From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wj0-f197.google.com (mail-wj0-f197.google.com [209.85.210.197]) by kanga.kvack.org (Postfix) with ESMTP id ACDED6B0038 for ; Mon, 5 Dec 2016 03:31:27 -0500 (EST) Received: by mail-wj0-f197.google.com with SMTP id he10so19963769wjc.6 for ; Mon, 05 Dec 2016 00:31:27 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id p123si11849323wmg.154.2016.12.05.00.31.26 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 05 Dec 2016 00:31:26 -0800 (PST) Received: from pps.filterd (m0098419.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uB58TStu112469 for ; Mon, 5 Dec 2016 03:31:25 -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 2750kgf4bg-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 05 Dec 2016 03:31:24 -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 01:31:24 -0700 Subject: Re: [RFC PATCH] mm: use ACCESS_ONCE in page_cpupid_xchg_last() References: <584523E4.9030600@huawei.com> From: Christian Borntraeger Date: Mon, 5 Dec 2016 09:31:18 +0100 MIME-Version: 1.0 In-Reply-To: <584523E4.9030600@huawei.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Message-Id: <26c66f28-d836-4d6e-fb40-3e2189a540ed@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 09:23 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). -- 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