From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-la0-f49.google.com (mail-la0-f49.google.com [209.85.215.49]) by kanga.kvack.org (Postfix) with ESMTP id 05BA46B0031 for ; Thu, 19 Dec 2013 01:37:34 -0500 (EST) Received: by mail-la0-f49.google.com with SMTP id er20so255243lab.36 for ; Wed, 18 Dec 2013 22:37:34 -0800 (PST) Received: from relay.parallels.com (relay.parallels.com. [195.214.232.42]) by mx.google.com with ESMTPS id k8si1121088lag.49.2013.12.18.22.37.33 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Wed, 18 Dec 2013 22:37:33 -0800 (PST) Message-ID: <52B29427.9010909@parallels.com> Date: Thu, 19 Dec 2013 10:37:27 +0400 From: Vladimir Davydov MIME-Version: 1.0 Subject: Re: [PATCH 3/6] memcg, slab: cleanup barrier usage when accessing memcg_caches References: <6f02b2d079ffd0990ae335339c803337b13ecd8c.1387372122.git.vdavydov@parallels.com> <20131218171411.GD31080@dhcp22.suse.cz> In-Reply-To: <20131218171411.GD31080@dhcp22.suse.cz> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, devel@openvz.org, Johannes Weiner , Glauber Costa , Christoph Lameter , Pekka Enberg , Andrew Morton On 12/18/2013 09:14 PM, Michal Hocko wrote: > On Wed 18-12-13 17:16:54, Vladimir Davydov wrote: >> 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. > Please be (much) more verbose on Why. Barriers are tricky and should be > documented accordingly. So if you say that we should issue a barrier > always be specific why we should do it. In short, we have kmem_cache::memcg_params::memcg_caches is an array of pointers to per-memcg caches. We access it lock-free so we should use memory barriers during initialization. Obviously we should place a write barrier just before we set the pointer in order to make sure nobody will see a partially initialized structure. Besides there must be a read barrier between reading the pointer and accessing the structure, to conform with the write barrier. It's all that similar to rcu_assign and rcu_deref. Currently the barrier usage looks rather strange: memcg_create_kmem_cache: initialize kmem set the pointer in memcg_caches wmb() // ??? __memcg_kmem_get_cache: <...> read_barrier_depends() // ??? cachep = root_cache->memcg_params->memcg_caches[memcg_id] <...> Nothing prevents some archs from moving initialization after setting the pointer, or reading data before reading the pointer to it. Of course, I will include a detailed description in the next version of this patch. Thanks. >> Signed-off-by: Vladimir Davydov >> Cc: Michal Hocko >> Cc: Johannes Weiner >> Cc: Glauber Costa >> Cc: Christoph Lameter >> Cc: Pekka Enberg >> Cc: Andrew Morton >> --- >> 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 >> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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