From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org ([63.228.1.57]:40428 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754243AbXHPVhT (ORCPT ); Thu, 16 Aug 2007 17:37:19 -0400 In-Reply-To: <46C2325C.8040900@redhat.com> References: <46C03885.7000109@redhat.com> <20070813110415.GA24018@shell.boston.redhat.com> <46C2325C.8040900@redhat.com> Mime-Version: 1.0 (Apple Message framework v623) Content-Type: text/plain; charset=US-ASCII; format=flowed Message-Id: <7913acdd6449613c49b86bc99da0d254@kernel.crashing.org> Content-Transfer-Encoding: 7bit From: Segher Boessenkool Subject: Re: [PATCH 1/23] document preferred use of volatile with atomic_t Date: Thu, 16 Aug 2007 23:36:49 +0200 Sender: linux-arch-owner@vger.kernel.org To: Chris Snook Cc: Linus Torvalds , "Robert P. J. Day" , paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, "Luck, Tony" , akpm@linux-foundation.org, linux-arch@vger.kernel.org, Christoph Lameter , Chris Friesen List-ID: > But barriers force a flush of *everything* in scope, Nonsense, whatever "flush" is supposed to mean here. > If you really, *really* distrust the compiler that much, you shouldn't > be using barrier, since that uses volatile under the hood too. You > should just go ahead and implement the atomic operations in assembler, > like Segher Boessenkool did for powerpc in response to my previous > patchset. Puh-lease. I DO NOT DISTRUST THE COMPILER, I just don't assume it will do whatever I would like it to do without telling it. It's a machine you know, and it is very well documented. (And most barriers don't (need to) use volatile). Implementing the atomic ops in asm loses exactly *no* semantics, and it doesn't add restrictions either; it does allow you however to access an atomic_t with normal loads/stores independently, if you so choose. The only valid issue brought up so far is Russell King's comment that GCC cannot schedule the machine instructions in and around an asm() perfectly, it doesn't look inside the asm() after all; but the situation isn't quite as sever as he suggests (GCC _does_ know you are performing loads/stores, etc.); more about that later, when I've finished researching the current state of that stuff. Segher