linux-kernel - Re: [BUG] TASK_DEAD task is able to be woken up in special condition

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Date:	Mon, 26 Dec 2011 18:11:51 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Yasunori Goto <y-goto@...fujitsu.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Hiroyuki KAMEZAWA <kamezawa.hiroyu@...fujitsu.com>,
	Motohiro Kosaki <kosaki.motohiro@...fujitsu.com>,
	Linux Kernel ML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] TASK_DEAD task is able to be woken up in special
	condition

On 12/26, Yasunori Goto wrote:
>
> >
> > IIRC, this was already discussed a bit. Say, try_to_wake_up(TASK_INTERRUPTIBLE)
> > can wakeup a TASK_UNINTERRUPTIBLE task if it temporary sets INTERRUPTIBLE but
> > doesn't call schedule() in this state.
>
> Oleg-san,
>
> Could you point the discussion?
> I don't understand yet how it occurred...

Suppose that the task T does

	set_current_state(TASK_INTERRUPTIBLE);

	set_current_state(TASK_UNINTERRUPTIBLE);
	schedule();

try_to_wake_up(TASK_UNINTERRUPTIBLE) in between can observe this task
in TASK_INTERRUPTIBLE state. Then it can set RUNNING/WAKING after T
sets ->state = TASK_UNINTERRUPTIBLE.

For example, this is possibly if T simply does wait_event() twice when
the the 1st wait_event() doesn't sleep.

Basically this is the same race you described, but I think you found
the case when we can't tolerate the spurious wakeup.

Oleg.

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