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.