From 2edefe1155b3ad3dc92065f6e1018d363525296e Mon Sep 17 00:00:00 2001 From: Vladimir Davydov Date: Thu, 23 Jan 2014 15:53:02 -0800 Subject: memcg, slab: fix races in per-memcg cache creation/destruction We obtain a per-memcg cache from a root kmem_cache by dereferencing an entry of the root cache's memcg_params::memcg_caches array. If we find no cache for a memcg there on allocation, we initiate the memcg cache creation (see memcg_kmem_get_cache()). The cache creation proceeds asynchronously in memcg_create_kmem_cache() in order to avoid lock clashes, so there can be several threads trying to create the same kmem_cache concurrently, but only one of them may succeed. However, due to a race in the code, it is not always true. The point is that the memcg_caches array can be relocated when we activate kmem accounting for a memcg (see memcg_update_all_caches(), memcg_update_cache_size()). If memcg_update_cache_size() and memcg_create_kmem_cache() proceed concurrently as described below, we can leak a kmem_cache. Asume two threads schedule creation of the same kmem_cache. One of them successfully creates it. Another one should fail then, but if memcg_create_kmem_cache() interleaves with memcg_update_cache_size() as follows, it won't: memcg_create_kmem_cache() memcg_update_cache_size() (called w/o mutexes held) (called with slab_mutex, set_limit_mutex held) ------------------------- ------------------------- mutex_lock(&memcg_cache_mutex) s->memcg_params=kzalloc(...) new_cachep=cache_from_memcg_idx(cachep,idx) // new_cachep==NULL => proceed to creation s->memcg_params->memcg_caches[i] =cur_params->memcg_caches[i] // kmem_cache_create_memcg takes slab_mutex // so we will hang around until // memcg_update_cache_size finishes, but // nothing will prevent it from succeeding so // memcg_caches[idx] will be overwritten in // memcg_register_cache! new_cachep = kmem_cache_create_memcg(...) mutex_unlock(&memcg_cache_mutex) Let's fix this by moving the check for existence of the memcg cache to kmem_cache_create_memcg() to be called under the slab_mutex and make it return NULL if so. A similar race is possible when destroying a memcg cache (see kmem_cache_destroy()). Since memcg_unregister_cache(), which clears the pointer in the memcg_caches array, is called w/o protection, we can race with memcg_update_cache_size() and omit clearing the pointer. Therefore memcg_unregister_cache() should be moved before we release the slab_mutex. Signed-off-by: Vladimir Davydov Cc: Michal Hocko Cc: Glauber Costa Cc: Johannes Weiner Cc: Balbir Singh Cc: KAMEZAWA Hiroyuki Cc: Pekka Enberg Cc: Christoph Lameter Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/slab_common.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) (limited to 'mm/slab_common.c') diff --git a/mm/slab_common.c b/mm/slab_common.c index db24ec48b946..f34707eeacc7 100644 --- a/mm/slab_common.c +++ b/mm/slab_common.c @@ -180,6 +180,18 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size, if (err) goto out_unlock; + if (memcg) { + /* + * Since per-memcg caches are created asynchronously on first + * allocation (see memcg_kmem_get_cache()), several threads can + * try to create the same cache, but only one of them may + * succeed. Therefore if we get here and see the cache has + * already been created, we silently return NULL. + */ + if (cache_from_memcg_idx(parent_cache, memcg_cache_id(memcg))) + goto out_unlock; + } + /* * Some allocators will constraint the set of valid flags to a subset * of all flags. We expect them to define CACHE_CREATE_MASK in this @@ -261,11 +273,11 @@ void kmem_cache_destroy(struct kmem_cache *s) list_del(&s->list); if (!__kmem_cache_shutdown(s)) { + memcg_unregister_cache(s); mutex_unlock(&slab_mutex); if (s->flags & SLAB_DESTROY_BY_RCU) rcu_barrier(); - memcg_unregister_cache(s); memcg_free_cache_params(s); kfree(s->name); kmem_cache_free(kmem_cache, s); -- cgit v1.2.3