From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752771AbbJFURF (ORCPT ); Tue, 6 Oct 2015 16:17:05 -0400 Received: from terminus.zytor.com ([198.137.202.10]:38162 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752337AbbJFURE (ORCPT ); Tue, 6 Oct 2015 16:17:04 -0400 Subject: Re: [PATCH] x86: cmpxchg_double: Add missing memory clobber To: Pranith Kumar , Thomas Gleixner , Ingo Molnar , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" References: <1444157670-19951-1-git-send-email-bobby.prani@gmail.com> From: "H. Peter Anvin" Message-ID: <56142C2E.20308@zytor.com> Date: Tue, 6 Oct 2015 13:16:46 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: <1444157670-19951-1-git-send-email-bobby.prani@gmail.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/06/2015 11:54 AM, Pranith Kumar wrote: > We are reading from memory locations pointed to by p1 and p2 in the asm > block. Add a memory clobber flag to make gcc aware of this. > > Signed-off-by: Pranith Kumar > --- > arch/x86/include/asm/cmpxchg.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h > index 4a2e5bc..3e83949 100644 > --- a/arch/x86/include/asm/cmpxchg.h > +++ b/arch/x86/include/asm/cmpxchg.h > @@ -214,7 +214,8 @@ extern void __add_wrong_size(void) > : "=a" (__ret), "+d" (__old2), \ > "+m" (*(p1)), "+m" (*(p2)) \ > : "i" (2 * sizeof(long)), "a" (__old1), \ > - "b" (__new1), "c" (__new2)); \ > + "b" (__new1), "c" (__new2) \ > + : "memory"); \ > __ret; \ > }) NAK. We already have the "+m" for exactly this reason; adding an explicit memory clobber should only be used to prevent movement of *other* memory operations around this one (i.e. a barrier). -hpa