From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures Date: Wed, 15 Aug 2007 17:39:48 -0700 Message-ID: <20070816003948.GY9645@linux.vnet.ibm.com> References: <20070809131423.GA9927@shell.boston.redhat.com> <46C2D6F3.3070707@s5r6.in-berlin.de> <18115.35524.56393.347841@cargo.ozlabs.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Paul Mackerras , Satyam Sharma , Stefan Richter , Chris Snook , Linux Kernel Mailing List , linux-arch@vger.kernel.org, Linus Torvalds , netdev@vger.kernel.org, Andrew Morton , ak@suse.de, heiko.carstens@de.ibm.com, davem@davemloft.net, 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, Herbert Xu To: Christoph Lameter Return-path: Received: from e4.ny.us.ibm.com ([32.97.182.144]:56331 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S937378AbXHPAjx (ORCPT ); Wed, 15 Aug 2007 20:39:53 -0400 Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Aug 15, 2007 at 05:26:34PM -0700, Christoph Lameter wrote: > On Thu, 16 Aug 2007, Paul Mackerras wrote: > > > In the kernel we use atomic variables in precisely those situations > > where a variable is potentially accessed concurrently by multiple > > CPUs, and where each CPU needs to see updates done by other CPUs in a > > timely fashion. That is what they are for. Therefore the compiler > > must not cache values of atomic variables in registers; each > > atomic_read must result in a load and each atomic_set must result in a > > store. Anything else will just lead to subtle bugs. > > This may have been the intend. However, today the visibility is controlled > using barriers. And we have barriers that we use with atomic operations. > Having volatile be the default just lead to confusion. Atomic read should > just read with no extras. Extras can be added by using variants like > atomic_read_volatile or so. Seems to me that we face greater chance of confusion without the volatile than with, particularly as compiler optimizations become more aggressive. Yes, we could simply disable optimization, but optimization can be quite helpful. Thanx, Paul