From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from moutng.kundenserver.de ([212.227.126.183]:55247 "EHLO moutng.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936464AbXHNWD4 convert rfc822-to-8bit (ORCPT ); Tue, 14 Aug 2007 18:03:56 -0400 From: Arnd Bergmann Subject: Re: [PATCH 6/24] make atomic_read() behave consistently on frv Date: Wed, 15 Aug 2007 00:01:54 +0200 References: <20070811042943.GA13410@linux.vnet.ibm.com> <46C13EE1.1000707@yahoo.com.au> <20070814170128.GA8243@linux.vnet.ibm.com> In-Reply-To: <20070814170128.GA8243@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 8BIT Content-Disposition: inline Message-Id: <200708150001.58217.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 Tuesday 14 August 2007, Paul E. McKenney wrote: > > #define order(x) asm volatile("" : "+m" (x)) > > There was something very similar discussed earlier in this thread, > with quite a bit of debate as to exactly what the "m" flag should > look like.  I suggested something similar named ACCESS_ONCE in the > context of RCU (http://lkml.org/lkml/2007/7/11/664): > >         #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) > > The nice thing about this is that it works for both loads and stores. > Not clear that order() above does this -- I get compiler errors when > I try something like "b = order(a)" or "order(a) = 1" using gcc 4.1.2. Well, it serves a different purpose: While your ACCESS_ONCE() macro is an lvalue, the order() macro is a statement that can be used in place of the barrier() macro. order() is the most lightweight barrier as it only enforces ordering on a single variable in the compiler, but does not have any side-effects visible to other threads, like the cache line access in ACCESS_ONCE has. Arnd <><