Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934342AbXHLJsv (ORCPT ); Sun, 12 Aug 2007 05:48:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1765250AbXHLJoK (ORCPT ); Sun, 12 Aug 2007 05:44:10 -0400 Received: from mtagate6.de.ibm.com ([195.212.29.155]:1312 "EHLO mtagate6.de.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1764990AbXHLJoA (ORCPT ); Sun, 12 Aug 2007 05:44:00 -0400 Subject: Re: [PATCH] make atomic_t volatile on all architectures From: Martin Schwidefsky Reply-To: schwidefsky@de.ibm.com To: Segher Boessenkool Cc: wjiang@resilience.com, Linus Torvalds , wensong@linux-vs.org, heiko.carstens@de.ibm.com, linux-kernel@vger.kernel.org, ak@suse.de, cfriesen@nortel.com, netdev@vger.kernel.org, horms@verge.net.au, akpm@linux-foundation.org, Chuck Ebbert , davem@davemloft.net, zlynx@acm.org, Chris Snook In-Reply-To: References: <20070808230733.GA17270@shell.boston.redhat.com> <46BAC2BE.1090106@redhat.com> <46BB508B.7050601@redhat.com> <1186683646.9669.20.camel@localhost> Content-Type: text/plain Organization: IBM Corporation Date: Sun, 12 Aug 2007 11:47:33 +0200 Message-Id: <1186912053.3852.9.camel@localhost> Mime-Version: 1.0 X-Mailer: Evolution 2.10.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1930 Lines: 48 On Sun, 2007-08-12 at 07:53 +0200, Segher Boessenkool wrote: > > Yes, though I would use "=m" on the output list and "m" on the input > > list. The reason is that I've seen gcc fall on its face with an ICE on > > s390 due to "+m". The explanation I've got from our compiler people was > > quite esoteric, as far as I remember gcc splits "+m" to an input > > operand > > and an output operand. Now it can happen that the compiler chooses two > > different registers to access the same memory location. "+m" requires > > that the two memory references are identical which causes the ICE if > > they are not. > > The problem is very nicely described here, last paragraph: > > > It's not a problem anymore in (very) recent GCC, although > that of course won't help you in the kernel (yet). So you are saying that gcc 3.x still has this problem ? > > I do not know if the current compilers still do this. Has > > anyone else seen this happen ? > > In recent GCC, it's actually documented: > > The ordinary output operands must be write-only; GCC will assume that > the values in these operands before the instruction are dead and need > not be generated. Extended asm supports input-output or read-write > operands. Use the constraint character `+' to indicate such an operand > and list it with the output operands. You should only use read-write > operands when the constraints for the operand (or the operand in which > only some of the bits are to be changed) allow a register. > > Note that last line. I see, thanks for the info. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/