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Date: Fri, 23 Dec 2011 03:27:45 +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 1/3] cgroup: remove tasklist_lock from cgroup_attach_proc
Message-ID: <20111223022742.GA28309@somewhere.redhat.com>
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On Thu, Dec 22, 2011 at 04:57:51PM -0800, Mandeep Singh Baines wrote:
> Since cgroup_attach_proc is protected by a threadgroup_lock, we
> no longer need a tasklist_lock to protect while_each_thread.
> To keep the complexity of the double-check locking in one place,
> I also moved the thread_group_leader check up into
> attach_task_by_pid.
> 
> While at it, also converted a couple of returns to gotos.
> 
> The suggestion was made here:
> 
> https://lkml.org/lkml/2011/12/22/86
> 
> Suggested-by: Frederic Weisbecker <fweisbec@gmail.com>
> 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: Oleg Nesterov <oleg@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Paul Menage <paul@paulmenage.org>
> ---
>  kernel/cgroup.c |   52 +++++++++++++++++++++-------------------------------
>  1 files changed, 21 insertions(+), 31 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 1042b3c..032139d 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2102,21 +2102,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
>  	if (retval)
>  		goto out_free_group_list;
>  
> -	/* prevent changes to the threadgroup list while we take a snapshot. */
> -	read_lock(&tasklist_lock);
> -	if (!thread_group_leader(leader)) {
> -		/*
> -		 * a race with de_thread from another thread's exec() may strip
> -		 * us of our leadership, making while_each_thread unsafe to use
> -		 * on this task. if this happens, there is no choice but to
> -		 * throw this task away and try again (from cgroup_procs_write);
> -		 * this is "double-double-toil-and-trouble-check locking".
> -		 */
> -		read_unlock(&tasklist_lock);
> -		retval = -EAGAIN;
> -		goto out_free_group_list;
> -	}
> -
>  	tsk = leader;
>  	i = 0;
>  	do {
> @@ -2145,7 +2130,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
>  	group_size = i;
>  	tset.tc_array = group;
>  	tset.tc_array_len = group_size;
> -	read_unlock(&tasklist_lock);

You still need rcu_read_lock()/rcu_read_unlock() around
	do {

	} while_each_thread()

because threadgroup_lock() doesn't lock the part that remove a thread from
its group on exit.