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>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 739383cd3f70d825c344d7d5f9a15672c25791eb..322d18dc17f0f9f5059923039ddcf9cd81be434b 100644 (file)
--- 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 0859c4241ba10b2ee602eca48af4e34dd82d6ae3..72d1f9df71bddba1463a1e4851918bb301fdad4c 100644 (file)
--- 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)