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Subject: Re: [BUG] TASK_DEAD task is able to be woken up in special
 condition
From: Peter Zijlstra <peterz@infradead.org>
To: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>, Yasunori Goto <y-goto@jp.fujitsu.com>,
        Ingo Molnar <mingo@elte.hu>,
        Hiroyuki KAMEZAWA <kamezawa.hiroyu@jp.fujitsu.com>,
        Motohiro Kosaki <kosaki.motohiro@jp.fujitsu.com>,
        Linux Kernel ML <linux-kernel@vger.kernel.org>
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Date: Tue, 24 Jan 2012 11:23:38 +0100
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On Wed, 2011-12-28 at 16:07 -0500, KOSAKI Motohiro wrote:
> I looked at scheduler code today briefly. now I'm afraid following code 
> have similar race.
> 
> 
>          if (task_contributes_to_load(p))
>                  rq->nr_uninterruptible--;
> 
> 
> 
> Can't following schenario be happen?
> 
> 
> CPU0                    CPU1
> --------------------------------------------------------
> deactivate_task()
>                         task->state = TASK_UNINTERRUPTIBLE;
> activate_task()
>    rq->nr_uninterruptible--;
> 
>                         schedule()
>                           deactivate_task()
>                             rq->nr_uninterruptible++;
> 
> Totally, nr_uninterruptible wasn't incremented.
> 
> 
> I'm still not sure. I need to read more sched code. 

You shouldn't ever set another tasks ->state.