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

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Date:	Thu, 3 Jun 2010 00:44:16 -0400
From:	Ben Blum <bblum@...rew.cmu.edu>
To:	Paul Menage <menage@...gle.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Ben Blum <bblum@...rew.cmu.edu>,
	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 Wed, Jun 02, 2010 at 03:03:49PM -0700, Paul Menage wrote:
> On Wed, Jun 2, 2010 at 2:38 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > cgroup_attach_task() does, and this time PF_EXITING is understandable.
> >
> 
> Oh, OK, I was talking about the one in cgroup_attach_task(), which is
> called by attach_task_by_pid(), and assumed that you were referring to
> the same one. I'd forgotten about the pre-check in
> attach_task_by_pid(). Yes, that one could probably be removed without
> affecting any final outcomes.
> 
> Paul

Yes, I agree. The check should be moved into cgroup_attach_task before
the ss->can_attach calls, so the "optimization" stays in the case where
it's wanted.

And the use of __task_cred after the exiting check seems safe too if the
check is gone - from cred.h: "The caller must make sure task doesn't go
away, either by holding a ref on task or by holding tasklist_lock to
prevent it from being unlinked." So that means it'd be safe to take a
reference, task sets exiting and does everything before unhashing, then
access the cred pointer; since we hold tasklist lock it's the same case.

-- Ben
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