Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932756AbXHPKbY (ORCPT ); Thu, 16 Aug 2007 06:31:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757115AbXHPKbH (ORCPT ); Thu, 16 Aug 2007 06:31:07 -0400 Received: from hp3.statik.tu-cottbus.de ([141.43.120.68]:41327 "EHLO hp3.statik.tu-cottbus.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753350AbXHPKbF (ORCPT ); Thu, 16 Aug 2007 06:31:05 -0400 Message-ID: <46C42767.4070104@s5r6.in-berlin.de> Date: Thu, 16 Aug 2007 12:31:03 +0200 From: Stefan Richter User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.8.1.6) Gecko/20070802 SeaMonkey/1.1.4 MIME-Version: 1.0 To: Herbert Xu CC: Paul Mackerras , Satyam Sharma , Christoph Lameter , "Paul E. McKenney" , 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 Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures References: <18115.44462.622801.683446@cargo.ozlabs.ibm.com> <20070816020042.GA30650@gondor.apana.org.au> <18115.45316.702491.681906@cargo.ozlabs.ibm.com> <18115.52863.638655.658466@cargo.ozlabs.ibm.com> <20070816053945.GB32442@gondor.apana.org.au> <18115.62741.807704.969977@cargo.ozlabs.ibm.com> <20070816070907.GA964@gondor.apana.org.au> <46C40587.7050708@s5r6.in-berlin.de> <20070816081049.GA1431@gondor.apana.org.au> <46C41EE4.9090806@s5r6.in-berlin.de> In-Reply-To: <46C41EE4.9090806@s5r6.in-berlin.de> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1626 Lines: 45 I wrote: > Herbert Xu wrote: >> On Thu, Aug 16, 2007 at 10:06:31AM +0200, Stefan Richter wrote: [...] >>> expectation that each >>> occurrence of atomic_read actually results in a load operation, i.e. is >>> not optimized away. [...] >> Can you find an actual atomic_read code snippet there that is >> broken without the volatile modifier? PS: Just to clarify, I'm not speaking for the volatile modifier. I'm not speaking for any particular implementation of atomic_t and its accessors at all. All I am saying is that - we use atomically accessed data types because we concurrently but locklessly access this data, - hence a read access to this data that could be optimized away makes *no sense at all*. The only sensible read accessor to an atomic datatype is a read accessor that will not be optimized away. So, the architecture guys can implement atomic_read however they want --- as long as it cannot be optimized away.* PPS: If somebody has code where he can afford to let the compiler coalesce atomic_read with a previous access to the same data, i.e. doesn't need and doesn't want all guarantees that the atomic_read API makes (or IMO should make), then he can replace the atomic_read by a local temporary variable. *) Exceptions: if (known_to_be_false) read_access(a); and the like. -- Stefan Richter -=====-=-=== =--- =---- http://arcgraph.de/sr/ - 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/