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Linux/Documentation/pi-futex.txt

  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.

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