linux-kernel - Re: [PATCH 2/5] cgroup: remove redundant get/put of task struct

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Message-ID: <20111220232326.GL10752@google.com>
Date:	Tue, 20 Dec 2011 15:23:26 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Mandeep Singh Baines <msb@...omium.org>
Cc:	Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Menage <paul@...lmenage.org>
Subject: Re: [PATCH 2/5] cgroup: remove redundant get/put of task struct

Hello,

On Tue, Dec 20, 2011 at 03:14:30PM -0800, Mandeep Singh Baines wrote:
> @@ -2287,14 +2280,12 @@ static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
>  			cgroup_unlock();
>  			return -EACCES;
>  		}
> -		get_task_struct(tsk);
>  		rcu_read_unlock();

This is before threadgroup is locked.  What guarantees @tsk doesn't go
away once rcu is unlocked?

Thanks.

-- 
tejun
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