Plain Text | 121 lines | 99 code | 22 blank | 0 comment | 0 complexity | 492ac92f69716bfaa0c40a909c992ade MD5 | raw file
Possible License(s): GPL-2.0, LGPL-2.0, AGPL-1.0
1Lightweight PI-futexes 2---------------------- 3 4We are calling them lightweight for 3 reasons: 5 6 - in the user-space fastpath a PI-enabled futex involves no kernel work 7 (or any other PI complexity) at all. No registration, no extra kernel 8 calls - just pure fast atomic ops in userspace. 9 10 - even in the slowpath, the system call and scheduling pattern is very 11 similar to normal futexes. 12 13 - the in-kernel PI implementation is streamlined around the mutex 14 abstraction, with strict rules that keep the implementation 15 relatively simple: only a single owner may own a lock (i.e. no 16 read-write lock support), only the owner may unlock a lock, no 17 recursive locking, etc. 18 19Priority Inheritance - why? 20--------------------------- 21 22The short reply: user-space PI helps achieving/improving determinism for 23user-space applications. In the best-case, it can help achieve 24determinism and well-bound latencies. Even in the worst-case, PI will 25improve the statistical distribution of locking related application 26delays. 27 28The longer reply: 29----------------- 30 31Firstly, sharing locks between multiple tasks is a common programming 32technique that often cannot be replaced with lockless algorithms. As we 33can see it in the kernel [which is a quite complex program in itself], 34lockless structures are rather the exception than the norm - the current 35ratio of lockless vs. locky code for shared data structures is somewhere 36between 1:10 and 1:100. Lockless is hard, and the complexity of lockless 37algorithms often endangers to ability to do robust reviews of said code. 38I.e. critical RT apps often choose lock structures to protect critical 39data structures, instead of lockless algorithms. Furthermore, there are 40cases (like shared hardware, or other resource limits) where lockless 41access is mathematically impossible. 42 43Media players (such as Jack) are an example of reasonable application 44design with multiple tasks (with multiple priority levels) sharing 45short-held locks: for example, a highprio audio playback thread is 46combined with medium-prio construct-audio-data threads and low-prio 47display-colory-stuff threads. Add video and decoding to the mix and 48we've got even more priority levels. 49 50So once we accept that synchronization objects (locks) are an 51unavoidable fact of life, and once we accept that multi-task userspace 52apps have a very fair expectation of being able to use locks, we've got 53to think about how to offer the option of a deterministic locking 54implementation to user-space. 55 56Most of the technical counter-arguments against doing priority 57inheritance only apply to kernel-space locks. But user-space locks are 58different, there we cannot disable interrupts or make the task 59non-preemptible in a critical section, so the 'use spinlocks' argument 60does not apply (user-space spinlocks have the same priority inversion 61problems as other user-space locking constructs). Fact is, pretty much 62the only technique that currently enables good determinism for userspace 63locks (such as futex-based pthread mutexes) is priority inheritance: 64 65Currently (without PI), if a high-prio and a low-prio task shares a lock 66[this is a quite common scenario for most non-trivial RT applications], 67even if all critical sections are coded carefully to be deterministic 68(i.e. all critical sections are short in duration and only execute a 69limited number of instructions), the kernel cannot guarantee any 70deterministic execution of the high-prio task: any medium-priority task 71could preempt the low-prio task while it holds the shared lock and 72executes the critical section, and could delay it indefinitely. 73 74Implementation: 75--------------- 76 77As mentioned before, the userspace fastpath of PI-enabled pthread 78mutexes involves no kernel work at all - they behave quite similarly to 79normal futex-based locks: a 0 value means unlocked, and a value==TID 80means locked. (This is the same method as used by list-based robust 81futexes.) Userspace uses atomic ops to lock/unlock these mutexes without 82entering the kernel. 83 84To handle the slowpath, we have added two new futex ops: 85 86 FUTEX_LOCK_PI 87 FUTEX_UNLOCK_PI 88 89If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to 90TID fails], then FUTEX_LOCK_PI is called. The kernel does all the 91remaining work: if there is no futex-queue attached to the futex address 92yet then the code looks up the task that owns the futex [it has put its 93own TID into the futex value], and attaches a 'PI state' structure to 94the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware, 95kernel-based synchronization object. The 'other' task is made the owner 96of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the 97futex value. Then this task tries to lock the rt-mutex, on which it 98blocks. Once it returns, it has the mutex acquired, and it sets the 99futex value to its own TID and returns. Userspace has no other work to 100perform - it now owns the lock, and futex value contains 101FUTEX_WAITERS|TID. 102 103If the unlock side fastpath succeeds, [i.e. userspace manages to do a 104TID -> 0 atomic transition of the futex value], then no kernel work is 105triggered. 106 107If the unlock fastpath fails (because the FUTEX_WAITERS bit is set), 108then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the 109behalf of userspace - and it also unlocks the attached 110pi_state->rt_mutex and thus wakes up any potential waiters. 111 112Note that under this approach, contrary to previous PI-futex approaches, 113there is no prior 'registration' of a PI-futex. [which is not quite 114possible anyway, due to existing ABI properties of pthread mutexes.] 115 116Also, under this scheme, 'robustness' and 'PI' are two orthogonal 117properties of futexes, and all four combinations are possible: futex, 118robust-futex, PI-futex, robust+PI-futex. 119 120More details about priority inheritance can be found in 121Documentation/rt-mutex.txt.