From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f50.google.com (mail-wm0-f50.google.com [74.125.82.50]) by kanga.kvack.org (Postfix) with ESMTP id E2DAA6B0005 for ; Wed, 30 Mar 2016 07:10:48 -0400 (EDT) Received: by mail-wm0-f50.google.com with SMTP id p65so66354439wmp.0 for ; Wed, 30 Mar 2016 04:10:48 -0700 (PDT) Received: from mail-wm0-f67.google.com (mail-wm0-f67.google.com. [74.125.82.67]) by mx.google.com with ESMTPS id w4si4398279wje.208.2016.03.30.04.10.45 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 30 Mar 2016 04:10:46 -0700 (PDT) Received: by mail-wm0-f67.google.com with SMTP id i204so11927732wmd.0 for ; Wed, 30 Mar 2016 04:10:45 -0700 (PDT) Date: Wed, 30 Mar 2016 13:10:44 +0200 From: Michal Hocko 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> Sender: owner-linux-mm@kvack.org List-ID: 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 On Wed 30-03-16 18:51:12, Hekuang wrote: > hi > > a?? 2016/3/30 18:38, Mel Gorman a??e??: > >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 -- 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