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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 4 Jun 2019 21:35:02 -0700
Message-ID: <CALvZod4F4FqO27Y+msXrxT9yaDLLN7njmBsRoTkmQSPE_7=FtQ@mail.gmail.com>
Subject: Re: [PATCH v6 01/10] mm: add missing smp read barrier on getting
 memcg kmem_cache pointer
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Waiman Long <longman@redhat.com>
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On Tue, Jun 4, 2019 at 7:45 PM Roman Gushchin <guro@fb.com> 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@cmpxchg.org>
> Signed-off-by: Roman Gushchin <guro@fb.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

This seems like independent to the series. Shouldn't this be Cc'ed stable?

> ---
>  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]);
>         rcu_read_unlock();
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 58251ba63e4a..8092bdfc05d5 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -652,7 +652,8 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
>          * allocation (see memcg_kmem_get_cache()), several threads can try to
>          * create the same cache, but only one of them may succeed.
>          */
> -       if (arr->entries[idx])
> +       smp_rmb();
> +       if (READ_ONCE(arr->entries[idx]))
>                 goto out_unlock;
>
>         cgroup_name(css->cgroup, memcg_name_buf, sizeof(memcg_name_buf));
> --
> 2.20.1
>