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authorVladimir Davydov <vdavydov@parallels.com>2014-01-23 15:52:59 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2014-01-23 16:36:51 -0800
commit959c8963fc6c8c9b97e80c55ce77105247040e7d (patch)
tree4f2bcf3e4a69f4a655fabf0fc178498f3d0c367f
parent1aa13254259bdef0bca723849ab3bab308d2f0c3 (diff)
downloadop-kernel-dev-959c8963fc6c8c9b97e80c55ce77105247040e7d.zip
op-kernel-dev-959c8963fc6c8c9b97e80c55ce77105247040e7d.tar.gz
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>
-rw-r--r--mm/memcontrol.c24
-rw-r--r--mm/slab.h12
2 files changed, 21 insertions, 15 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 739383c..322d18d 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 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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