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 873686B025E for ; Mon, 5 Dec 2016 04:24:44 -0500 (EST) Received: by mail-wj0-f197.google.com with SMTP id xr1so61046522wjb.7 for ; Mon, 05 Dec 2016 01:24:44 -0800 (PST) Received: from szxga03-in.huawei.com (szxga03-in.huawei.com. [119.145.14.66]) by mx.google.com with ESMTPS id n6si14201475wjk.207.2016.12.05.01.24.41 for (version=TLS1 cipher=AES128-SHA bits=128/128); Mon, 05 Dec 2016 01:24:43 -0800 (PST) Message-ID: <584531CF.9030204@huawei.com> Date: Mon, 5 Dec 2016 17:22:23 +0800 From: Xishi Qiu MIME-Version: 1.0 Subject: Re: [RFC PATCH] mm: use ACCESS_ONCE in page_cpupid_xchg_last() References: <584523E4.9030600@huawei.com> <26c66f28-d836-4d6e-fb40-3e2189a540ed@de.ibm.com> <0cc3c2bb-e292-2d7b-8d44-16c8e6c19899@de.ibm.com> In-Reply-To: <0cc3c2bb-e292-2d7b-8d44-16c8e6c19899@de.ibm.com> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Christian Borntraeger Cc: Andrew Morton , Mel Gorman , Yaowei Bai , Linux MM , LKML , Yisheng Xie On 2016/12/5 16:50, Christian Borntraeger wrote: > On 12/05/2016 09:31 AM, Christian Borntraeger wrote: >> 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). >> > > Reading the code again, you might be right, but I think your patch description > is somewhat misleading. I think the problem is that old_flags and flags are > not necessarily the same. > > So what about > > 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. > Hi Christian, I'll resend v2, thanks! > > -- 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