From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from moutng.kundenserver.de ([212.227.126.186]:49785 "EHLO moutng.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1765163AbXHOOv7 (ORCPT ); Wed, 15 Aug 2007 10:51:59 -0400 From: Arnd Bergmann Subject: Re: [PATCH 6/24] make atomic_read() behave consistently on frv Date: Wed, 15 Aug 2007 15:29:43 +0200 References: <20070811042943.GA13410@linux.vnet.ibm.com> <200708150001.58217.arnd@arndb.de> <20070814224354.GE8243@linux.vnet.ibm.com> In-Reply-To: <20070814224354.GE8243@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200708151529.46780.arnd@arndb.de> Sender: linux-arch-owner@vger.kernel.org To: paulmck@linux.vnet.ibm.com Cc: Nick Piggin , Herbert Xu , csnook@redhat.com, dhowells@redhat.com, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, torvalds@linux-foundation.org, netdev@vger.kernel.org, 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 List-ID: On Wednesday 15 August 2007, Paul E. McKenney wrote: > ACCESS_ONCE() is indeed intended to be used when actually loading or > storing the variable. That said, I must admit that it is not clear to me > why you would want to add an extra order() rather than ACCESS_ONCE()ing > one or both of the adjacent accesses to that same variable. > > So, what am I missing? You're probably right, the only case I can construct is something like if (ACCESS_ONCE(x)) { ... ACCESS_ONCE(x)++; } which would be slightly less efficient than if (x) x++; order(x); because in the first case, you need to do two ordered accesses but only one in the second case. However, I can't think of a case where this actually makes a noticable difference in real life. Arnd <><