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diff --git a/Documentation/pi-futex.txt b/Documentation/pi-futex.txt new file mode 100644 index 000000000000..5d61dacd21f6 --- /dev/null +++ b/Documentation/pi-futex.txt @@ -0,0 +1,121 @@ +Lightweight PI-futexes +---------------------- + +We are calling them lightweight for 3 reasons: + + - in the user-space fastpath a PI-enabled futex involves no kernel work + (or any other PI complexity) at all. No registration, no extra kernel + calls - just pure fast atomic ops in userspace. + + - even in the slowpath, the system call and scheduling pattern is very + similar to normal futexes. + + - the in-kernel PI implementation is streamlined around the mutex + abstraction, with strict rules that keep the implementation + relatively simple: only a single owner may own a lock (i.e. no + read-write lock support), only the owner may unlock a lock, no + recursive locking, etc. + +Priority Inheritance - why? +--------------------------- + +The short reply: user-space PI helps achieving/improving determinism for +user-space applications. In the best-case, it can help achieve +determinism and well-bound latencies. Even in the worst-case, PI will +improve the statistical distribution of locking related application +delays. + +The longer reply: +----------------- + +Firstly, sharing locks between multiple tasks is a common programming +technique that often cannot be replaced with lockless algorithms. As we +can see it in the kernel [which is a quite complex program in itself], +lockless structures are rather the exception than the norm - the current +ratio of lockless vs. locky code for shared data structures is somewhere +between 1:10 and 1:100. Lockless is hard, and the complexity of lockless +algorithms often endangers to ability to do robust reviews of said code. +I.e. critical RT apps often choose lock structures to protect critical +data structures, instead of lockless algorithms. Furthermore, there are +cases (like shared hardware, or other resource limits) where lockless +access is mathematically impossible. + +Media players (such as Jack) are an example of reasonable application +design with multiple tasks (with multiple priority levels) sharing +short-held locks: for example, a highprio audio playback thread is +combined with medium-prio construct-audio-data threads and low-prio +display-colory-stuff threads. Add video and decoding to the mix and +we've got even more priority levels. + +So once we accept that synchronization objects (locks) are an +unavoidable fact of life, and once we accept that multi-task userspace +apps have a very fair expectation of being able to use locks, we've got +to think about how to offer the option of a deterministic locking +implementation to user-space. + +Most of the technical counter-arguments against doing priority +inheritance only apply to kernel-space locks. But user-space locks are +different, there we cannot disable interrupts or make the task +non-preemptible in a critical section, so the 'use spinlocks' argument +does not apply (user-space spinlocks have the same priority inversion +problems as other user-space locking constructs). Fact is, pretty much +the only technique that currently enables good determinism for userspace +locks (such as futex-based pthread mutexes) is priority inheritance: + +Currently (without PI), if a high-prio and a low-prio task shares a lock +[this is a quite common scenario for most non-trivial RT applications], +even if all critical sections are coded carefully to be deterministic +(i.e. all critical sections are short in duration and only execute a +limited number of instructions), the kernel cannot guarantee any +deterministic execution of the high-prio task: any medium-priority task +could preempt the low-prio task while it holds the shared lock and +executes the critical section, and could delay it indefinitely. + +Implementation: +--------------- + +As mentioned before, the userspace fastpath of PI-enabled pthread +mutexes involves no kernel work at all - they behave quite similarly to +normal futex-based locks: a 0 value means unlocked, and a value==TID +means locked. (This is the same method as used by list-based robust +futexes.) Userspace uses atomic ops to lock/unlock these mutexes without +entering the kernel. + +To handle the slowpath, we have added two new futex ops: + + FUTEX_LOCK_PI + FUTEX_UNLOCK_PI + +If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to +TID fails], then FUTEX_LOCK_PI is called. The kernel does all the +remaining work: if there is no futex-queue attached to the futex address +yet then the code looks up the task that owns the futex [it has put its +own TID into the futex value], and attaches a 'PI state' structure to +the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware, +kernel-based synchronization object. The 'other' task is made the owner +of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the +futex value. Then this task tries to lock the rt-mutex, on which it +blocks. Once it returns, it has the mutex acquired, and it sets the +futex value to its own TID and returns. Userspace has no other work to +perform - it now owns the lock, and futex value contains +FUTEX_WAITERS|TID. + +If the unlock side fastpath succeeds, [i.e. userspace manages to do a +TID -> 0 atomic transition of the futex value], then no kernel work is +triggered. + +If the unlock fastpath fails (because the FUTEX_WAITERS bit is set), +then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the +behalf of userspace - and it also unlocks the attached +pi_state->rt_mutex and thus wakes up any potential waiters. + +Note that under this approach, contrary to previous PI-futex approaches, +there is no prior 'registration' of a PI-futex. [which is not quite +possible anyway, due to existing ABI properties of pthread mutexes.] + +Also, under this scheme, 'robustness' and 'PI' are two orthogonal +properties of futexes, and all four combinations are possible: futex, +robust-futex, PI-futex, robust+PI-futex. + +More details about priority inheritance can be found in +Documentation/rtmutex.txt. |