linux-kernel - Re: [PATCH 3/4] threadgroup: extend threadgroup

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Date:	Sun, 18 Sep 2011 20:46:13 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <htejun@...il.com>
Cc:	rjw@...k.pl, paul@...lmenage.org, lizf@...fujitsu.com,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, fweisbec@...il.com,
	matthltc@...ibm.com, akpm@...ux-foundation.org,
	Tejun Heo <tj@...nel.org>, Paul Menage <menage@...gle.com>,
	Ben Blum <bblum@...rew.cmu.edu>
Subject: Re: [PATCH 3/4] threadgroup: extend threadgroup_lock() to cover
	exit and exec

On 09/18, Oleg Nesterov wrote:
>
> Of course I am in no position to ack the changes in this code, I do not
> fell I understand it enough. But afaics this series is fine.
  ^^^^
I do not feel ;)

> If I am right, afaics the only change 4/4 needs is that it should not add
> WARN_ON_ONCE(tsk->flags & PF_EXITING) into cgroup_task_migrate().

Hmm. Unless I missed something this WARN_ON_ONCE() is not right anyway,
because

> - * Call holding cgroup_mutex. May take task_lock of
> - * the task 'tsk' during call.
> + * Call with cgroup_mutex and threadgroup locked. May take task_lock of
> + * @tsk during call.
>   */
>  int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)

this is not true, cgroup_attach_proc() has other callers.

cgroup_attach_task_all() is probably fine, tsk should be current.

but cpuset_do_move_task() doesn't look fine at first glance, we can race
with the exiting task.

And this leads to another question about the cgroup_mutex/threadgroup_lock
nesting...

Oleg.

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