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

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Message-ID: <4F1EF1E8.4090909@jp.fujitsu.com>
Date:	Tue, 24 Jan 2012 13:01:12 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	peterz@...radead.org
CC:	kosaki.motohiro@...il.com, oleg@...hat.com, y-goto@...fujitsu.com,
	mingo@...e.hu, kamezawa.hiroyu@...fujitsu.com,
	linux-kernel@...r.kernel.org
Subject: Re: [BUG] TASK_DEAD task is able to be woken up in special condition

On 1/24/2012 5:23 AM, Peter Zijlstra wrote:
> 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. 

I'm sorry. I haven't catch your point. I think following step is
valid kernel code. Do you disagree?

>>                         task->state = TASK_UNINTERRUPTIBLE;
>>                         schedule()

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