From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751494AbcLEIb1 (ORCPT ); Mon, 5 Dec 2016 03:31:27 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:38908 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751039AbcLEIbZ (ORCPT ); Mon, 5 Dec 2016 03:31:25 -0500 Subject: Re: [RFC PATCH] mm: use ACCESS_ONCE in page_cpupid_xchg_last() To: Xishi Qiu , Andrew Morton , Mel Gorman , Yaowei Bai References: <584523E4.9030600@huawei.com> Cc: Linux MM , LKML , Yisheng Xie From: Christian Borntraeger Date: Mon, 5 Dec 2016 09:31:18 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0 MIME-Version: 1.0 In-Reply-To: <584523E4.9030600@huawei.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16120508-0012-0000-0000-0000114E3573 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006192; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000193; SDB=6.00789478; UDB=6.00382216; IPR=6.00567218; BA=6.00004938; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00013533; XFM=3.00000011; UTC=2016-12-05 08:31:23 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16120508-0013-0000-0000-000047BC0FCD Message-Id: <26c66f28-d836-4d6e-fb40-3e2189a540ed@de.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-12-05_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 suspectscore=0 malwarescore=0 phishscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1609300000 definitions=main-1612050154 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/05/2016 09:23 AM, Xishi Qiu wrote: > By reading the code, I find the following code maybe optimized by > compiler, maybe page->flags and old_flags use the same register, > so use ACCESS_ONCE in page_cpupid_xchg_last() to fix the problem. please use READ_ONCE instead of ACCESS_ONCE for future patches. > > Signed-off-by: Xishi Qiu > --- > mm/mmzone.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/mmzone.c b/mm/mmzone.c > index 5652be8..e0b698e 100644 > --- a/mm/mmzone.c > +++ b/mm/mmzone.c > @@ -102,7 +102,7 @@ int page_cpupid_xchg_last(struct page *page, int cpupid) > int last_cpupid; > > do { > - old_flags = flags = page->flags; > + old_flags = flags = ACCESS_ONCE(page->flags); > last_cpupid = page_cpupid_last(page); > > flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); I dont thing that this is actually a problem. The code below does } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags)) and the cmpxchg should be an atomic op that should already take care of everything (page->flags is passed as a pointer).