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Date: Mon, 19 Dec 2011 03:01:49 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Mandeep Singh Baines <msb@chromium.org>
Cc: Tejun Heo <tj@kernel.org>, Li Zefan <lizf@cn.fujitsu.com>,
        linux-kernel@vger.kernel.org, containers@lists.linux-foundation.org,
        cgroups@vger.kernel.org,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Paul Menage <paul@paulmenage.org>
Subject: Re: [PATCH] cgroup: remove redundate get/put of old css_set from
 migrate
Message-ID: <20111219020147.GA13171@somewhere>
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On Fri, Dec 16, 2011 at 08:38:31AM -0800, Mandeep Singh Baines wrote:
> We can now assume that the css_set reference held by the task
> will not go away for an exiting task. PF_EXITING state can be
> trusted throughout migration by checking it after locking
> threadgroup.
> 
> This patch depends on:
> 
> commit cd3d095275374220921fcf0d4e0c16584b26ddbc
> Author: Tejun Heo <tj@kernel.org>
> Date:   Mon Dec 12 18:12:21 2011 -0800
> 
>     cgroup: always lock threadgroup during migration
> 
> Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Li Zefan <lizf@cn.fujitsu.com>
> Cc: containers@lists.linux-foundation.org
> Cc: cgroups@vger.kernel.org
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Paul Menage <paul@paulmenage.org>
> ---
>  kernel/cgroup.c |   25 ++++++-------------------
>  1 files changed, 6 insertions(+), 19 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 1b3b841..eb95e32 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1856,7 +1856,6 @@ static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
>  	 */
>  	task_lock(tsk);
>  	oldcg = tsk->cgroups;
> -	get_css_set(oldcg);
>  	task_unlock(tsk);

Seems we don't need these task_lock() anymore. I'm cooking a patch to clean that up.