From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752468AbcC3LK6 (ORCPT ); Wed, 30 Mar 2016 07:10:58 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:34143 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751723AbcC3LK5 (ORCPT ); Wed, 30 Mar 2016 07:10:57 -0400 Date: Wed, 30 Mar 2016 13:10:44 +0200 From: Michal Hocko To: Hekuang Cc: Mel Gorman , akpm@linux-foundation.org, vbabka@suse.cz, rientjes@google.com, cody@linux.vnet.ibm.com, gilad@benyossef.com, kosaki.motohiro@gmail.com, mgorman@suse.de, penberg@kernel.org, lizefan@huawei.com, wangnan0@huawei.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] Revert "mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE" Message-ID: <20160330111044.GA4324@dhcp22.suse.cz> References: <1459333327-89720-1-git-send-email-hekuang@huawei.com> <20160330103839.GA4773@techsingularity.net> <56FBAFA0.3010604@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <56FBAFA0.3010604@huawei.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 30-03-16 18:51:12, Hekuang wrote: > hi > > 在 2016/3/30 18:38, Mel Gorman 写道: > >On Wed, Mar 30, 2016 at 10:22:07AM +0000, He Kuang wrote: > >>This reverts commit 998d39cb236fe464af86a3492a24d2f67ee1efc2. > >> > >>When local irq is disabled, a percpu variable does not change, so we can > >>remove the access macros and let the compiler optimize the code safely. > >> > >batch can be changed from other contexts. Why is this safe? > > > I've mistakenly thought that per_cpu variable can only be accessed by that > cpu. git blame would point you to 998d39cb236f ("mm/page_alloc: protect pcp->batch accesses with ACCESS_ONCE"). I haven't looked into the code deeply to confirm this is still the case but it would be a good lead that this is not that simple. ACCESS_ONCE resp. {READ,WRITE}_ONCE are usually quite subtle so I would encourage you or anybody else who try to remove them to study the code and the history deeper before removing them. -- Michal Hocko SUSE Labs