From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lb0-f173.google.com (mail-lb0-f173.google.com [209.85.217.173]) by kanga.kvack.org (Postfix) with ESMTP id C48236B0031 for ; Thu, 19 Dec 2013 04:16:15 -0500 (EST) Received: by mail-lb0-f173.google.com with SMTP id z5so335293lbh.32 for ; Thu, 19 Dec 2013 01:16:15 -0800 (PST) Received: from relay.parallels.com (relay.parallels.com. [195.214.232.42]) by mx.google.com with ESMTPS id 6si1282050lby.157.2013.12.19.01.16.14 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Thu, 19 Dec 2013 01:16:14 -0800 (PST) Message-ID: <52B2B951.5080809@parallels.com> Date: Thu, 19 Dec 2013 13:16:01 +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> <52B29427.9010909@parallels.com> <20131219091007.GC9331@dhcp22.suse.cz> In-Reply-To: <20131219091007.GC9331@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 12/19/2013 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. > 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. -- 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