From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx165.postini.com [74.125.245.165]) by kanga.kvack.org (Postfix) with SMTP id EE2556B004F for ; Thu, 19 Jan 2012 04:28:36 -0500 (EST) Date: Thu, 19 Jan 2012 10:28:34 +0100 From: Michal Hocko Subject: Re: [RFC] [PATCH 2/7 v2] memcg: add memory barrier for checking account move. Message-ID: <20120119092833.GA13932@tiehlicka.suse.cz> References: <20120113173001.ee5260ca.kamezawa.hiroyu@jp.fujitsu.com> <20120113173347.6231f510.kamezawa.hiroyu@jp.fujitsu.com> <20120117152635.GA22142@tiehlicka.suse.cz> <20120118090656.83268b3e.kamezawa.hiroyu@jp.fujitsu.com> <20120118123759.GB31112@tiehlicka.suse.cz> <20120119111727.6337bde4.kamezawa.hiroyu@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120119111727.6337bde4.kamezawa.hiroyu@jp.fujitsu.com> Sender: owner-linux-mm@kvack.org List-ID: To: KAMEZAWA Hiroyuki Cc: "linux-mm@kvack.org" , Ying Han , "hugh.dickins@tiscali.co.uk" , "hannes@cmpxchg.org" , cgroups@vger.kernel.org, "bsingharora@gmail.com" On Thu 19-01-12 11:17:27, KAMEZAWA Hiroyuki wrote: > On Wed, 18 Jan 2012 13:37:59 +0100 > Michal Hocko wrote: > > > On Wed 18-01-12 09:06:56, KAMEZAWA Hiroyuki wrote: > > > On Tue, 17 Jan 2012 16:26:35 +0100 > > > Michal Hocko wrote: > > > > > > > On Fri 13-01-12 17:33:47, KAMEZAWA Hiroyuki wrote: > > > > > I think this bugfix is needed before going ahead. thoughts? > > > > > == > > > > > From 2cb491a41782b39aae9f6fe7255b9159ac6c1563 Mon Sep 17 00:00:00 2001 > > > > > From: KAMEZAWA Hiroyuki > > > > > Date: Fri, 13 Jan 2012 14:27:20 +0900 > > > > > Subject: [PATCH 2/7] memcg: add memory barrier for checking account move. > > > > > > > > > > At starting move_account(), source memcg's per-cpu variable > > > > > MEM_CGROUP_ON_MOVE is set. The page status update > > > > > routine check it under rcu_read_lock(). But there is no memory > > > > > barrier. This patch adds one. > > > > > > > > OK this would help to enforce that the CPU would see the current value > > > > but what prevents us from the race with the value update without the > > > > lock? This is as racy as it was before AFAICS. > > > > > > > > > > Hm, do I misunderstand ? > > > == > > > update reference > > > > > > CPU A CPU B > > > set value rcu_read_lock() > > > smp_wmb() smp_rmb() > > > read_value > > > rcu_read_unlock() > > > synchronize_rcu(). > > > == > > > I expect > > > If synchronize_rcu() is called before rcu_read_lock() => move_lock_xxx will be held. > > > If synchronize_rcu() is called after rcu_read_lock() => update will be delayed. > > > > Ahh, OK I can see it now. Readers are not that important because it is > > actually the updater who is delayed until all preexisting rcu read > > sections are finished. > > > > In that case. Why do we need both barriers? spin_unlock is a full > > barrier so maybe we just need smp_rmb before we read value to make sure > > that we do not get stalled value when we start rcu_read section after > > synchronize_rcu? > > > > I doubt .... If no barrier, this case happens > > == > update reference > CPU A CPU B > set value > synchronize_rcu() rcu_read_lock() > read_value <= find old value > rcu_read_unlock() > do no lock > == OK, I have looked at Documentation/memory-barriers.txt again and spin_unlock is not a full barrier so we cannot rely on it. Anyway you still could do the barrier once after we set all values? -- Michal Hocko SUSE Labs SUSE LINUX s.r.o. Lihovarska 1060/12 190 00 Praha 9 Czech Republic -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org