linux-kernel - Re: Possible race between cgroup_attach_proc and de_thread, and questionable code in de

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Message-ID: <20110814175119.GC2381@redhat.com>
Date:	Sun, 14 Aug 2011 19:51:19 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	NeilBrown <neilb@...e.de>, Ben Blum <bblum@...rew.cmu.edu>,
	Paul Menage <menage@...gle.com>,
	Li Zefan <lizf@...fujitsu.com>,
	containers@...ts.linux-foundation.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Possible race between cgroup_attach_proc and de_thread, and
	questionable code in de_thread.

On 07/28, Paul E. McKenney wrote:
>
> On Thu, Jul 28, 2011 at 11:08:13AM +1000, NeilBrown wrote:
> >
> > I disagree.  It also requires - by virtue of the use of while_each_thread() -
> > that 'g' remains on the list that 't' is walking along.
>
> Doesn't the following code in the loop body deal with this possibilty?
>
> 	/* Exit if t or g was unhashed during refresh. */
> 	if (t->state == TASK_DEAD || g->state == TASK_DEAD)
> 		goto unlock;

This code is completely wrong even if while_each_thread() was fine.

I sent the patch but it was ignored.

	[PATCH] fix the racy check_hung_uninterruptible_tasks()->rcu_lock_break()
	http://marc.info/?l=linux-kernel&m=127688790019041

Oleg.

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