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

[Shakeel Butt <shakeelb@google.com>]

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
>