From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752575AbcLGItK (ORCPT ); Wed, 7 Dec 2016 03:49:10 -0500 Received: from mx2.suse.de ([195.135.220.15]:34961 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752243AbcLGItJ (ORCPT ); Wed, 7 Dec 2016 03:49:09 -0500 Subject: Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last() To: Michal Hocko , Xishi Qiu References: <584523E4.9030600@huawei.com> <58461A0A.3070504@huawei.com> <20161207084305.GA20350@dhcp22.suse.cz> Cc: Andrew Morton , Mel Gorman , Yaowei Bai , Christian Borntraeger , Linux MM , LKML , Yisheng Xie From: Vlastimil Babka Message-ID: <7b74a021-e472-a21e-7936-6741e07906b5@suse.cz> Date: Wed, 7 Dec 2016 09:48:52 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.0 MIME-Version: 1.0 In-Reply-To: <20161207084305.GA20350@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? > Or this doesn't matter? I think it would matter. >> >> flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); >> -- >> 1.8.3.1 >> >