From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751281AbdAQQht (ORCPT ); Tue, 17 Jan 2017 11:37:49 -0500 Received: from mail-pg0-f68.google.com ([74.125.83.68]:35050 "EHLO mail-pg0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751134AbdAQQhs (ORCPT ); Tue, 17 Jan 2017 11:37:48 -0500 Date: Tue, 17 Jan 2017 08:37:45 -0800 From: Tejun Heo To: Joonsoo Kim Cc: Vladimir Davydov , cl@linux.com, penberg@kernel.org, rientjes@google.com, akpm@linux-foundation.org, jsvana@fb.com, hannes@cmpxchg.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 2/9] slab: remove synchronous rcu_barrier() call in memcg cache release path Message-ID: <20170117163745.GA8352@mtj.duckdns.org> References: <20170114055449.11044-1-tj@kernel.org> <20170114055449.11044-3-tj@kernel.org> <20170114131939.GA2668@esperanza> <20170114151921.GA32693@mtj.duckdns.org> <20170117000754.GA25218@js1304-P5Q-DELUXE> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170117000754.GA25218@js1304-P5Q-DELUXE> User-Agent: Mutt/1.7.1 (2016-10-04) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Joonsoo. On Tue, Jan 17, 2017 at 09:07:54AM +0900, Joonsoo Kim wrote: > Long time no see! :) Yeah, happy new year! > IIUC, rcu_barrier() here prevents to destruct the kmem_cache until all > slab pages in it are freed. These slab pages are freed through call_rcu(). Hmm... why do we need that tho? SLAB_DESTROY_BY_RCU only needs to protect the slab pages, not kmem cache struct. I thought that this was because kmem cache destruction is allowed to release pages w/o RCU delaying it. > Your patch changes it to another call_rcu() and, I think, if sequence of > executing rcu callbacks is the same with sequence of adding rcu > callbacks, it would work. However, I'm not sure that it is > guaranteed by RCU API. Am I missing something? The call sequence doesn't matter. Whether you're using call_rcu() or rcu_barrier(), you're just waiting for a grace period to pass before continuing. It doens't give any other ordering guarantees, so the new code should be equivalent to the old one except for being asynchronous. Thanks. -- tejun