From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752263AbcLEJoZ (ORCPT ); Mon, 5 Dec 2016 04:44:25 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:58551 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751244AbcLEJoR (ORCPT ); Mon, 5 Dec 2016 04:44:17 -0500 Subject: Re: [RFC PATCH v2] mm: use ACCESS_ONCE in page_cpupid_xchg_last() To: Xishi Qiu , Andrew Morton , Mel Gorman , Yaowei Bai References: <584523E4.9030600@huawei.com> <26c66f28-d836-4d6e-fb40-3e2189a540ed@de.ibm.com> <0cc3c2bb-e292-2d7b-8d44-16c8e6c19899@de.ibm.com> <584532DF.7080805@huawei.com> Cc: Linux MM , LKML , Yisheng Xie From: Christian Borntraeger Date: Mon, 5 Dec 2016 10:44:09 +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: <584532DF.7080805@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: 16120509-0044-0000-0000-000001F08DED X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006197; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000194; SDB=6.00789502; UDB=6.00382230; IPR=6.00567242; 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=3.00013539; XFM=3.00000011; UTC=2016-12-05 09:44:14 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16120509-0045-0000-0000-0000061C98D4 Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-12-05_06:,, 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-1612050173 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/05/2016 10:26 AM, Xishi Qiu wrote: > A compiler could re-read "old_flags" from the memory location after reading > and calculation "flags" and passes a newer value into the cmpxchg making > the comparison succeed while it should actually fail. > > Signed-off-by: Xishi Qiu > Suggested-by: Christian Borntraeger > --- > 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); please use READ_ONCE. > last_cpupid = page_cpupid_last(page); > > flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); >