From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id D442B6B0038 for ; Wed, 7 Dec 2016 05:03:36 -0500 (EST) Received: by mail-pf0-f198.google.com with SMTP id 17so594923696pfy.2 for ; Wed, 07 Dec 2016 02:03:36 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id 7si23410659pgh.30.2016.12.07.02.03.35 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 07 Dec 2016 02:03:36 -0800 (PST) Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.17/8.16.0.17) with SMTP id uB7A0eY5114964 for ; Wed, 7 Dec 2016 05:03:35 -0500 Received: from e37.co.us.ibm.com (e37.co.us.ibm.com [32.97.110.158]) by mx0a-001b2d01.pphosted.com with ESMTP id 276exh3q4f-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Wed, 07 Dec 2016 05:03:35 -0500 Received: from localhost by e37.co.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 7 Dec 2016 03:03:34 -0700 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> <20161207095943.GF17136@dhcp22.suse.cz> From: Christian Borntraeger Date: Wed, 7 Dec 2016 11:03:29 +0100 MIME-Version: 1.0 In-Reply-To: <20161207095943.GF17136@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Message-Id: <5d4accd3-e26b-d23f-5417-debe9ad7148a@de.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Vlastimil Babka , Xishi Qiu , Andrew Morton , Mel Gorman , Yaowei Bai , Linux MM , LKML , Yisheng Xie On 12/07/2016 10:59 AM, Michal Hocko wrote: > 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. Hmmm, just rereading C, I am no longer sure... I cannot find anything right now, that adds a sequence point in here. Still looking... -- 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