From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from py-out-1112.google.com ([64.233.166.183]:3667 "EHLO py-out-1112.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754536AbXIJK4q convert rfc822-to-8bit (ORCPT ); Mon, 10 Sep 2007 06:56:46 -0400 Received: by py-out-1112.google.com with SMTP id u77so2408751pyb for ; Mon, 10 Sep 2007 03:56:45 -0700 (PDT) From: Denys Vlasenko Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures Date: Mon, 10 Sep 2007 11:56:29 +0100 References: <18115.52863.638655.658466@cargo.ozlabs.ibm.com> <200709091902.55388.vda.linux@googlemail.com> <20070909191839.1fa10e88@laptopd505.fenrus.org> In-Reply-To: <20070909191839.1fa10e88@laptopd505.fenrus.org> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 8BIT Content-Disposition: inline Message-Id: <200709101156.30010.vda.linux@googlemail.com> Sender: linux-arch-owner@vger.kernel.org To: Arjan van de Ven Cc: Linus Torvalds , Nick Piggin , Satyam Sharma , Herbert Xu , Paul Mackerras , Christoph Lameter , Chris Snook , Ilpo Jarvinen , "Paul E. McKenney" , Stefan Richter , Linux Kernel Mailing List , linux-arch@vger.kernel.org, Netdev , Andrew Morton , ak@suse.de, heiko.carstens@de.ibm.com, David Miller , schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org, rpjday@mindspring.com, jesper.juhl@gmail.com, segher@kernel.crashing.org List-ID: On Sunday 09 September 2007 19:18, Arjan van de Ven wrote: > On Sun, 9 Sep 2007 19:02:54 +0100 > Denys Vlasenko wrote: > > > Why is all this fixation on "volatile"? I don't think > > people want "volatile" keyword per se, they want atomic_read(&x) to > > _always_ compile into an memory-accessing instruction, not register > > access. > > and ... why is that? > is there any valid, non-buggy code sequence that makes that a > reasonable requirement? Well, if you insist on having it again: Waiting for atomic value to be zero:         while (atomic_read(&x))                 continue; gcc may happily convert it into: reg = atomic_read(&x); while (reg) continue; Expecting every driver writer to remember that atomic_read is not in fact a "read from memory" is naive. That won't happen. Face it, majority of driver authors are a bit less talented than Ingo Molnar or Arjan van de Ven ;) The name of the macro is saying that it's a read. We are confusing users here. It's doubly confusing that cpy_relax(), which says _nothing_ about barriers in its name, is actually a barrier you need to insert here. -- vda