From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id 57C006B0261 for ; Mon, 5 Dec 2016 03:50:12 -0500 (EST) Received: by mail-wm0-f72.google.com with SMTP id a20so15577982wme.5 for ; Mon, 05 Dec 2016 00:50:12 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id l66si11921397wma.114.2016.12.05.00.50.10 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 05 Dec 2016 00:50:11 -0800 (PST) Received: from pps.filterd (m0098414.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uB58mmif096778 for ; Mon, 5 Dec 2016 03:50:10 -0500 Received: from e35.co.us.ibm.com (e35.co.us.ibm.com [32.97.110.153]) by mx0b-001b2d01.pphosted.com with ESMTP id 27548h1xpy-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 05 Dec 2016 03:50:09 -0500 Received: from localhost by e35.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 5 Dec 2016 01:50:09 -0700 Subject: Re: [RFC PATCH] mm: use ACCESS_ONCE in page_cpupid_xchg_last() References: <584523E4.9030600@huawei.com> <26c66f28-d836-4d6e-fb40-3e2189a540ed@de.ibm.com> From: Christian Borntraeger Date: Mon, 5 Dec 2016 09:50:02 +0100 MIME-Version: 1.0 In-Reply-To: <26c66f28-d836-4d6e-fb40-3e2189a540ed@de.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Message-Id: <0cc3c2bb-e292-2d7b-8d44-16c8e6c19899@de.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Xishi Qiu , Andrew Morton , Mel Gorman , Yaowei Bai Cc: Linux MM , LKML , Yisheng Xie On 12/05/2016 09:31 AM, Christian Borntraeger wrote: > 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). > Reading the code again, you might be right, but I think your patch description is somewhat misleading. I think the problem is that old_flags and flags are not necessarily the same. So what about 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. -- 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