From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lb0-f169.google.com (mail-lb0-f169.google.com [209.85.217.169]) by kanga.kvack.org (Postfix) with ESMTP id 63FA66B0055 for ; Mon, 6 Jan 2014 03:45:40 -0500 (EST) Received: by mail-lb0-f169.google.com with SMTP id u14so9645977lbd.14 for ; Mon, 06 Jan 2014 00:45:39 -0800 (PST) Received: from relay.parallels.com (relay.parallels.com. [195.214.232.42]) by mx.google.com with ESMTPS id y9si35743164laa.85.2014.01.06.00.45.38 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Mon, 06 Jan 2014 00:45:39 -0800 (PST) From: Vladimir Davydov Subject: [PATCH RESEND 04/11] memcg, slab: fix barrier usage when accessing memcg_caches Date: Mon, 6 Jan 2014 12:44:55 +0400 Message-ID: In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain Sender: owner-linux-mm@kvack.org List-ID: To: mhocko@suse.cz, akpm@linux-foundation.org Cc: glommer@gmail.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, devel@openvz.org, Johannes Weiner , Balbir Singh , KAMEZAWA Hiroyuki , Pekka Enberg , Christoph Lameter 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 Cc: Michal Hocko Cc: Glauber Costa Cc: Johannes Weiner Cc: Balbir Singh Cc: KAMEZAWA Hiroyuki Cc: Pekka Enberg Cc: Christoph Lameter Cc: Andrew Morton --- 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); 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) @@ -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; VM_BUG_ON(!cachep->memcg_params); VM_BUG_ON(!cachep->memcg_params->is_root_cache); @@ -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) -- 1.7.10.4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org