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-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/rt-mutex.txt.