From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752644AbcLGJlL (ORCPT ); Wed, 7 Dec 2016 04:41:11 -0500 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:56960 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750813AbcLGJlK (ORCPT ); Wed, 7 Dec 2016 04:41:10 -0500 Subject: Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last() To: Vlastimil Babka , Michal Hocko References: <584523E4.9030600@huawei.com> <58461A0A.3070504@huawei.com> <20161207084305.GA20350@dhcp22.suse.cz> <7b74a021-e472-a21e-7936-6741e07906b5@suse.cz> <20161207085809.GD17136@dhcp22.suse.cz> Cc: Xishi Qiu , Andrew Morton , Mel Gorman , Yaowei Bai , Linux MM , LKML , Yisheng Xie From: Christian Borntraeger Date: Wed, 7 Dec 2016 10:40:47 +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: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Content-Scanned: Fidelis XPS MAILER x-cbid: 16120709-0020-0000-0000-00000A7397B8 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00006208; HX=3.00000240; KW=3.00000007; PH=3.00000004; SC=3.00000194; SDB=6.00790462; UDB=6.00382806; IPR=6.00568201; BA=6.00004950; NDR=6.00000001; ZLA=6.00000005; ZF=6.00000009; ZB=6.00000000; ZP=6.00000000; ZH=6.00000000; ZU=6.00000002; MB=3.00013563; XFM=3.00000011; UTC=2016-12-07 09:40:52 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 16120709-0021-0000-0000-000057E44F93 Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2016-12-07_02:,, 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-1612070167 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/07/2016 10:29 AM, Vlastimil Babka wrote: > On 12/07/2016 09:58 AM, Michal Hocko wrote: >> On Wed 07-12-16 09:48:52, Vlastimil Babka wrote: >>> On 12/07/2016 09:43 AM, Michal Hocko wrote: >>>> On Tue 06-12-16 09:53:14, 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 = READ_ONCE(page->flags); >>>>> last_cpupid = page_cpupid_last(page); >>>> >>>> what prevents compiler from doing? >>>> old_flags = READ_ONCE(page->flags); >>>> flags = READ_ONCE(page->flags); >>> >>> AFAIK, READ_ONCE tells the compiler that page->flags is volatile. It >>> can't read from volatile location more times than being told? >> >> But those are two different variables which we assign to so what >> prevents the compiler from applying READ_ONCE on each of them >> separately? > > I would naively expect that it's assigned to flags first, and then from > flags to old_flags. But I don't know exactly the C standard evaluation > rules that apply here. > >> Anyway, this could be addressed easily by > > Yes, that way there should be no doubt. That change would make it clearer, but the code is correct anyway, as assignments in C are done from right to left, so old_flags = flags = READ_ONCE(page->flags); is equivalent to flags = READ_ONCE(page->flags); old_flags = flags; > >> diff --git a/mm/mmzone.c b/mm/mmzone.c >> index 5652be858e5e..b4e093dd24c1 100644 >> --- a/mm/mmzone.c >> +++ b/mm/mmzone.c >> @@ -102,10 +102,10 @@ int page_cpupid_xchg_last(struct page *page, int cpupid) >> int last_cpupid; >> >> do { >> - old_flags = flags = page->flags; >> + old_flags = READ_ONCE(page->flags); >> last_cpupid = page_cpupid_last(page); >> >> - flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); >> + flags = old_flags & ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); >> flags |= (cpupid & LAST_CPUPID_MASK) << LAST_CPUPID_PGSHIFT; >> } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags)); >> >> >>>> Or this doesn't matter? >>> >>> I think it would matter. >>> >>>>> >>>>> flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); >>>>> -- >>>>> 1.8.3.1 >>>>> >>>> >> >