Date: Fri, 26 Feb 2010 14:01:35 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Minchan Kim <minchan.kim@gmail.com>
Cc: Andrea Righi <arighi@develer.com>, Vivek Goyal <vgoyal@redhat.com>,
       David Rientjes <rientjes@google.com>,
       Balbir Singh <balbir@linux.vnet.ibm.com>,
       Suleiman Souhlal <suleiman@google.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       containers@lists.linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] memcg: dirty pages accounting and limiting 
 infrastructure
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Hi,

On Fri, 26 Feb 2010 13:50:04 +0900
Minchan Kim <minchan.kim@gmail.com> wrote:

> > Hm ? I don't read the whole thread but can_attach() is called under
> > cgroup_mutex(). So, it doesn't need to use RCU.
> 
> Vivek mentioned memcg is protected by RCU if I understand his intention right.
> So I commented that without enough knowledge of memcg.
> After your comment, I dive into the code.
> 
> Just out of curiosity.
> 
> Really, memcg is protected by RCU?
yes. All cgroup subsystem is protected by RCU.

> I think most of RCU around memcg is for protecting task_struct and
> cgroup_subsys_state.
> The memcg is protected by cgroup_mutex as you mentioned.
> Am I missing something?

There are several levels of protections.

cgroup subsystem's ->destroy() call back is finally called by

As this.

 768                 synchronize_rcu();
 769 
 770                 mutex_lock(&cgroup_mutex);
 771                 /*
 772                  * Release the subsystem state objects.
 773                  */
 774                 for_each_subsys(cgrp->root, ss)
 775                         ss->destroy(ss, cgrp);
 776 
 777                 cgrp->root->number_of_cgroups--;
 778                 mutex_unlock(&cgroup_mutex);

Before here, 
	- there are no tasks under this cgroup (cgroup's refcnt is 0)
          && cgroup is marked as REMOVED.

Then, this access
	rcu_read_lock();
	mem = mem_cgroup_from_task(task);
	if (css_tryget(mem->css))   <===============checks cgroup refcnt
		....
	rcu_read_unlock()
is O.K.

And, it's graranteed that we don't have to do complicated fine-grain check
if cgroup_mutex() is held.

Because cgroup_mutex() is system-wide heavy lock, this refcnt+RCU trick is
used and works quite well.

Thanks,
-Kame

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