From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Snook Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha Date: Thu, 09 Aug 2007 15:05:17 -0400 Message-ID: <46BB656D.6090408@redhat.com> References: <20070809132442.GA13042@shell.boston.redhat.com> <20070809143255.GA8424@linux.vnet.ibm.com> <46BB2A5A.5090006@redhat.com> <20070809150445.GB8424@linux.vnet.ibm.com> <46BB31A6.4080507@redhat.com> <3bfabd7472d6f019aa1880b14013f7a1@kernel.crashing.org> <46BB3ECF.2070100@redhat.com> <1abc2c621d6b62b3ac9f489d4d18806a@kernel.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: wjiang@resilience.com, cfriesen@nortel.com, wensong@linux-vs.org, heiko.carstens@de.ibm.com, linux-kernel@vger.kernel.org, ak@suse.de, netdev@vger.kernel.org, paulmck@linux.vnet.ibm.com, horms@verge.net.au, akpm@linux-foundation.org, linux-arch@vger.kernel.org, jesper.juhl@gmail.com, torvalds@linux-foundation.org, zlynx@acm.org, rpjday@mindspring.com, schwidefsky@de.ibm.com, davem@davemloft.net To: Segher Boessenkool Return-path: Received: from mx1.redhat.com ([66.187.233.31]:37597 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751043AbXHITKw (ORCPT ); Thu, 9 Aug 2007 15:10:52 -0400 In-Reply-To: <1abc2c621d6b62b3ac9f489d4d18806a@kernel.crashing.org> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Segher Boessenkool wrote: >>> The only safe way to get atomic accesses is to write >>> assembler code. Are there any downsides to that? I don't >>> see any. >> >> The assumption that aligned word reads and writes are atomic, and that >> words are aligned unless explicitly packed otherwise, is endemic in >> the kernel. No sane compiler violates this assumption. It's true >> that we're not portable to insane compilers after this patch, but we >> never were in the first place. > > You didn't answer my question: are there any downsides to using > explicit coded-in-assembler accesses for atomic accesses? You > can handwave all you want that it should "just work" with > volatile accesses, but volatility != atomicity, volatile in C > is really badly defined, GCC never officially gave stronger > guarantees, and we have a bugzilla full of PRs to show what a > minefield it is. > > So, why not use the well-defined alternative? Because we don't need to, and it hurts performance.