From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f72.google.com (mail-wm0-f72.google.com [74.125.82.72]) by kanga.kvack.org (Postfix) with ESMTP id 87C536B0038 for ; Wed, 7 Dec 2016 04:30:02 -0500 (EST) Received: by mail-wm0-f72.google.com with SMTP id w13so34921872wmw.0 for ; Wed, 07 Dec 2016 01:30:02 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id n6si23587022wjk.207.2016.12.07.01.30.01 for (version=TLS1 cipher=AES128-SHA bits=128/128); Wed, 07 Dec 2016 01:30:01 -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> <7b74a021-e472-a21e-7936-6741e07906b5@suse.cz> <20161207085809.GD17136@dhcp22.suse.cz> From: Vlastimil Babka Message-ID: Date: Wed, 7 Dec 2016 10:29:44 +0100 MIME-Version: 1.0 In-Reply-To: <20161207085809.GD17136@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 Cc: Xishi Qiu , Andrew Morton , Mel Gorman , Yaowei Bai , Christian Borntraeger , Linux MM , LKML , Yisheng Xie 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. > diff --git a/mm/mmzone.c b/mm/mmzone.c > index 5652be858e5e..b4e093dd24c1 100644 > --- a/mm/mmzone.c > +++ b/mm/mmzone.c > @@ -102,10 +102,10 @@ int page_cpupid_xchg_last(struct page *page, int cpupid) > int last_cpupid; > > do { > - old_flags = flags = page->flags; > + old_flags = READ_ONCE(page->flags); > last_cpupid = page_cpupid_last(page); > > - flags &= ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); > + flags = old_flags & ~(LAST_CPUPID_MASK << LAST_CPUPID_PGSHIFT); > flags |= (cpupid & LAST_CPUPID_MASK) << LAST_CPUPID_PGSHIFT; > } while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags)); > > >>> 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