Re: [PATCH] sched: remove false-positive warning from wake_up_process()

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From	Linus Torvalds <torvalds@linux-foundation.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] sched: remove false-positive warning from wake_up_process()
Date	Tue, 01 Dec 2015 02:50:01 +0100
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On Mon, Nov 30, 2015 at 5:34 PM, Sasha Levin <sasha.levin@oracle.com> wrote:
> Futex can have a spurious wake up before we actually wake it up on our own,
> which will trigger this warning if the task is still stopped.

Actually, I think it would presumably be the other way around: a
spurious stale futex wakeup happens *after* the process has been woken
up for some other reason and moved to stopped state.

(The "wake up and move to stopped state" could be for the same reason:
a SIGSTOP signal).

So the setup is presumably something like this:

 - on cpu1: futex code is about to go to sleep, adds itself to the
futex hash chains, but then gets interrupted by a SIGSTOP

 - in the meantime, on cpu2, the futex is changed, and the wakup code
sees the process from cpu1 on the futex hash chains

 - on cpu1, the process has now removed itself from the hash chains,
and goes through the signal code that sets the state to STOPPED

 - in the meantime, on cpu2, the futex code now gets around to waking
things up, and sees that stopped state

Roughly.

               Linus
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Thread

[PATCH] sched: remove false-positive warning from wake_up_process() Sasha Levin <sasha.levin@oracle.com> - 2015-12-01 02:40 +0100
  Re: [PATCH] sched: remove false-positive warning from wake_up_process() Linus Torvalds <torvalds@linux-foundation.org> - 2015-12-01 02:50 +0100
    Re: [PATCH] sched: remove false-positive warning from  wake_up_process() Rik van Riel <riel@redhat.com> - 2015-12-01 03:50 +0100
      Re: [PATCH] sched: remove false-positive warning from wake_up_process() Linus Torvalds <torvalds@linux-foundation.org> - 2015-12-01 04:20 +0100
  Re: [PATCH] sched: remove false-positive warning from  wake_up_process() Peter Zijlstra <peterz@infradead.org> - 2015-12-03 13:40 +0100
    Re: [PATCH] sched: remove false-positive warning from wake_up_process() Linus Torvalds <torvalds@linux-foundation.org> - 2015-12-03 19:20 +0100
  [tip:locking/core] sched/core:   Remove false-positive warning from wake_up_process() tip-bot for Sasha Levin <tipbot@zytor.com> - 2015-12-04 13:00 +0100

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