linux-kernel - Re: [PATCH v3 06/16] memcg: infrastructure to match an allocation to the right cache

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Date:	Mon, 24 Sep 2012 10:58:01 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	kamezawa.hiroyu@...fujitsu.com, devel@...nvz.org,
	linux-mm@...ck.org, Suleiman Souhlal <suleiman@...gle.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v3 06/16] memcg: infrastructure to match an allocation
 to the right cache

On Mon, Sep 24, 2012 at 12:17:37PM +0400, Glauber Costa wrote:
> On 09/22/2012 12:52 AM, Tejun Heo wrote:
> > Missed some stuff.
> > 
> > On Tue, Sep 18, 2012 at 06:12:00PM +0400, Glauber Costa wrote:
> >> +static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
> >> +						  struct kmem_cache *cachep)
> >> +{
> > ...
> >> +	memcg->slabs[idx] = new_cachep;
> > ...
> >> +struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
> >> +					  gfp_t gfp)
> >> +{
> > ...
> >> +	return memcg->slabs[idx];
> > 
> > I think you need memory barriers for the above pair.
> > 
> > Thanks.
> > 
> 
> Why is that?
> 
> We'll either see a value, or NULL. If we see NULL, we assume the cache
> is not yet created. Not a big deal.

Because when you see !NULL cache pointer you want to be able to see
the cache fully initialized.  You need wmb between cache creation and
pointer assignment and at least read_barrier_depends() between
fetching the cache pointer and dereferencing it.

Thanks.

-- 
tejun
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