Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752237AbcLEJXv (ORCPT ); Mon, 5 Dec 2016 04:23:51 -0500 Received: from szxga03-in.huawei.com ([119.145.14.66]:30640 "EHLO szxga03-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751955AbcLEJXT (ORCPT ); Mon, 5 Dec 2016 04:23:19 -0500 Message-ID: <584531CF.9030204@huawei.com> Date: Mon, 5 Dec 2016 17:22:23 +0800 From: Xishi Qiu User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:12.0) Gecko/20120428 Thunderbird/12.0.1 MIME-Version: 1.0 To: Christian Borntraeger CC: Andrew Morton , Mel Gorman , Yaowei Bai , Linux MM , LKML , "Yisheng Xie" 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> <0cc3c2bb-e292-2d7b-8d44-16c8e6c19899@de.ibm.com> In-Reply-To: <0cc3c2bb-e292-2d7b-8d44-16c8e6c19899@de.ibm.com> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.25.179] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1781 Lines: 58 On 2016/12/5 16:50, Christian Borntraeger wrote: > 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. > Hi Christian, I'll resend v2, thanks! > >