linux-kernel - Re: [PATCH v6 01/10] mm: add missing smp read barrier on getting memcg kmem

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Date:   Mon, 10 Jun 2019 16:33:44 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Vladimir Davydov <vdavydov.dev@...il.com>
Cc:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Shakeel Butt <shakeelb@...gle.com>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v6 01/10] mm: add missing smp read barrier on getting
 memcg kmem_cache pointer

On Sun, Jun 09, 2019 at 03:10:52PM +0300, Vladimir Davydov wrote:
> On Tue, Jun 04, 2019 at 07:44:45PM -0700, Roman Gushchin wrote:
> > Johannes noticed that reading the memcg kmem_cache pointer in
> > cache_from_memcg_idx() is performed using READ_ONCE() macro,
> > which doesn't implement a SMP barrier, which is required
> > by the logic.
> > 
> > Add a proper smp_rmb() to be paired with smp_wmb() in
> > memcg_create_kmem_cache().
> > 
> > The same applies to memcg_create_kmem_cache() itself,
> > which reads the same value without barriers and READ_ONCE().
> > 
> > Suggested-by: Johannes Weiner <hannes@...xchg.org>
> > Signed-off-by: Roman Gushchin <guro@...com>
> > ---
> >  mm/slab.h        | 1 +
> >  mm/slab_common.c | 3 ++-
> >  2 files changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/slab.h b/mm/slab.h
> > index 739099af6cbb..1176b61bb8fc 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -260,6 +260,7 @@ cache_from_memcg_idx(struct kmem_cache *s, int idx)
> >  	 * memcg_caches issues a write barrier to match this (see
> >  	 * memcg_create_kmem_cache()).
> >  	 */
> > +	smp_rmb();
> >  	cachep = READ_ONCE(arr->entries[idx]);
> 
> Hmm, we used to have lockless_dereference() here, but it was replaced
> with READ_ONCE some time ago. The commit message claims that READ_ONCE
> has an implicit read barrier in it.

Thanks for catching this Vladimir. I wasn't aware of this change to
the memory model. Indeed, we don't need to change anything here.