From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f49.google.com (mail-ee0-f49.google.com [74.125.83.49]) by kanga.kvack.org (Postfix) with ESMTP id DAFFB6B0031 for ; Thu, 19 Dec 2013 04:43:34 -0500 (EST) Received: by mail-ee0-f49.google.com with SMTP id c41so341476eek.8 for ; Thu, 19 Dec 2013 01:43:34 -0800 (PST) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id r9si3518585eeo.107.2013.12.19.01.43.34 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Thu, 19 Dec 2013 01:43:34 -0800 (PST) Date: Thu, 19 Dec 2013 10:43:33 +0100 From: Michal Hocko Subject: Re: [PATCH 6/6] memcg, slab: RCU protect memcg_params for root caches Message-ID: <20131219094333.GB10855@dhcp22.suse.cz> References: <6f02b2d079ffd0990ae335339c803337b13ecd8c.1387372122.git.vdavydov@parallels.com> <20131219092836.GH9331@dhcp22.suse.cz> <52B2BE2A.2080509@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <52B2BE2A.2080509@parallels.com> Sender: owner-linux-mm@kvack.org List-ID: To: Vladimir Davydov 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 Thu 19-12-13 13:36:42, Vladimir Davydov wrote: > On 12/19/2013 01:28 PM, Michal Hocko wrote: > > On Wed 18-12-13 17:16:57, Vladimir Davydov wrote: [...] > >> diff --git a/mm/slab.h b/mm/slab.h > >> index 1d8b53f..53b81a9 100644 > >> --- a/mm/slab.h > >> +++ b/mm/slab.h > >> @@ -164,10 +164,16 @@ static inline struct kmem_cache * > >> cache_from_memcg_idx(struct kmem_cache *s, int idx) > >> { > >> struct kmem_cache *cachep; > >> + struct memcg_cache_params *params; > >> > >> if (!s->memcg_params) > >> return NULL; > >> - cachep = s->memcg_params->memcg_caches[idx]; > >> + > >> + rcu_read_lock(); > >> + params = rcu_dereference(s->memcg_params); > >> + cachep = params->memcg_caches[idx]; > >> + rcu_read_unlock(); > >> + > > Consumer has to be covered by the same rcu section otherwise > > memcg_params might be freed right after rcu unlock here. > > No. We protect only accesses to kmem_cache::memcg_params, which can > potentially be relocated for root caches. Hmm, ok. So memcg_params might change (a new memcg is accounted) but pointers at idx will be same, right? > But as soon as we get the > pointer to a kmem_cache from this array, we can freely dereference it, > because the cache cannot be freed when we use it. This is, because we > access a kmem_cache either under the slab_mutex or > memcg->slab_caches_mutex, or when we allocate/free from it. While doing > the latter, the cache can't go away, it would be a bug. IMO. That expects that cache_from_memcg_idx is always called with slab_mutex or slab_caches_mutex held, right? Please document it. > > Thanks. -- Michal Hocko SUSE Labs -- 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