From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f197.google.com (mail-pf0-f197.google.com [209.85.192.197]) by kanga.kvack.org (Postfix) with ESMTP id A462A6B0038 for ; Wed, 7 Dec 2016 04:40:55 -0500 (EST) Received: by mail-pf0-f197.google.com with SMTP id c4so593237311pfb.7 for ; Wed, 07 Dec 2016 01:40:55 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id z3si23365727pfd.61.2016.12.07.01.40.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 07 Dec 2016 01:40:54 -0800 (PST) Received: from pps.filterd (m0098394.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uB79cgIU006468 for ; Wed, 7 Dec 2016 04:40:54 -0500 Received: from e36.co.us.ibm.com (e36.co.us.ibm.com [32.97.110.154]) by mx0a-001b2d01.pphosted.com with ESMTP id 276arh9f5d-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 07 Dec 2016 04:40:54 -0500 Received: from localhost by e36.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 7 Dec 2016 02:40:53 -0700 Subject: Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last() 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> From: Christian Borntraeger Date: Wed, 7 Dec 2016 10:40:47 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Message-Id: Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka , Michal Hocko Cc: Xishi Qiu , Andrew Morton , Mel Gorman , Yaowei Bai , Linux MM , LKML , Yisheng Xie 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 >>>>> >>>> >> > -- 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