On Mon, Jun 10, 2019 at 04:33:44PM -0400, Johannes Weiner wrote: > On Sun, Jun 09, 2019 at 03:10:52PM +0300, Vladimir Davydov wrote: > > 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@xxxxxxxxxxx> > > > Signed-off-by: Roman Gushchin <guro@xxxxxx> > > > --- > > > 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. > > Thanks for catching this Vladimir. I wasn't aware of this change to > the memory model. Indeed, we don't need to change anything here. Cool, I'm dropping this patch. Thanks!
