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

[Andrea Righi <arighi-vWjgImWzx8FBDgjK7y7TUQ@public.gmane.org>]

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