From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ee0-f43.google.com (mail-ee0-f43.google.com [74.125.83.43]) by kanga.kvack.org (Postfix) with ESMTP id CFE956B0039 for ; Thu, 19 Dec 2013 04:21:39 -0500 (EST) Received: by mail-ee0-f43.google.com with SMTP id c13so326846eek.2 for ; Thu, 19 Dec 2013 01:21:39 -0800 (PST) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id u49si3476973eep.43.2013.12.19.01.21.39 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Thu, 19 Dec 2013 01:21:39 -0800 (PST) Date: Thu, 19 Dec 2013 10:21:37 +0100 From: Michal Hocko Subject: Re: [PATCH 3/6] memcg, slab: cleanup barrier usage when accessing memcg_caches Message-ID: <20131219092137.GG9331@dhcp22.suse.cz> References: <6f02b2d079ffd0990ae335339c803337b13ecd8c.1387372122.git.vdavydov@parallels.com> <20131218171411.GD31080@dhcp22.suse.cz> <52B29427.9010909@parallels.com> <20131219091007.GC9331@dhcp22.suse.cz> <52B2B951.5080809@parallels.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <52B2B951.5080809@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 13:16:01, Vladimir Davydov wrote: > On 12/19/2013 01:10 PM, Michal Hocko wrote: > > On Thu 19-12-13 10:37:27, Vladimir Davydov wrote: > >> 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] > >> <...> > > Why do we need explicit memory barriers when we can use RCU? > > __memcg_kmem_get_cache already dereferences within rcu_read_lock. > > Because it's not RCU, IMO. RCU implies freeing the old version after a > grace period, while kmem_caches are freed immediately. We simply want to > be sure the kmem_cache is fully initialized. And we do not require > calling this in an RCU critical section. And you can use rcu_dereference and rcu_assign for that as well. It hides all the juicy details about memory barriers. Besides that nothing prevents us from freeing from rcu callback. Or? > > Btw. cache_from_memcg_idx is desperately asking for a comment about > > required locking. > > Actually, I placed a reference to the comment there ;-) but no problem, > I move it to cache_from_memcg_idx(). > > 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