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

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Date:	Tue, 27 Dec 2011 15:48:29 +0900
From:	Yasunori Goto <y-goto@...fujitsu.com>
To:	Oleg Nesterov <oleg@...hat.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.
> 
> 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.

Thanks.

I suppose your idea which uses cmpxchg(old_state, RUNNING) is best way.
I'll remake my patch.



> 
> Oleg.
> 

-- 
Yasunori Goto 


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