From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx137.postini.com [74.125.245.137]) by kanga.kvack.org (Postfix) with SMTP id 534C66B004D for ; Tue, 17 Jan 2012 19:08:14 -0500 (EST) Received: from m1.gw.fujitsu.co.jp (unknown [10.0.50.71]) by fgwmail6.fujitsu.co.jp (Postfix) with ESMTP id 7A5493EE0BC for ; Wed, 18 Jan 2012 09:08:12 +0900 (JST) Received: from smail (m1 [127.0.0.1]) by outgoing.m1.gw.fujitsu.co.jp (Postfix) with ESMTP id 63E6045DE5D for ; Wed, 18 Jan 2012 09:08:12 +0900 (JST) Received: from s1.gw.fujitsu.co.jp (s1.gw.fujitsu.co.jp [10.0.50.91]) by m1.gw.fujitsu.co.jp (Postfix) with ESMTP id 4724745DE5B for ; Wed, 18 Jan 2012 09:08:12 +0900 (JST) Received: from s1.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s1.gw.fujitsu.co.jp (Postfix) with ESMTP id 3AF0F1DB8045 for ; Wed, 18 Jan 2012 09:08:12 +0900 (JST) Received: from m107.s.css.fujitsu.com (m107.s.css.fujitsu.com [10.240.81.147]) by s1.gw.fujitsu.co.jp (Postfix) with ESMTP id E23BC1DB8044 for ; Wed, 18 Jan 2012 09:08:11 +0900 (JST) Date: Wed, 18 Jan 2012 09:06:56 +0900 From: KAMEZAWA Hiroyuki Subject: Re: [RFC] [PATCH 2/7 v2] memcg: add memory barrier for checking account move. Message-Id: <20120118090656.83268b3e.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20120117152635.GA22142@tiehlicka.suse.cz> References: <20120113173001.ee5260ca.kamezawa.hiroyu@jp.fujitsu.com> <20120113173347.6231f510.kamezawa.hiroyu@jp.fujitsu.com> <20120117152635.GA22142@tiehlicka.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: "linux-mm@kvack.org" , Ying Han , "hugh.dickins@tiscali.co.uk" , "hannes@cmpxchg.org" , cgroups@vger.kernel.org, "bsingharora@gmail.com" 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. Here, cpu B needs to read most recently updated value. Thanks, -Kame -- 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