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

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Message-ID: <28c262361002250736k57543379j8291e0dfb8df194e@mail.gmail.com>
Date:	Fri, 26 Feb 2010 00:36:15 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Andrea Righi <arighi@...eler.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	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

Hi

On Tue, Feb 23, 2010 at 8:58 PM, Andrea Righi <arighi@...eler.com> wrote:
> 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.

So, Doesn't mem_cgroup_from_task in mem_cgroup_can_attach need RCU, too?


-- 
Kind regards,
Minchan Kim
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