linux-kernel - Re: [PATCH] sched: remove false-positive warning from wake_up

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Message-ID: <565D0A88.5060902@redhat.com>
Date:	Mon, 30 Nov 2015 21:48:40 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] sched: remove false-positive warning from
 wake_up_process()

On 11/30/2015 08:47 PM, Linus Torvalds wrote:
> On Mon, Nov 30, 2015 at 5:34 PM, Sasha Levin <sasha.levin@...cle.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.

What would the correct behaviour in that case be?

Does waking up the task while it is being traced, and ptrace
(or gdb) is not expecting a wakeup, break the tracing?

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