read_lock_sched()</tt>, -<tt>rcu_read_unlock_sched()</tt>, -<tt>rcu_read_lock_sched_notrace()</tt>, -<tt>rcu_read_unlock_sched_notrace()</tt>, -<tt>rcu_dereference_sched()</tt>, -<tt>rcu_dereference_sched_check()</tt>, -<tt>synchronize_sched()</tt>, -<tt>synchronize_rcu_sched_expedited()</tt>, -<tt>call_rcu_sched()</tt>, -<tt>rcu_barrier_sched()</tt>, and -<tt>rcu_read_lock_sched_held()</tt>. -However, anything that disables preemption also marks an RCU-sched -read-side critical section, including -<tt>preempt_disable()</tt> and <tt>preempt_enable()</tt>, -<tt>local_irq_save()</tt> and <tt>local_irq_restore()</tt>, -and so on. - -<h3><a name="Sleepable RCU">Sleepable RCU</a></h3> - -<p> -For well over a decade, someone saying “I need to block within -an RCU read-side critical section” was a reliable indication -that this someone did not understand RCU. -After all, if you are always blocking in an RCU read-side critical -section, you can probably afford to use a higher-overhead synchronization -mechanism. -However, that changed with the advent of the Linux kernel's notifiers, -whose RCU read-side critical -sections almost never sleep, but sometimes need to. -This resulted in the introduction of -<a href="https://lwn.net/Articles/202847/">sleepable RCU</a>, -or <i>SRCU</i>. - -<p> -SRCU allows different domains to be defined, with each such domain -defined by an instance of an <tt>srcu_struct</tt> structure. -A pointer to this structure must be passed in to each SRCU function, -for example, <tt>synchronize_srcu(&ss)</tt>, where -<tt>ss</tt> is the <tt>srcu_struct</tt> structure. -The key benefit of these domains is that a slow SRCU reader in one -domain does not delay an SRCU grace period in some other domain. -That said, one consequence of these domains is that read-side code -must pass a “cookie” from <tt>srcu_read_lock()</tt> -to <tt>srcu_read_unlock()</tt>, for example, as follows: - -<blockquote> -<pre> - 1 int idx; - 2 - 3 idx = srcu_read_lock(&ss); - 4 do_something(); - 5 srcu_read_unlock(&ss, idx); -</pre> -</blockquote> - -<p> -As noted above, it is legal to block within SRCU read-side critical sections, -however, with great power comes great responsibility. -If you block forever in one of a given domain's SRCU read-side critical -sections, then that domain's grace periods will also be blocked forever. -Of course, one good way to block forever is to deadlock, which can -happen if any operation in a given domain's SRCU read-side critical -section can block waiting, either directly or indirectly, for that domain's -grace period to elapse. -For example, this results in a self-deadlock: - -<blockquote> -<pre> - 1 int idx; - 2 - 3 idx = srcu_read_lock(&ss); - 4 do_something(); - 5 synchronize_srcu(&ss); - 6 srcu_read_unlock(&ss, idx); -</pre> -</blockquote> - -<p> -However, if line 5 acquired a mutex that was held across -a <tt>synchronize_srcu()</tt> for domain <tt>ss</tt>, -deadlock would still be possible. -Furthermore, if line 5 acquired a mutex that was held across -a <tt>synchronize_srcu()</tt> for some other domain <tt>ss1</tt>, -and if an <tt>ss1</tt>-domain SRCU read-side critical section -acquired another mutex that was held across as <tt>ss</tt>-domain -<tt>synchronize_srcu()</tt>, -deadlock would again be possible. -Such a deadlock cycle could extend across an arbitrarily large number -of different SRCU domains. -Again, with great power comes great responsibility. - -<p> -Unlike the other RCU flavors, SRCU read-side critical sections can -run on idle and even offline CPUs. -This ability requires that <tt>srcu_read_lock()</tt> and -<tt>srcu_read_unlock()</tt> contain memory barriers, which means -that SRCU readers will run a bit slower than would RCU readers. -It also motivates the <tt>smp_mb__after_srcu_read_unlock()</tt> -API, which, in combination with <tt>srcu_read_unlock()</tt>, -guarantees a full memory barrier. - -<p> -The -<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">SRCU API</a> -includes -<tt>srcu_read_lock()</tt>, -<tt>srcu_read_unlock()</tt>, -<tt>srcu_dereference()</tt>, -<tt>srcu_dereference_check()</tt>, -<tt>synchronize_srcu()</tt>, -<tt>synchronize_srcu_expedited()</tt>, -<tt>call_srcu()</tt>, -<tt>srcu_barrier()</tt>, and -<tt>srcu_read_lock_held()</tt>. -It also includes -<tt>DEFINE_SRCU()</tt>, -<tt>DEFINE_STATIC_SRCU()</tt>, and -<tt>init_srcu_struct()</tt> -APIs for defining and initializing <tt>srcu_struct</tt> structures. - -<h3><a name="Tasks RCU">Tasks RCU</a></h3> - -<p> -Some forms of tracing use “tramopolines” to handle the -binary rewriting required to install different types of probes. -It would be good to be able to free old trampolines, which sounds -like a job for some form of RCU. -However, because it is necessary to be able to install a trace -anywhere in the code, it is not possible to use read-side markers -such as <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>. -In addition, it does not work to have these markers in the trampoline -itself, because there would need to be instructions following -<tt>rcu_read_unlock()</tt>. -Although <tt>synchronize_rcu()</tt> would guarantee that execution -reached the <tt>rcu_read_unlock()</tt>, it would not be able to -guarantee that execution had completely left the trampoline. - -<p> -The solution, in the form of -<a href="https://lwn.net/Articles/607117/"><i>Tasks RCU</i></a>, -is to have implicit -read-side critical sections that are delimited by voluntary context -switches, that is, calls to <tt>schedule()</tt>, -<tt>cond_resched_rcu_qs()</tt>, and -<tt>synchronize_rcu_tasks()</tt>. -In addition, transitions to and from userspace execution also delimit -tasks-RCU read-side critical sections. - -<p> -The tasks-RCU API is quite compact, consisting only of -<tt>call_rcu_tasks()</tt>, -<tt>synchronize_rcu_tasks()</tt>, and -<tt>rcu_barrier_tasks()</tt>. - -<h2><a name="Possible Future Changes">Possible Future Changes</a></h2> - -<p> -One of the tricks that RCU uses to attain update-side scalability is -to increase grace-period latency with increasing numbers of CPUs. -If this becomes a serious problem, it will be necessary to rework the -grace-period state machine so as to avoid the need for the additional -latency. - -<p> -Expedited grace periods scan the CPUs, so their latency and overhead -increases with increasing numbers of CPUs. -If this becomes a serious problem on large systems, it will be necessary -to do some redesign to avoid this scalability problem. - -<p> -RCU disables CPU hotplug in a few places, perhaps most notably in the -expedited grace-period and <tt>rcu_barrier()</tt> operations. -If there is a strong reason to use expedited grace periods in CPU-hotplug -notifiers, it will be necessary to avoid disabling CPU hotplug. -This would introduce some complexity, so there had better be a <i>very</i> -good reason. - -<p> -The tradeoff between grace-period latency on the one hand and interruptions -of other CPUs on the other hand may need to be re-examined. -The desire is of course for zero grace-period latency as well as zero -interprocessor interrupts undertaken during an expedited grace period -operation. -While this ideal is unlikely to be achievable, it is quite possible that -further improvements can be made. - -<p> -The multiprocessor implementations of RCU use a combining tree that -groups CPUs so as to reduce lock contention and increase cache locality. -However, this combining tree does not spread its memory across NUMA -nodes nor does it align the CPU groups with hardware features such -as sockets or cores. -Such spreading and alignment is currently believed to be unnecessary -because the hotpath read-side primitives do not access the combining -tree, nor does <tt>call_rcu()</tt> in the common case. -If you believe that your architecture needs such spreading and alignment, -then your architecture should also benefit from the -<tt>rcutree.rcu_fanout_leaf</tt> boot parameter, which can be set -to the number of CPUs in a socket, NUMA node, or whatever. -If the number of CPUs is too large, use a fraction of the number of -CPUs. -If the number of CPUs is a large prime number, well, that certainly -is an “interesting” architectural choice! -More flexible arrangements might be considered, but only if -<tt>rcutree.rcu_fanout_leaf</tt> has proven inadequate, and only -if the inadequacy has been demonstrated by a carefully run and -realistic system-level workload. - -<p> -Please note that arrangements that require RCU to remap CPU numbers will -require extremely good demonstration of need and full exploration of -alternatives. - -<p> -There is an embarrassingly large number of flavors of RCU, and this -number has been increasing over time. -Perhaps it will be possible to combine some at some future date. - -<p> -RCU's various kthreads are reasonably recent additions. -It is quite likely that adjustments will be required to more gracefully -handle extreme loads. -It might also be necessary to be able to relate CPU utilization by -RCU's kthreads and softirq handlers to the code that instigated this -CPU utilization. -For example, RCU callback overhead might be charged back to the -originating <tt>call_rcu()</tt> instance, though probably not -in production kernels. - -<h2><a name="Summary">Summary</a></h2> - -<p> -This document has presented more than two decade's worth of RCU -requirements. -Given that the requirements keep changing, this will not be the last -word on this subject, but at least it serves to get an important -subset of the requirements set forth. - -<h2><a name="Acknowledgments">Acknowledgments</a></h2> - -I am grateful to Steven Rostedt, Lai Jiangshan, Ingo Molnar, -Oleg Nesterov, Borislav Petkov, Peter Zijlstra, Boqun Feng, and -Andy Lutomirski for their help in rendering -this article human readable, and to Michelle Rankin for her support -of this effort. -Other contributions are acknowledged in the Linux kernel's git archive. -The cartoon is copyright (c) 2013 by Melissa Broussard, -and is provided -under the terms of the Creative Commons Attribution-Share Alike 3.0 -United States license. - -<p>@@QQAL@@ - -</body></html> diff --git a/Documentation/RCU/Design/htmlqqz.sh b/Documentation/RCU/Design/htmlqqz.sh deleted file mode 100755 index d354f069559b..000000000000 --- a/Documentation/RCU/Design/htmlqqz.sh +++ /dev/null @@ -1,108 +0,0 @@ -#!/bin/sh -# -# Usage: sh htmlqqz.sh file -# -# Extracts and converts quick quizzes in a proto-HTML document file.htmlx. -# Commands, all of which must be on a line by themselves: -# -# "<p>@@QQ@@": Start of a quick quiz. -# "<p>@@QQA@@": Start of a quick-quiz answer. -# "<p>@@QQE@@": End of a quick-quiz answer, and thus of the quick quiz. -# "<p>@@QQAL@@": Place to put quick-quiz answer list. -# -# Places the result in file.html. -# -# This program is free software; you can redistribute it and/or modify -# it under the terms of the GNU General Public License as published by -# the Free Software Foundation; either version 2 of the License, or -# (at your option) any later version. -# -# This program is distributed in the hope that it will be useful, -# but WITHOUT ANY WARRANTY; without even the implied warranty of -# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the -# GNU General Public License for more details. -# -# You should have received a copy of the GNU General Public License -# along with this program; if not, you can access it online at -# http://www.gnu.org/licenses/gpl-2.0.html. -# -# Copyright (c) 2013 Paul E. McKenney, IBM Corporation. - -fn=$1 -if test ! -r $fn.htmlx -then - echo "Error: $fn.htmlx unreadable." - exit 1 -fi - -echo "<!-- DO NOT HAND EDIT. -->" > $fn.html -echo "<!-- Instead, edit $fn.htmlx and run 'sh htmlqqz.sh $fn' -->" >> $fn.html -awk < $fn.htmlx >> $fn.html ' - -state == "" && $1 != "<p>@@QQ@@" && $1 != "<p>@@QQAL@@" { - print $0; - if ($0 ~ /^<p>@@QQ/) - print "Bad Quick Quiz command: " NR " (expected <p>@@QQ@@ or <p>@@QQAL@@)." > "/dev/stderr" - next; -} - -state == "" && $1 == "<p>@@QQ@@" { - qqn++; - qqlineno = NR; - haveqq = 1; - state = "qq"; - print "<p><a name=\"Quick Quiz " qqn "\"><b>Quick Quiz " qqn "</b>:</a>" - next; -} - -state == "qq" && $1 != "<p>@@QQA@@" { - qq[qqn] = qq[qqn] $0 "\n"; - print $0 - if ($0 ~ /^<p>@@QQ/) - print "Bad Quick Quiz command: " NR ". (expected <p>@@QQA@@)" > "/dev/stderr" - next; -} - -state == "qq" && $1 == "<p>@@QQA@@" { - state = "qqa"; - print "<br><a href=\"#qq" qqn "answer\">Answer</a>" - next; -} - -state == "qqa" && $1 != "<p>@@QQE@@" { - qqa[qqn] = qqa[qqn] $0 "\n"; - if ($0 ~ /^<p>@@QQ/) - print "Bad Quick Quiz command: " NR " (expected <p>@@QQE@@)." > "/dev/stderr" - next; -} - -state == "qqa" && $1 == "<p>@@QQE@@" { - state = ""; - next; -} - -state == "" && $1 == "<p>@@QQAL@@" { - haveqq = ""; - print "<h3><a name=\"Answers to Quick Quizzes\">" - print "Answers to Quick Quizzes</a></h3>" - print ""; - for (i = 1; i <= qqn; i++) { - print "<a name=\"qq" i "answer\"></a>" - print "<p><b>Quick Quiz " i "</b>:" - print qq[i]; - print ""; - print "</p><p><b>Answer</b>:" - print qqa[i]; - print ""; - print "</p><p><a href=\"#Quick%20Quiz%20" i "\"><b>Back to Quick Quiz " i "</b>.</a>" - print ""; - } - next; -} - -END { - if (state != "") - print "Unterminated Quick Quiz: " qqlineno "." > "/dev/stderr" - else if (haveqq) - print "Missing \"<p>@@QQAL@@\", no Quick Quiz." > "/dev/stderr" -}' |
