Based on kernel version 4.15. Page generated on 2018-01-29 10:00 EST.
1 ====================== 2 Lightweight PI-futexes 3 ====================== 4 5 We are calling them lightweight for 3 reasons: 6 7 - in the user-space fastpath a PI-enabled futex involves no kernel work 8 (or any other PI complexity) at all. No registration, no extra kernel 9 calls - just pure fast atomic ops in userspace. 10 11 - even in the slowpath, the system call and scheduling pattern is very 12 similar to normal futexes. 13 14 - the in-kernel PI implementation is streamlined around the mutex 15 abstraction, with strict rules that keep the implementation 16 relatively simple: only a single owner may own a lock (i.e. no 17 read-write lock support), only the owner may unlock a lock, no 18 recursive locking, etc. 19 20 Priority Inheritance - why? 21 --------------------------- 22 23 The short reply: user-space PI helps achieving/improving determinism for 24 user-space applications. In the best-case, it can help achieve 25 determinism and well-bound latencies. Even in the worst-case, PI will 26 improve the statistical distribution of locking related application 27 delays. 28 29 The longer reply 30 ---------------- 31 32 Firstly, sharing locks between multiple tasks is a common programming 33 technique that often cannot be replaced with lockless algorithms. As we 34 can see it in the kernel [which is a quite complex program in itself], 35 lockless structures are rather the exception than the norm - the current 36 ratio of lockless vs. locky code for shared data structures is somewhere 37 between 1:10 and 1:100. Lockless is hard, and the complexity of lockless 38 algorithms often endangers to ability to do robust reviews of said code. 39 I.e. critical RT apps often choose lock structures to protect critical 40 data structures, instead of lockless algorithms. Furthermore, there are 41 cases (like shared hardware, or other resource limits) where lockless 42 access is mathematically impossible. 43 44 Media players (such as Jack) are an example of reasonable application 45 design with multiple tasks (with multiple priority levels) sharing 46 short-held locks: for example, a highprio audio playback thread is 47 combined with medium-prio construct-audio-data threads and low-prio 48 display-colory-stuff threads. Add video and decoding to the mix and 49 we've got even more priority levels. 50 51 So once we accept that synchronization objects (locks) are an 52 unavoidable fact of life, and once we accept that multi-task userspace 53 apps have a very fair expectation of being able to use locks, we've got 54 to think about how to offer the option of a deterministic locking 55 implementation to user-space. 56 57 Most of the technical counter-arguments against doing priority 58 inheritance only apply to kernel-space locks. But user-space locks are 59 different, there we cannot disable interrupts or make the task 60 non-preemptible in a critical section, so the 'use spinlocks' argument 61 does not apply (user-space spinlocks have the same priority inversion 62 problems as other user-space locking constructs). Fact is, pretty much 63 the only technique that currently enables good determinism for userspace 64 locks (such as futex-based pthread mutexes) is priority inheritance: 65 66 Currently (without PI), if a high-prio and a low-prio task shares a lock 67 [this is a quite common scenario for most non-trivial RT applications], 68 even if all critical sections are coded carefully to be deterministic 69 (i.e. all critical sections are short in duration and only execute a 70 limited number of instructions), the kernel cannot guarantee any 71 deterministic execution of the high-prio task: any medium-priority task 72 could preempt the low-prio task while it holds the shared lock and 73 executes the critical section, and could delay it indefinitely. 74 75 Implementation 76 -------------- 77 78 As mentioned before, the userspace fastpath of PI-enabled pthread 79 mutexes involves no kernel work at all - they behave quite similarly to 80 normal futex-based locks: a 0 value means unlocked, and a value==TID 81 means locked. (This is the same method as used by list-based robust 82 futexes.) Userspace uses atomic ops to lock/unlock these mutexes without 83 entering the kernel. 84 85 To handle the slowpath, we have added two new futex ops: 86 87 - FUTEX_LOCK_PI 88 - FUTEX_UNLOCK_PI 89 90 If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to 91 TID fails], then FUTEX_LOCK_PI is called. The kernel does all the 92 remaining work: if there is no futex-queue attached to the futex address 93 yet then the code looks up the task that owns the futex [it has put its 94 own TID into the futex value], and attaches a 'PI state' structure to 95 the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware, 96 kernel-based synchronization object. The 'other' task is made the owner 97 of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the 98 futex value. Then this task tries to lock the rt-mutex, on which it 99 blocks. Once it returns, it has the mutex acquired, and it sets the 100 futex value to its own TID and returns. Userspace has no other work to 101 perform - it now owns the lock, and futex value contains 102 FUTEX_WAITERS|TID. 103 104 If the unlock side fastpath succeeds, [i.e. userspace manages to do a 105 TID -> 0 atomic transition of the futex value], then no kernel work is 106 triggered. 107 108 If the unlock fastpath fails (because the FUTEX_WAITERS bit is set), 109 then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the 110 behalf of userspace - and it also unlocks the attached 111 pi_state->rt_mutex and thus wakes up any potential waiters. 112 113 Note that under this approach, contrary to previous PI-futex approaches, 114 there is no prior 'registration' of a PI-futex. [which is not quite 115 possible anyway, due to existing ABI properties of pthread mutexes.] 116 117 Also, under this scheme, 'robustness' and 'PI' are two orthogonal 118 properties of futexes, and all four combinations are possible: futex, 119 robust-futex, PI-futex, robust+PI-futex. 120 121 More details about priority inheritance can be found in 122 Documentation/locking/rt-mutex.txt.