From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-arch-owner@vger.kernel.org>
Received: from ozlabs.org ([203.10.76.45]:45444 "EHLO ozlabs.org"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753556AbXHPBvt (ORCPT <rfc822;linux-arch@vger.kernel.org>);
	Wed, 15 Aug 2007 21:51:49 -0400
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Message-ID: <18115.44462.622801.683446@cargo.ozlabs.ibm.com>
Date: Thu, 16 Aug 2007 11:51:42 +1000
From: Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures
In-Reply-To: <Pine.LNX.4.64.0708151740570.10497@schroedinger.engr.sgi.com>
References: <20070809131423.GA9927@shell.boston.redhat.com>
	<Pine.LNX.4.64.0708141529310.32693@schroedinger.engr.sgi.com>
	<alpine.LFD.0.999.0708150417240.6482@enigma.security.iitk.ac.in>
	<46C2D6F3.3070707@s5r6.in-berlin.de>
	<alpine.LFD.0.999.0708151713080.16414@enigma.security.iitk.ac.in>
	<18115.35524.56393.347841@cargo.ozlabs.ibm.com>
	<Pine.LNX.4.64.0708151725130.10497@schroedinger.engr.sgi.com>
	<20070816003948.GY9645@linux.vnet.ibm.com>
	<Pine.LNX.4.64.0708151740570.10497@schroedinger.engr.sgi.com>
Sender: linux-arch-owner@vger.kernel.org
To: Christoph Lameter <clameter@sgi.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Satyam Sharma <satyam@infradead.org>, Stefan Richter <stefanr@s5r6.in-berlin.de>, Chris Snook <csnook@redhat.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-arch@vger.kernel.org, Linus Torvalds <torvalds@linux-foundation.org>, netdev@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>, 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 <herbert@gondor.apana.org.au>
List-ID: <linux-arch.vger.kernel.org>

Christoph Lameter writes:

> A volatile default would disable optimizations for atomic_read. 
> atomic_read without volatile would allow for full optimization by the 
> compiler. Seems that this is what one wants in many cases.

Name one such case.

An atomic_read should do a load from memory.  If the programmer puts
an atomic_read() in the code then the compiler should emit a load for
it, not re-use a value returned by a previous atomic_read.  I do not
believe it would ever be useful for the compiler to collapse two
atomic_read statements into a single load.

Paul.