[PATCH 3/6] memcg, slab: cleanup barrier usage when accessing memcg_caches

[Vladimir Davydov <vdavydov@parallels.com>]

First, in memcg_create_kmem_cache() we should issue the write barrier
after the kmem_cache is initialized, but before storing the pointer to
it in its parent's memcg_params.

Second, we should always issue the read barrier after
cache_from_memcg_idx() to conform with the write barrier.

Third, its better to use smp_* versions of barriers, because we don't
need them on UP systems.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c |   24 ++++++++++--------------
 mm/slab.h       |    6 +++++-
 2 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e6ad6ff..e37fdb5 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3429,12 +3429,14 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
 
 	atomic_set(&new_cachep->memcg_params->nr_pages , 0);
 
-	cachep->memcg_params->memcg_caches[idx] = new_cachep;
 	/*
-	 * 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();
+
+	cachep->memcg_params->memcg_caches[idx] = new_cachep;
 out:
 	mutex_unlock(&memcg_cache_mutex);
 	return new_cachep;
@@ -3573,7 +3575,7 @@ 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);
@@ -3587,15 +3589,9 @@ 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 0859c42..1d8b53f 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -163,9 +163,13 @@ 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];
+	smp_read_barrier_depends();	/* see memcg_register_cache() */
+	return cachep;
 }
 
 static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
-- 
1.7.10.4

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