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

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Date:	Wed, 2 Jun 2010 14:12:20 -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 1:58 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>> 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.

Not quite - something is required for correctness, and the PF_EXITING
check provides that correctness, with a very small window (between
setting PF_EXITING and calling cgroup_exit) where we might arguably
have been able to move the thread but decline to do so because it's
simpler not to do so and no-one cares. That's the optimization that I
meant - the data structures are slightly simpler since there's no way
to tell when a task has passed cgroup_exit(), and instead we just see
if they've passed PF_EXITING.

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

Yes, the important part is that they haven't set it *before* the check
in attach_task_by_pid(). If they have set it before that, then they
could be anywhere in the exit path after PF_EXITING, and we decline to
move them since it's possible that they've already passed
cgroup_exit(). If the exiting task has not yet set PF_EXITING, then it
can't possibly get into the critical section in cgroup_exit() since
attach_task_by_pid() holds task->alloc_lock.

Paul
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