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

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Message-ID: <20100602205855.GA30039@redhat.com>
Date:	Wed, 2 Jun 2010 22:58:55 +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 1:20 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > 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.
>
> This check has been in cgroups for quite a while and no-one's
> complained so far...

Sure. because currently attach_task_by_pid() works with the single
thread. But Ben's patch reuses this code to call cgroup_attach_proc().

> > 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.
>
> The "it" that you're proposing to remove is in fact the code that
> handles those races.

In that case I confused, and I thought we already agreed that
the PF_EXITING check in attach_task_by_pid() is not strictly needed
for correctness.

Once again, the task can call do_exit() and set PF_EXITING right
after the check.

> Anyway, I think this issue is orthogonal to the movability of entire
> processes - with Ben's patch, an exited-but-not-reaped group leader is
> immovable either via "tasks" or "cgroup.procs".

Hmm. As I said, I didn't really read this patch. But I thought that
cgroup_attach_proc() tries to handle this.

Anyway I agree, this is minor and I never pretended I understand
this code.

Hmm. The usage of ->thread_group in ->can_attach() methods doesn't
look safe to me... but currently bool threadgroup is always false.

Oleg.

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