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

[Vladimir Davydov <vdavydov.dev@gmail.com>]

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@cmpxchg.org>
> Signed-off-by: Roman Gushchin <guro@fb.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.

commit 506458efaf153c1ea480591c5602a5a3ba5a3b76
Author: Will Deacon <will.deacon@arm.com>
Date:   Tue Oct 24 11:22:48 2017 +0100

    locking/barriers: Convert users of lockless_dereference() to READ_ONCE()

    READ_ONCE() now has an implicit smp_read_barrier_depends() call, so it
    can be used instead of lockless_dereference() without any change in
    semantics.

    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/1508840570-22169-4-git-send-email-will.deacon@arm.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>

commit 76ebbe78f7390aee075a7f3768af197ded1bdfbb
Author: Will Deacon <will.deacon@arm.com>
Date:   Tue Oct 24 11:22:47 2017 +0100

    locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()

    In preparation for the removal of lockless_dereference(), which is the
    same as READ_ONCE() on all architectures other than Alpha, add an
    implicit smp_read_barrier_depends() to READ_ONCE() so that it can be
    used to head dependency chains on all architectures.

    Signed-off-by: Will Deacon <will.deacon@arm.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/1508840570-22169-3-git-send-email-will.deacon@arm.com
    Signed-off-by: Ingo Molnar <mingo@kernel.org>