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From:   Roman Gushchin <guro@fb.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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        "Michal Hocko" <mhocko@kernel.org>,
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        Shakeel Butt <shakeelb@google.com>,
        Christoph Lameter <cl@linux.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
        Waiman Long <longman@redhat.com>
Subject: Re: [PATCH v5 5/7] mm: rework non-root kmem_cache lifecycle
 management
Thread-Topic: [PATCH v5 5/7] mm: rework non-root kmem_cache lifecycle
 management
Thread-Index: AQHVFaFKV+dsG3FY9UWPVf9hOznuvqaBHi0A
Date:   Tue, 28 May 2019 22:28:51 +0000
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References: <20190521200735.2603003-1-guro@fb.com>
 <20190521200735.2603003-6-guro@fb.com> <20190528220353.GC26614@cmpxchg.org>
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On Tue, May 28, 2019 at 06:03:53PM -0400, Johannes Weiner wrote:
> On Tue, May 21, 2019 at 01:07:33PM -0700, Roman Gushchin wrote:
> > +	arr =3D rcu_dereference(cachep->memcg_params.memcg_caches);
> > +
> > +	/*
> > +	 * Make sure we will access the up-to-date value. The code updating
> > +	 * memcg_caches issues a write barrier to match this (see
> > +	 * memcg_create_kmem_cache()).
> > +	 */
> > +	memcg_cachep =3D READ_ONCE(arr->entries[kmemcg_id]);
>=20
> READ_ONCE() isn't an SMP barrier, it just prevents compiler
> muckery. This needs an explicit smp_rmb() to pair with the smp_wmb()
> on the other side.

I believe rcu_dereference()/rcu_assign_pointer()/... are better replacement=
s.

>=20
> I realize you're only moving this code, but it would be good to fix
> that up while you're there.

Right. I'll try to fix it with new-ish rcu API in a separate patch
preceding this one.

Thank you for looking into the series!