From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wj0-f200.google.com (mail-wj0-f200.google.com [209.85.210.200]) by kanga.kvack.org (Postfix) with ESMTP id 482BC6B0038 for ; Wed, 7 Dec 2016 04:59:47 -0500 (EST) Received: by mail-wj0-f200.google.com with SMTP id bk3so81832869wjc.4 for ; Wed, 07 Dec 2016 01:59:47 -0800 (PST) Received: from mail-wm0-f67.google.com (mail-wm0-f67.google.com. [74.125.82.67]) by mx.google.com with ESMTPS id y130si7651190wmc.29.2016.12.07.01.59.45 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 07 Dec 2016 01:59:45 -0800 (PST) Received: by mail-wm0-f67.google.com with SMTP id g23so26770828wme.1 for ; Wed, 07 Dec 2016 01:59:45 -0800 (PST) Date: Wed, 7 Dec 2016 10:59:44 +0100 From: Michal Hocko Subject: Re: [RFC PATCH v3] mm: use READ_ONCE in page_cpupid_xchg_last() Message-ID: <20161207095943.GF17136@dhcp22.suse.cz> References: <584523E4.9030600@huawei.com> <58461A0A.3070504@huawei.com> <20161207084305.GA20350@dhcp22.suse.cz> <7b74a021-e472-a21e-7936-6741e07906b5@suse.cz> <20161207085809.GD17136@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Christian Borntraeger Cc: Vlastimil Babka , Xishi Qiu , Andrew Morton , Mel Gorman , Yaowei Bai , Linux MM , LKML , Yisheng Xie On Wed 07-12-16 10:40:47, Christian Borntraeger wrote: > On 12/07/2016 10:29 AM, Vlastimil Babka wrote: > > On 12/07/2016 09:58 AM, Michal Hocko wrote: > >> On Wed 07-12-16 09:48:52, Vlastimil Babka wrote: > >>> 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? > >> > >> But those are two different variables which we assign to so what > >> prevents the compiler from applying READ_ONCE on each of them > >> separately? > > > > I would naively expect that it's assigned to flags first, and then from > > flags to old_flags. But I don't know exactly the C standard evaluation > > rules that apply here. > > > >> Anyway, this could be addressed easily by > > > > Yes, that way there should be no doubt. > > That change would make it clearer, but the code is correct anyway, > as assignments in C are done from right to left, so > old_flags = flags = READ_ONCE(page->flags); > > is equivalent to > > flags = READ_ONCE(page->flags); > old_flags = flags; OK, I guess you are right. For some reason I thought that the compiler is free to bypass flags and split an assignment a = b = c; into b = c; a = c which would still follow from right to left rule. I guess I am over speculating here though, so sorry for the noise. -- Michal Hocko SUSE Labs -- 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