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diff --git a/Documentation/locking/crossrelease.txt b/Documentation/locking/crossrelease.txt deleted file mode 100644 index bdf1423d5f99..000000000000 --- a/Documentation/locking/crossrelease.txt +++ /dev/null @@ -1,874 +0,0 @@ -Crossrelease -============ - -Started by Byungchul Park <byungchul.park@lge.com> - -Contents: - - (*) Background - - - What causes deadlock - - How lockdep works - - (*) Limitation - - - Limit lockdep - - Pros from the limitation - - Cons from the limitation - - Relax the limitation - - (*) Crossrelease - - - Introduce crossrelease - - Introduce commit - - (*) Implementation - - - Data structures - - How crossrelease works - - (*) Optimizations - - - Avoid duplication - - Lockless for hot paths - - (*) APPENDIX A: What lockdep does to work aggresively - - (*) APPENDIX B: How to avoid adding false dependencies - - -========== -Background -========== - -What causes deadlock --------------------- - -A deadlock occurs when a context is waiting for an event to happen, -which is impossible because another (or the) context who can trigger the -event is also waiting for another (or the) event to happen, which is -also impossible due to the same reason. - -For example: - - A context going to trigger event C is waiting for event A to happen. - A context going to trigger event A is waiting for event B to happen. - A context going to trigger event B is waiting for event C to happen. - -A deadlock occurs when these three wait operations run at the same time, -because event C cannot be triggered if event A does not happen, which in -turn cannot be triggered if event B does not happen, which in turn -cannot be triggered if event C does not happen. After all, no event can -be triggered since any of them never meets its condition to wake up. - -A dependency might exist between two waiters and a deadlock might happen -due to an incorrect releationship between dependencies. Thus, we must -define what a dependency is first. A dependency exists between them if: - - 1. There are two waiters waiting for each event at a given time. - 2. The only way to wake up each waiter is to trigger its event. - 3. Whether one can be woken up depends on whether the other can. - -Each wait in the example creates its dependency like: - - Event C depends on event A. - Event A depends on event B. - Event B depends on event C. - - NOTE: Precisely speaking, a dependency is one between whether a - waiter for an event can be woken up and whether another waiter for - another event can be woken up. However from now on, we will describe - a dependency as if it's one between an event and another event for - simplicity. - -And they form circular dependencies like: - - -> C -> A -> B - - / \ - \ / - ---------------- - - where 'A -> B' means that event A depends on event B. - -Such circular dependencies lead to a deadlock since no waiter can meet -its condition to wake up as described. - -CONCLUSION - -Circular dependencies cause a deadlock. - - -How lockdep works ------------------ - -Lockdep tries to detect a deadlock by checking dependencies created by -lock operations, acquire and release. Waiting for a lock corresponds to -waiting for an event, and releasing a lock corresponds to triggering an -event in the previous section. - -In short, lockdep does: - - 1. Detect a new dependency. - 2. Add the dependency into a global graph. - 3. Check if that makes dependencies circular. - 4. Report a deadlock or its possibility if so. - -For example, consider a graph built by lockdep that looks like: - - A -> B - - \ - -> E - / - C -> D - - - where A, B,..., E are different lock classes. - -Lockdep will add a dependency into the graph on detection of a new -dependency. For example, it will add a dependency 'E -> C' when a new -dependency between lock E and lock C is detected. Then the graph will be: - - A -> B - - \ - -> E - - / \ - -> C -> D - \ - / / - \ / - ------------------ - - where A, B,..., E are different lock classes. - -This graph contains a subgraph which demonstrates circular dependencies: - - -> E - - / \ - -> C -> D - \ - / / - \ / - ------------------ - - where C, D and E are different lock classes. - -This is the condition under which a deadlock might occur. Lockdep -reports it on detection after adding a new dependency. This is the way -how lockdep works. - -CONCLUSION - -Lockdep detects a deadlock or its possibility by checking if circular -dependencies were created after adding each new dependency. - - -========== -Limitation -========== - -Limit lockdep -------------- - -Limiting lockdep to work on only typical locks e.g. spin locks and -mutexes, which are released within the acquire context, the -implementation becomes simple but its capacity for detection becomes -limited. Let's check pros and cons in next section. - - -Pros from the limitation ------------------------- - -Given the limitation, when acquiring a lock, locks in a held_locks -cannot be released if the context cannot acquire it so has to wait to -acquire it, which means all waiters for the locks in the held_locks are -stuck. It's an exact case to create dependencies between each lock in -the held_locks and the lock to acquire. - -For example: - - CONTEXT X - --------- - acquire A - acquire B /* Add a dependency 'A -> B' */ - release B - release A - - where A and B are different lock classes. - -When acquiring lock A, the held_locks of CONTEXT X is empty thus no -dependency is added. But when acquiring lock B, lockdep detects and adds -a new dependency 'A -> B' between lock A in the held_locks and lock B. -They can be simply added whenever acquiring each lock. - -And data required by lockdep exists in a local structure, held_locks -embedded in task_struct. Forcing to access the data within the context, -lockdep can avoid racy problems without explicit locks while handling -the local data. - -Lastly, lockdep only needs to keep locks currently being held, to build -a dependency graph. However, relaxing the limitation, it needs to keep -even locks already released, because a decision whether they created -dependencies might be long-deferred. - -To sum up, we can expect several advantages from the limitation: - - 1. Lockdep can easily identify a dependency when acquiring a lock. - 2. Races are avoidable while accessing local locks in a held_locks. - 3. Lockdep only needs to keep locks currently being held. - -CONCLUSION - -Given the limitation, the implementation becomes simple and efficient. - - -Cons from the limitation ------------------------- - -Given the limitation, lockdep is applicable only to typical locks. For -example, page locks for page access or completions for synchronization -cannot work with lockdep. - -Can we detect deadlocks below, under the limitation? - -Example 1: - - CONTEXT X CONTEXT Y CONTEXT Z - --------- --------- ---------- - mutex_lock A - lock_page B - lock_page B - mutex_lock A /* DEADLOCK */ - unlock_page B held by X - unlock_page B - mutex_unlock A - mutex_unlock A - - where A and B are different lock classes. - -No, we cannot. - -Example 2: - - CONTEXT X CONTEXT Y - --------- --------- - mutex_lock A - mutex_lock A - wait_for_complete B /* DEADLOCK */ - complete B - mutex_unlock A - mutex_unlock A - - where A is a lock class and B is a completion variable. - -No, we cannot. - -CONCLUSION - -Given the limitation, lockdep cannot detect a deadlock or its -possibility caused by page locks or completions. - - -Relax the limitation --------------------- - -Under the limitation, things to create dependencies are limited to -typical locks. However, synchronization primitives like page locks and -completions, which are allowed to be released in any context, also -create dependencies and can cause a deadlock. So lockdep should track -these locks to do a better job. We have to relax the limitation for -these locks to work with lockdep. - -Detecting dependencies is very important for lockdep to work because -adding a dependency means adding an opportunity to check whether it -causes a deadlock. The more lockdep adds dependencies, the more it -thoroughly works. Thus Lockdep has to do its best to detect and add as -many true dependencies into a graph as possible. - -For example, considering only typical locks, lockdep builds a graph like: - - A -> B - - \ - -> E - / - C -> D - - - where A, B,..., E are different lock classes. - -On the other hand, under the relaxation, additional dependencies might -be created and added. Assuming additional 'FX -> C' and 'E -> GX' are -added thanks to the relaxation, the graph will be: - - A -> B - - \ - -> E -> GX - / - FX -> C -> D - - - where A, B,..., E, FX and GX are different lock classes, and a suffix - 'X' is added on non-typical locks. - -The latter graph gives us more chances to check circular dependencies -than the former. However, it might suffer performance degradation since -relaxing the limitation, with which design and implementation of lockdep -can be efficient, might introduce inefficiency inevitably. So lockdep -should provide two options, strong detection and efficient detection. - -Choosing efficient detection: - - Lockdep works with only locks restricted to be released within the - acquire context. However, lockdep works efficiently. - -Choosing strong detection: - - Lockdep works with all synchronization primitives. However, lockdep - suffers performance degradation. - -CONCLUSION - -Relaxing the limitation, lockdep can add additional dependencies giving -additional opportunities to check circular dependencies. - - -============ -Crossrelease -============ - -Introduce crossrelease ----------------------- - -In order to allow lockdep to handle additional dependencies by what -might be released in any context, namely 'crosslock', we have to be able -to identify those created by crosslocks. The proposed 'crossrelease' -feature provoides a way to do that. - -Crossrelease feature has to do: - - 1. Identify dependencies created by crosslocks. - 2. Add the dependencies into a dependency graph. - -That's all. Once a meaningful dependency is added into graph, then -lockdep would work with the graph as it did. The most important thing -crossrelease feature has to do is to correctly identify and add true -dependencies into the global graph. - -A dependency e.g. 'A -> B' can be identified only in the A's release -context because a decision required to identify the dependency can be -made only in the release context. That is to decide whether A can be -released so that a waiter for A can be woken up. It cannot be made in -other than the A's release context. - -It's no matter for typical locks because each acquire context is same as -its release context, thus lockdep can decide whether a lock can be -released in the acquire context. However for crosslocks, lockdep cannot -make the decision in the acquire context but has to wait until the -release context is identified. - -Therefore, deadlocks by crosslocks cannot be detected just when it -happens, because those cannot be identified until the crosslocks are -released. However, deadlock possibilities can be detected and it's very -worth. See 'APPENDIX A' section to check why. - -CONCLUSION - -Using crossrelease feature, lockdep can work with what might be released -in any context, namely crosslock. - - -Introduce commit ----------------- - -Since crossrelease defers the work adding true dependencies of -crosslocks until they are actually released, crossrelease has to queue -all acquisitions which might create dependencies with the crosslocks. -Then it identifies dependencies using the queued data in batches at a -proper time. We call it 'commit'. - -There are four types of dependencies: - -1. TT type: 'typical lock A -> typical lock B' - - Just when acquiring B, lockdep can see it's in the A's release - context. So the dependency between A and B can be identified - immediately. Commit is unnecessary. - -2. TC type: 'typical lock A -> crosslock BX' - - Just when acquiring BX, lockdep can see it's in the A's release - context. So the dependency between A and BX can be identified - immediately. Commit is unnecessary, too. - -3. CT type: 'crosslock AX -> typical lock B' - - When acquiring B, lockdep cannot identify the dependency because - there's no way to know if it's in the AX's release context. It has - to wait until the decision can be made. Commit is necessary. - -4. CC type: 'crosslock AX -> crosslock BX' - - When acquiring BX, lockdep cannot identify the dependency because - there's no way to know if it's in the AX's release context. It has - to wait until the decision can be made. Commit is necessary. - But, handling CC type is not implemented yet. It's a future work. - -Lockdep can work without commit for typical locks, but commit step is -necessary once crosslocks are involved. Introducing commit, lockdep -performs three steps. What lockdep does in each step is: - -1. Acquisition: For typical locks, lockdep does what it originally did - and queues the lock so that CT type dependencies can be checked using - it at the commit step. For crosslocks, it saves data which will be - used at the commit step and increases a reference count for it. - -2. Commit: No action is reauired for typical locks. For crosslocks, - lockdep adds CT type dependencies using the data saved at the - acquisition step. - -3. Release: No changes are required for typical locks. When a crosslock - is released, it decreases a reference count for it. - -CONCLUSION - -Crossrelease introduces commit step to handle dependencies of crosslocks -in batches at a proper time. - - -============== -Implementation -============== - -Data structures ---------------- - -Crossrelease introduces two main data structures. - -1. hist_lock - - This is an array embedded in task_struct, for keeping lock history so - that dependencies can be added using them at the commit step. Since - it's local data, it can be accessed locklessly in the owner context. - The array is filled at the acquisition step and consumed at the - commit step. And it's managed in circular manner. - -2. cross_lock - - One per lockdep_map exists. This is for keeping data of crosslocks - and used at the commit step. - - -How crossrelease works ----------------------- - -It's the key of how crossrelease works, to defer necessary works to an -appropriate point in time and perform in at once at the commit step. -Let's take a look with examples step by step, starting from how lockdep -works without crossrelease for typical locks. - - acquire A /* Push A onto held_locks */ - acquire B /* Push B onto held_locks and add 'A -> B' */ - acquire C /* Push C onto held_locks and add 'B -> C' */ - release C /* Pop C from held_locks */ - release B /* Pop B from held_locks */ - release A /* Pop A from held_locks */ - - where A, B and C are different lock classes. - - NOTE: This document assumes that readers already understand how - lockdep works without crossrelease thus omits details. But there's - one thing to note. Lockdep pretends to pop a lock from held_locks - when releasing it. But it's subtly different from the original pop - operation because lockdep allows other than the top to be poped. - -In this case, lockdep adds 'the top of held_locks -> the lock to acquire' -dependency every time acquiring a lock. - -After adding 'A -> B', a dependency graph will be: - - A -> B - - where A and B are different lock classes. - -And after adding 'B -> C', the graph will be: - - A -> B -> C - - where A, B and C are different lock classes. - -Let's performs commit step even for typical locks to add dependencies. -Of course, commit step is not necessary for them, however, it would work -well because this is a more general way. - - acquire A - /* - * Queue A into hist_locks - * - * In hist_locks: A - * In graph: Empty - */ - - acquire B - /* - * Queue B into hist_locks - * - * In hist_locks: A, B - * In graph: Empty - */ - - acquire C - /* - * Queue C into hist_locks - * - * In hist_locks: A, B, C - * In graph: Empty - */ - - commit C - /* - * Add 'C -> ?' - * Answer the following to decide '?' - * What has been queued since acquire C: Nothing - * - * In hist_locks: A, B, C - * In graph: Empty - */ - - release C - - commit B - /* - * Add 'B -> ?' - * Answer the following to decide '?' - * What has been queued since acquire B: C - * - * In hist_locks: A, B, C - * In graph: 'B -> C' - */ - - release B - - commit A - /* - * Add 'A -> ?' - * Answer the following to decide '?' - * What has been queued since acquire A: B, C - * - * In hist_locks: A, B, C - * In graph: 'B -> C', 'A -> B', 'A -> C' - */ - - release A - - where A, B and C are different lock classes. - -In this case, dependencies are added at the commit step as described. - -After commits for A, B and C, the graph will be: - - A -> B -> C - - where A, B and C are different lock classes. - - NOTE: A dependency 'A -> C' is optimized out. - -We can see the former graph built without commit step is same as the -latter graph built using commit steps. Of course the former way leads to -earlier finish for building the graph, which means we can detect a -deadlock or its possibility sooner. So the former way would be prefered -when possible. But we cannot avoid using the latter way for crosslocks. - -Let's look at how commit steps work for crosslocks. In this case, the -commit step is performed only on crosslock AX as real. And it assumes -that the AX release context is different from the AX acquire context. - - BX RELEASE CONTEXT BX ACQUIRE CONTEXT - ------------------ ------------------ - acquire A - /* - * Push A onto held_locks - * Queue A into hist_locks - * - * In held_locks: A - * In hist_locks: A - * In graph: Empty - */ - - acquire BX - /* - * Add 'the top of held_locks -> BX' - * - * In held_locks: A - * In hist_locks: A - * In graph: 'A -> BX' - */ - - ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - It must be guaranteed that the following operations are seen after - acquiring BX globally. It can be done by things like barrier. - ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - - acquire C - /* - * Push C onto held_locks - * Queue C into hist_locks - * - * In held_locks: C - * In hist_locks: C - * In graph: 'A -> BX' - */ - - release C - /* - * Pop C from held_locks - * - * In held_locks: Empty - * In hist_locks: C - * In graph: 'A -> BX' - */ - acquire D - /* - * Push D onto held_locks - * Queue D into hist_locks - * Add 'the top of held_locks -> D' - * - * In held_locks: A, D - * In hist_locks: A, D - * In graph: 'A -> BX', 'A -> D' - */ - acquire E - /* - * Push E onto held_locks - * Queue E into hist_locks - * - * In held_locks: E - * In hist_locks: C, E - * In graph: 'A -> BX', 'A -> D' - */ - - release E - /* - * Pop E from held_locks - * - * In held_locks: Empty - * In hist_locks: D, E - * In graph: 'A -> BX', 'A -> D' - */ - release D - /* - * Pop D from held_locks - * - * In held_locks: A - * In hist_locks: A, D - * In graph: 'A -> BX', 'A -> D' - */ - commit BX - /* - * Add 'BX -> ?' - * What has been queued since acquire BX: C, E - * - * In held_locks: Empty - * In hist_locks: D, E - * In graph: 'A -> BX', 'A -> D', - * 'BX -> C', 'BX -> E' - */ - - release BX - /* - * In held_locks: Empty - * In hist_locks: D, E - * In graph: 'A -> BX', 'A -> D', - * 'BX -> C', 'BX -> E' - */ - release A - /* - * Pop A from held_locks - * - * In held_locks: Empty - * In hist_locks: A, D - * In graph: 'A -> BX', 'A -> D', - * 'BX -> C', 'BX -> E' - */ - - where A, BX, C,..., E are different lock classes, and a suffix 'X' is - added on crosslocks. - -Crossrelease considers all acquisitions after acqiuring BX are -candidates which might create dependencies with BX. True dependencies -will be determined when identifying the release context of BX. Meanwhile, -all typical locks are queued so that they can be used at the commit step. -And then two dependencies 'BX -> C' and 'BX -> E' are added at the -commit step when identifying the release context. - -The final graph will be, with crossrelease: - - -> C - / - -> BX - - / \ - A - -> E - \ - -> D - - where A, BX, C,..., E are different lock classes, and a suffix 'X' is - added on crosslocks. - -However, the final graph will be, without crossrelease: - - A -> D - - where A and D are different lock classes. - -The former graph has three more dependencies, 'A -> BX', 'BX -> C' and -'BX -> E' giving additional opportunities to check if they cause -deadlocks. This way lockdep can detect a deadlock or its possibility -caused by crosslocks. - -CONCLUSION - -We checked how crossrelease works with several examples. - - -============= -Optimizations -============= - -Avoid duplication ------------------ - -Crossrelease feature uses a cache like what lockdep already uses for -dependency chains, but this time it's for caching CT type dependencies. -Once that dependency is cached, the same will never be added again. - - -Lockless for hot paths ----------------------- - -To keep all locks for later use at the commit step, crossrelease adopts -a local array embedded in task_struct, which makes access to the data -lockless by forcing it to happen only within the owner context. It's -like how lockdep handles held_locks. Lockless implmentation is important -since typical locks are very frequently acquired and released. - - -================================================= -APPENDIX A: What lockdep does to work aggresively -================================================= - -A deadlock actually occurs when all wait operations creating circular -dependencies run at the same time. Even though they don't, a potential -deadlock exists if the problematic dependencies exist. Thus it's -meaningful to detect not only an actual deadlock but also its potential -possibility. The latter is rather valuable. When a deadlock occurs -actually, we can identify what happens in the system by some means or -other even without lockdep. However, there's no way to detect possiblity -without lockdep unless the whole code is parsed in head. It's terrible. -Lockdep does the both, and crossrelease only focuses on the latter. - -Whether or not a deadlock actually occurs depends on several factors. -For example, what order contexts are switched in is a factor. Assuming -circular dependencies exist, a deadlock would occur when contexts are -switched so that all wait operations creating the dependencies run -simultaneously. Thus to detect a deadlock possibility even in the case -that it has not occured yet, lockdep should consider all possible -combinations of dependencies, trying to: - -1. Use a global dependency graph. - - Lockdep combines all dependencies into one global graph and uses them, - regardless of which context generates them or what order contexts are - switched in. Aggregated dependencies are only considered so they are - prone to be circular if a problem exists. - -2. Check dependencies between classes instead of instances. - - What actually causes a deadlock are instances of lock. However, - lockdep checks dependencies between classes instead of instances. - This way lockdep can detect a deadlock which has not happened but - might happen in future by others but the same class. - -3. Assume all acquisitions lead to waiting. - - Although locks might be acquired without waiting which is essential - to create dependencies, lockdep assumes all acquisitions lead to - waiting since it might be true some time or another. - -CONCLUSION - -Lockdep detects not only an actual deadlock but also its possibility, -and the latter is more valuable. - - -================================================== -APPENDIX B: How to avoid adding false dependencies -================================================== - -Remind what a dependency is. A dependency exists if: - - 1. There are two waiters waiting for each event at a given time. - 2. The only way to wake up each waiter is to trigger its event. - 3. Whether one can be woken up depends on whether the other can. - -For example: - - acquire A - acquire B /* A dependency 'A -> B' exists */ - release B - release A - - where A and B are different lock classes. - -A depedency 'A -> B' exists since: - - 1. A waiter for A and a waiter for B might exist when acquiring B. - 2. Only way to wake up each is to release what it waits for. - 3. Whether the waiter for A can be woken up depends on whether the - other can. IOW, TASK X cannot release A if it fails to acquire B. - -For another example: - - TASK X TASK Y - ------ ------ - acquire AX - acquire B /* A dependency 'AX -> B' exists */ - release B - release AX held by Y - - where AX and B are different lock classes, and a suffix 'X' is added - on crosslocks. - -Even in this case involving crosslocks, the same rule can be applied. A -depedency 'AX -> B' exists since: - - 1. A waiter for AX and a waiter for B might exist when acquiring B. - 2. Only way to wake up each is to release what it waits for. - 3. Whether the waiter for AX can be woken up depends on whether the - other can. IOW, TASK X cannot release AX if it fails to acquire B. - -Let's take a look at more complicated example: - - TASK X TASK Y - ------ ------ - acquire B - release B - fork Y - acquire AX - acquire C /* A dependency 'AX -> C' exists */ - release C - release AX held by Y - - where AX, B and C are different lock classes, and a suffix 'X' is - added on crosslocks. - -Does a dependency 'AX -> B' exist? Nope. - -Two waiters are essential to create a dependency. However, waiters for -AX and B to create 'AX -> B' cannot exist at the same time in this -example. Thus the dependency 'AX -> B' cannot be created. - -It would be ideal if the full set of true ones can be considered. But -we can ensure nothing but what actually happened. Relying on what -actually happens at runtime, we can anyway add only true ones, though -they might be a subset of true ones. It's similar to how lockdep works -for typical locks. There might be more true dependencies than what -lockdep has detected in runtime. Lockdep has no choice but to rely on -what actually happens. Crossrelease also relies on it. - -CONCLUSION - -Relying on what actually happens, lockdep can avoid adding false -dependencies. |
