Documentation/locking/pi-futex.rst

Source file repositories/reference/linux-study-clean/Documentation/locking/pi-futex.rst

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Documentation/locking/pi-futex.rst
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======================
Lightweight PI-futexes
======================

We are calling them lightweight for 3 reasons:

 - in the user-space fastpath a PI-enabled futex involves no kernel work
   (or any other PI complexity) at all. No registration, no extra kernel
   calls - just pure fast atomic ops in userspace.

 - even in the slowpath, the system call and scheduling pattern is very
   similar to normal futexes.

 - the in-kernel PI implementation is streamlined around the mutex
   abstraction, with strict rules that keep the implementation
   relatively simple: only a single owner may own a lock (i.e. no
   read-write lock support), only the owner may unlock a lock, no
   recursive locking, etc.

Priority Inheritance - why?
---------------------------

The short reply: user-space PI helps achieving/improving determinism for
user-space applications. In the best-case, it can help achieve
determinism and well-bound latencies. Even in the worst-case, PI will
improve the statistical distribution of locking related application
delays.

The longer reply
----------------

Firstly, sharing locks between multiple tasks is a common programming
technique that often cannot be replaced with lockless algorithms. As we
can see it in the kernel [which is a quite complex program in itself],
lockless structures are rather the exception than the norm - the current
ratio of lockless vs. locky code for shared data structures is somewhere
between 1:10 and 1:100. Lockless is hard, and the complexity of lockless
algorithms often endangers to ability to do robust reviews of said code.
I.e. critical RT apps often choose lock structures to protect critical
data structures, instead of lockless algorithms. Furthermore, there are
cases (like shared hardware, or other resource limits) where lockless
access is mathematically impossible.

Media players (such as Jack) are an example of reasonable application
design with multiple tasks (with multiple priority levels) sharing
short-held locks: for example, a highprio audio playback thread is
combined with medium-prio construct-audio-data threads and low-prio
display-colory-stuff threads. Add video and decoding to the mix and
we've got even more priority levels.

So once we accept that synchronization objects (locks) are an
unavoidable fact of life, and once we accept that multi-task userspace
apps have a very fair expectation of being able to use locks, we've got
to think about how to offer the option of a deterministic locking
implementation to user-space.

Most of the technical counter-arguments against doing priority
inheritance only apply to kernel-space locks. But user-space locks are
different, there we cannot disable interrupts or make the task
non-preemptible in a critical section, so the 'use spinlocks' argument
does not apply (user-space spinlocks have the same priority inversion
problems as other user-space locking constructs). Fact is, pretty much
the only technique that currently enables good determinism for userspace
locks (such as futex-based pthread mutexes) is priority inheritance:

Currently (without PI), if a high-prio and a low-prio task shares a lock
[this is a quite common scenario for most non-trivial RT applications],
even if all critical sections are coded carefully to be deterministic
(i.e. all critical sections are short in duration and only execute a
limited number of instructions), the kernel cannot guarantee any

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