From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wj0-f199.google.com (mail-wj0-f199.google.com [209.85.210.199]) by kanga.kvack.org (Postfix) with ESMTP id EDF066B0038 for ; Wed, 7 Dec 2016 03:49:08 -0500 (EST) Received: by mail-wj0-f199.google.com with SMTP id hb5so81130848wjc.2 for ; Wed, 07 Dec 2016 00:49:08 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id xs6si23455578wjc.244.2016.12.07.00.49.07 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 07 Dec 2016 00:49:07 -0800 (PST) Subject: Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last() References: <584523E4.9030600@huawei.com> <58461A0A.3070504@huawei.com> <20161207084305.GA20350@dhcp22.suse.cz> From: Vlastimil Babka Message-ID: <7b74a021-e472-a21e-7936-6741e07906b5@suse.cz> Date: Wed, 7 Dec 2016 09:48:52 +0100 MIME-Version: 1.0 In-Reply-To: <20161207084305.GA20350@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko , Xishi Qiu Cc: Andrew Morton , Mel Gorman , Yaowei Bai , Christian Borntraeger , Linux MM , LKML , Yisheng Xie 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 >> > -- 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