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

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Message-Id: <20100226092339.1f639cbf.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 26 Feb 2010 09:23:39 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrea Righi <arighi@...eler.com>, Vivek Goyal <vgoyal@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.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 Fri, 26 Feb 2010 00:36:15 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> 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?
> 
Hm ? I don't read the whole thread but can_attach() is called under
cgroup_mutex(). So, it doesn't need to use RCU.

Thanks,
-Kame


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