linux-kernel - Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid at once

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Message-ID: <6599ad830908031855t3ad772e4w4c52e5d7bf4d2508@mail.gmail.com>
Date:	Mon, 3 Aug 2009 18:55:20 -0700
From:	Paul Menage <menage@...gle.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Benjamin Blum <bblum@...gle.com>, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	serue@...ibm.com
Subject: Re: [PATCH 6/6] Makes procs file writable to move all threads by tgid 
	at once

On Mon, Aug 3, 2009 at 6:45 PM, Li Zefan<lizf@...fujitsu.com> wrote:
>
> No. Note task exiting is not protected by cgroup_lock, so this can
> happen:
>
>                               | cgroup_attach_task()
>                               |   oldcg = tsk->cgroups;
>                               |   (tasks->flags & TASK_EXISING == 0)
>                               |   rcu_assign_pointer(tsk->cgroups, newcg);
> cgroup_exit()                  |
>  oldcg = tsk->cgroups;        |
>  put_css_set_taskexit(oldcg); |
>  (now ref of olcg is 0)       |
>                               |   put_css_set(oldcg);
>

But in both cases, reading tsk->cgroups occurs with task->alloc_lock held.

Paul
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