linux-kernel - Re: [RFC] [PATCH 2/2] cgroups: make procs file writable

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Date:	Thu, 3 Jun 2010 16:48:10 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ben Blum <bblum@...rew.cmu.edu>
Cc:	Paul Menage <menage@...gle.com>, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	ebiederm@...ssion.com, lizf@...fujitsu.com, matthltc@...ibm.com
Subject: Re: [RFC] [PATCH 2/2] cgroups: make procs file writable

On 06/03, Ben Blum wrote:
>
> On Wed, Jun 02, 2010 at 10:58:55PM +0200, Oleg Nesterov wrote:
> > Hmm. The usage of ->thread_group in ->can_attach() methods doesn't
> > look safe to me... but currently bool threadgroup is always false.
>
> I recall putting a rcu_read_lock() around that part and being assured
> that made it safe. But I don't remember the details. Maybe taking
> tasklist_lock is necessary?

rcu_read_lock() is not enough, see another email I sent.

Once again.

	rcu_read_lock()
	list_for_each_rcu(tsk->thread_group)

assumes that at least tsk->thread_group->next can't point to nowhere,
this is not true. This memory can go away _before_ we take rcu lock.

Oleg.

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