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