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    SubjectRe: [PATCH v6 01/10] mm: add missing smp read barrier on getting memcg kmem_cache pointer
    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
    >

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