[PATCH RESEND 04/11] memcg, slab: fix barrier usage when accessing memcg_caches

From: Vladimir Davydov
Date: Mon Jan 06 2014 - 03:45:47 EST

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

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@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxx>
Cc: Glauber Costa <glommer@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Balbir Singh <bsingharora@xxxxxxxxx>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
mm/memcontrol.c | 24 ++++++++++--------------
mm/slab.h | 12 +++++++++++-
2 files changed, 21 insertions(+), 15 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f8eb994..999e7d4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3238,12 +3238,14 @@ void memcg_register_cache(struct kmem_cache *s)
list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);

- 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)
@@ -3569,7 +3571,7 @@ 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;

@@ -3583,15 +3585,9 @@ 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..72d1f9d 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)

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