linux-kernel - Re: + cgroups-more-safe-tasklist-locking-in-cgroup_attach

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Message-ID: <20110902155534.GA4595@redhat.com>
Date:	Fri, 2 Sep 2011 17:55:34 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ben Blum <bblum@...rew.cmu.edu>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	fweisbec@...il.com, neilb@...e.de, paul@...lmenage.org,
	paulmck@...ux.vnet.ibm.com
Subject: Re: +
	cgroups-more-safe-tasklist-locking-in-cgroup_attach_proc.patch
	added to -mm tree

On 09/02, Ben Blum wrote:
>
> On Fri, Sep 02, 2011 at 04:00:15PM +0200, Oleg Nesterov wrote:
> > Forgot to mention, sorry...
> >
> > That said, I believe the patch is correct and should fix the problem.
>
> Thanks!
>
> But I don't think the check becomes pointless? If a sub-thread execs
> right before read_lock(&tasklist_lock) (but after the find_task_by_vpid
> in attach_task_by_pid), that causes the case that the comment refers to.

How so? The comment says:

	* a race with de_thread from another thread's exec() may strip
	* us of our leadership, making while_each_thread unsafe

This is not true.

And. Given that ->group_leader can be changed right after we drop tasklist
this check is pointless. Yes, it can detect the case when this task_struct
has nothing to do with this process sometimes, but not in general. (This
connects to other problems I mentioned).

IOW, personally I think it would be better to update the patch. But I
won't insist.

Oleg.

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