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

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Message-ID: <AANLkTin-T6ij3xf0ceGoCamwcoayxyf0GdAKY6yXDMVl@mail.gmail.com>
Date:	Wed, 2 Jun 2010 12:53:33 -0700
From:	Paul Menage <menage@...gle.com>
To:	Oleg Nesterov <oleg@...hat.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 Wed, Jun 2, 2010 at 7:06 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> 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.

Right.

>
> 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?

For the value of correctness that was relevant when writing that code
originally, I think it's fine. But ...

> 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.

Hmm, maybe. I could see this being argued both ways. Can the process
hang around indefinitely with the leader as a zombie and the other
threads still running?

It wouldn't be that hard to make it possible to avoid relying on
PF_EXITING as the check - instead we'd have an exiting_css_set object
that have the same pointer set as init_css_set, but would be
distinguishable from it as a task->cgroups pointer by virtue of being
a different object. Then cgroup_exit() can reassign tasks to point to
exiting_css_set rather than init_css_set, and the various checks that
are currently made for (task->flags & PF_EXITING) could check for
(task->cgroups == exiting_css_set) instead. This would allow task
movement further into the exit process.

Paul
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