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Date: Tue, 20 Dec 2011 15:23:26 -0800
From: Tejun Heo <tj@kernel.org>
To: Mandeep Singh Baines <msb@chromium.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>, LKML <linux-kernel@vger.kernel.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        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 2/5] cgroup: remove redundant get/put of task struct
Message-ID: <20111220232326.GL10752@google.com>
References: <1324422873-31001-1-git-send-email-msb@chromium.org>
 <1324422873-31001-3-git-send-email-msb@chromium.org>
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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