linux-kernel - Re: [PATCH 1/2] memcg: dirty pages accounting and limiting infrastructure

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Message-ID: <20100223115846.GI1882@linux>
Date:	Tue, 23 Feb 2010 12:58:46 +0100
From:	Andrea Righi <arighi@...eler.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	containers@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] memcg: dirty pages accounting and limiting
 infrastructure

On Mon, Feb 22, 2010 at 01:07:32PM -0500, Vivek Goyal wrote:
> > > > +unsigned long mem_cgroup_dirty_bytes(void)
> > > > +{
> > > > +	struct mem_cgroup *memcg;
> > > > +	unsigned long dirty_bytes;
> > > > +
> > > > +	if (mem_cgroup_disabled())
> > > > +		return vm_dirty_bytes;
> > > > +
> > > > +	rcu_read_lock();
> > > > +	memcg = mem_cgroup_from_task(current);
> > > > +	if (memcg == NULL)
> > > > +		dirty_bytes = vm_dirty_bytes;
> > > > +	else
> > > > +		dirty_bytes = get_dirty_bytes(memcg);
> > > > +	rcu_read_unlock();
> > > 
> > > The rcu_read_lock() isn't protecting anything here.
> > 
> > Right!
> 
> Are we not protecting "memcg" pointer using rcu here?

Vivek, you are right:

 mem_cgroup_from_task() -> task_subsys_state() -> rcu_dereference()

So, this *must* be RCU protected.

Thanks!
-Andrea
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