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

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Message-ID: <20100602202048.GB28663@redhat.com>
Date:	Wed, 2 Jun 2010 22:20:49 +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 06/02, Paul Menage wrote:
>
> On Wed, Jun 2, 2010 at 7:06 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > 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?

Yes sure. The main thread can exit via sys_exit(), this doesn't affect
the thread group. Of course, I am not saying this is common practice,
perhaps no "important" app does this.

> 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.

It is too late for me to even try to understand the above ;)

But I still can't understand why we can't just remove it. Both
cgroup_attach_task() and cgroup_attach_proc() should handle the
possible races correctly anyway.

Oleg.

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