Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752629AbcLGIjt (ORCPT ); Wed, 7 Dec 2016 03:39:49 -0500 Received: from mx2.suse.de ([195.135.220.15]:34101 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752110AbcLGIjs (ORCPT ); Wed, 7 Dec 2016 03:39:48 -0500 Subject: Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last() To: Xishi Qiu , Andrew Morton , Mel Gorman , Yaowei Bai , Christian Borntraeger References: <584523E4.9030600@huawei.com> <58461A0A.3070504@huawei.com> Cc: Linux MM , LKML , Yisheng Xie From: Vlastimil Babka Message-ID: <70a14036-a8e7-473f-3dc1-2517ffbe27e9@suse.cz> Date: Wed, 7 Dec 2016 09:39:32 +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: <58461A0A.3070504@huawei.com> 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 Content-Length: 912 Lines: 28 On 12/06/2016 02:53 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 Acked-by: Vlastimil Babka > --- > 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); > > flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); >