linux-kernel - Re: [ckrm-tech] [PATCH 1/7] containers (V7): Generic container system abstracted from cpusets code

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Date:	Sun, 25 Mar 2007 09:46:02 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Paul Jackson <pj@....com>
Cc:	sekharan@...ibm.com, ckrm-tech@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, xemul@...ru,
	containers@...ts.osdl.org, ebiederm@...ssion.com,
	mbligh@...gle.com, winget@...gle.com, rohitseth@...gle.com,
	serue@...ibm.com, menage@...gle.com, dev@...ru, devel@...nvz.org
Subject: Re: [ckrm-tech] [PATCH 1/7] containers (V7): Generic container system abstracted from cpusets code

On Sun, Mar 25, 2007 at 07:58:16AM +0530, Srivatsa Vaddagiri wrote:
> Not just this, continuing further we have more trouble:
> 
> --------------------------------------------------------------------
> CPU0 (attach_task T1 to CS2)			CPU1 (T1 is exiting)
> --------------------------------------------------------------------
> 
> synchronize_rcu()
> 						    atomic_dec(&CS1->count);
> 						    [CS1->count = 0]
> 
> if atomic_dec_and_test(&oldcs->count))
> 	[CS1->count = -1]
> 
> 
> 
> We now have CS1->count negative. Is that good? I am uncomfortable ..
> 
> We need a task_lock() in cpuset_exit to avoid this race.

2nd race is tricky. We probably need to do this to avoid it:

	task_lock(tsk);

	/* Check if tsk->cpuset is still same. We may have raced with 
	 * cpuset_exit changing tsk->cpuset again under our feet.
	 */
	if (tsk->cpuset == cs && atomic_dec_and_test(&oldcs->count)) {
		task_unlock(tsk);
                check_for_release(oldcs, ppathbuf);
		goto done;
	}

	task_unlock(tsk);

done:
	return 0;



-- 
Regards,
vatsa
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