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

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Date:	Wed, 2 Jun 2010 16:06:59 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Paul Menage <menage@...gle.com>
Cc:	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 06/01, Paul Menage wrote:
>
> On Mon, May 31, 2010 at 11:04 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > And, forgot to mention, I do not understand the PF_EXITING check in
> > attach_task_by_pid() (and some others).
> >
> > At first glance, it buys nothing. PF_EXITING can be set right after
> > the check.
>
> It can, but it's a benign race.

Yes,

> Moving a non-current thread into a cgroup takes task->alloc_lock and
> checks for PF_EXITING before manipulating that thread's cgroup links.
> The exit procedure sets PF_EXITING and then (somewhat later, but
> guaranteed) moves current to the root cgroups while holding
> alloc_lock.

Yes sure, I understand this part. cgroup_attach_task() correctly checks
PF_EXITING under task_lock(), this protects from the case when this
task has already passed cgroup_exit() which takes this lock too.

But. This exactly means that the PF_EXITING check in attach_task_by_pid()
is not necessary from the correctness pov (while probably can be considered
as optimization), right?

And,

> so it's fine to refuse to move it to a new cgroup.

I am not sure. It doesn't hurt when we try to move a thread. But if
we want to move the whole thread group, we should proceed even if the
group leader has already exited and thus has PF_EXITING bit set.

That was my question.

Oleg.

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