memcg, slab: fix barrier usage when accessing memcg_caches

Each root kmem_cache has pointers to per-memcg caches stored in its
memcg_params::memcg_caches array.  Whenever we want to allocate a slab
for a memcg, we access this array to get per-memcg cache to allocate
from (see memcg_kmem_get_cache()).  The access must be lock-free for
performance reasons, so we should use barriers to assert the kmem_cache
is up-to-date.

First, we should place a write barrier immediately before setting the
pointer to it in the memcg_caches array in order to make sure nobody
will see a partially initialized object.  Second, we should issue a read
barrier before dereferencing the pointer to conform to the write
barrier.

However, currently the barrier usage looks rather strange.  We have a
write barrier *after* setting the pointer and a read barrier *before*
reading the pointer, which is incorrect.  This patch fixes this.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 21 insertions, 15 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 739383cd3f70..322d18dc17f0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3274,12 +3274,14 @@ void memcg_register_cache(struct kmem_cache *s)
         list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);
         mutex_unlock(&memcg->slab_caches_mutex);
 
-        root->memcg_params->memcg_caches[id] = s;
         /*
-         * the readers won't lock, make sure everybody sees the updated value,
-         * so they won't put stuff in the queue again for no reason
+         * Since readers won't lock (see cache_from_memcg_idx()), we need a
+         * barrier here to ensure nobody will see the kmem_cache partially
+         * initialized.
          */
-        wmb();
+        smp_wmb();
+
+        root->memcg_params->memcg_caches[id] = s;
 }
 
 void memcg_unregister_cache(struct kmem_cache *s)
@@ -3605,7 +3607,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
                                           gfp_t gfp)
 {
         struct mem_cgroup *memcg;
-        int idx;
+        struct kmem_cache *memcg_cachep;
 
         VM_BUG_ON(!cachep->memcg_params);
         VM_BUG_ON(!cachep->memcg_params->is_root_cache);
@@ -3619,15 +3621,9 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
         if (!memcg_can_account_kmem(memcg))
                 goto out;
 
-        idx = memcg_cache_id(memcg);
-
-        /*
-         * barrier to mare sure we're always seeing the up to date value.  The
-         * code updating memcg_caches will issue a write barrier to match this.
-         */
-        read_barrier_depends();
-        if (likely(cache_from_memcg_idx(cachep, idx))) {
-                cachep = cache_from_memcg_idx(cachep, idx);
+        memcg_cachep = cache_from_memcg_idx(cachep, memcg_cache_id(memcg));
+        if (likely(memcg_cachep)) {
+                cachep = memcg_cachep;
                 goto out;
         }
 
diff --git a/mm/slab.h b/mm/slab.h
index 0859c4241ba1..72d1f9df71bd 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -163,9 +163,19 @@ static inline const char *cache_name(struct kmem_cache *s)
 static inline struct kmem_cache *
 cache_from_memcg_idx(struct kmem_cache *s, int idx)
 {
+        struct kmem_cache *cachep;
+
         if (!s->memcg_params)
                 return NULL;
-        return s->memcg_params->memcg_caches[idx];
+        cachep = s->memcg_params->memcg_caches[idx];
+
+        /*
+         * Make sure we will access the up-to-date value. The code updating
+         * memcg_caches issues a write barrier to match this (see
+         * memcg_register_cache()).
+         */
+        smp_read_barrier_depends();
+        return cachep;
 }
 
 static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)