From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id 8603E6B0055 for ; Thu, 25 Jun 2009 20:57:31 -0400 (EDT) Received: from m6.gw.fujitsu.co.jp ([10.0.50.76]) by fgwmail5.fujitsu.co.jp (Fujitsu Gateway) with ESMTP id n5Q0xJox004748 for (envelope-from kamezawa.hiroyu@jp.fujitsu.com); Fri, 26 Jun 2009 09:59:19 +0900 Received: from smail (m6 [127.0.0.1]) by outgoing.m6.gw.fujitsu.co.jp (Postfix) with ESMTP id E008E45DE53 for ; Fri, 26 Jun 2009 09:59:18 +0900 (JST) Received: from s6.gw.fujitsu.co.jp (s6.gw.fujitsu.co.jp [10.0.50.96]) by m6.gw.fujitsu.co.jp (Postfix) with ESMTP id BD30545DE50 for ; Fri, 26 Jun 2009 09:59:18 +0900 (JST) Received: from s6.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s6.gw.fujitsu.co.jp (Postfix) with ESMTP id 9AB021DB803A for ; Fri, 26 Jun 2009 09:59:18 +0900 (JST) Received: from m108.s.css.fujitsu.com (m108.s.css.fujitsu.com [10.249.87.108]) by s6.gw.fujitsu.co.jp (Postfix) with ESMTP id 555A51DB8038 for ; Fri, 26 Jun 2009 09:59:18 +0900 (JST) Date: Fri, 26 Jun 2009 09:57:45 +0900 From: KAMEZAWA Hiroyuki 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> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org 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 List-ID: 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, 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