Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753753AbZFZA7d (ORCPT ); Thu, 25 Jun 2009 20:59:33 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752119AbZFZA7R (ORCPT ); Thu, 25 Jun 2009 20:59:17 -0400 Received: from fgwmail7.fujitsu.co.jp ([192.51.44.37]:44535 "EHLO fgwmail7.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751878AbZFZA7Q (ORCPT ); Thu, 25 Jun 2009 20:59:16 -0400 Date: Fri, 26 Jun 2009 09:57:45 +0900 From: KAMEZAWA Hiroyuki To: KAMEZAWA Hiroyuki Cc: Andrew Morton , balbir@linux.vnet.ibm.com, kamezawa.hiroyuki@jp.fujitsu.com, nishimura@mxp.nes.nec.co.jp, lizf@cn.fujitsu.com, menage@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH] memcg: add commens for expaing memory barrier (Was Re: Low overhead patches for the memory cgroup controller (v5) Message-Id: <20090626095745.01cef410.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20090623090116.556d4f97.kamezawa.hiroyu@jp.fujitsu.com> References: <20090615043900.GF23577@balbir.in.ibm.com> <20090622154343.9cdbf23a.akpm@linux-foundation.org> <20090623090116.556d4f97.kamezawa.hiroyu@jp.fujitsu.com> Organization: FUJITSU Co. LTD. X-Mailer: Sylpheed 2.5.0 (GTK+ 2.10.14; i686-pc-mingw32) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1739 Lines: 52 On Tue, 23 Jun 2009 09:01:16 +0900 KAMEZAWA Hiroyuki wrote: > > Do we still need the smp_wmb()? > > > > It's hard to say, because we forgot to document it :( > > > Sorry for lack of documentation. > > pc->mem_cgroup should be visible before SetPageCgroupUsed(). Othrewise, > A routine believes USED bit will see bad pc->mem_cgroup. > > I'd like to add a comment later (againt new mmotm.) > Ok, it's now. == From: KAMEZAWA Hiroyuki Add comments for the reason of smp_wmb() in mem_cgroup_commit_charge(). Cc: Daisuke Nishimura Cc: Balbir Singh Signed-off-by: KAMEZAWA Hiroyuki --- mm/memcontrol.c | 7 +++++++ 1 file changed, 7 insertions(+) Index: mmotm-2.6.31-Jun25/mm/memcontrol.c =================================================================== --- mmotm-2.6.31-Jun25.orig/mm/memcontrol.c +++ mmotm-2.6.31-Jun25/mm/memcontrol.c @@ -1134,6 +1134,13 @@ static void __mem_cgroup_commit_charge(s } pc->mem_cgroup = mem; + /* + * We access a page_cgroup asynchronously without lock_page_cgroup(). + * Especially when a page_cgroup is taken from a page, pc->mem_cgroup + * is accessed after testing USED bit. To make pc->mem_cgroup visible + * before USED bit, we need memory barrier here. + * See mem_cgroup_add_lru_list(), etc. + */ smp_wmb(); switch (ctype) { case MEM_CGROUP_CHARGE_TYPE_CACHE: -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/