From mboxrd@z Thu Jan 1 00:00:00 1970 From: Segher Boessenkool Subject: Re: [PATCH 24/24] document volatile atomic_read() behavior Date: Fri, 10 Aug 2007 00:23:47 +0200 Message-ID: <9452870655b1844283961f8e9e406464@kernel.crashing.org> References: <20070809142430.GA19817@shell.boston.redhat.com> <8f6bb8a9e4f2819a161d732bdb6c70c0@kernel.crashing.org> <46BB403D.10202@redhat.com> <0a08872e608cf5f7a3d9c0fc746a1051@kernel.crashing.org> <46BB74B9.4070702@nortel.com> Mime-Version: 1.0 (Apple Message framework v623) Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Cc: wjiang@resilience.com, rpjday@mindspring.com, wensong@linux-vs.org, heiko.carstens@de.ibm.com, linux-kernel@vger.kernel.org, ak@suse.de, netdev@vger.kernel.org, horms@verge.net.au, akpm@linux-foundation.org, linux-arch@vger.kernel.org, jesper.juhl@gmail.com, torvalds@linux-foundation.org, schwidefsky@de.ibm.com, davem@davemloft.net, zlynx@acm.org, Chris Snook To: "Chris Friesen" Return-path: Received: from gate.crashing.org ([63.228.1.57]:39499 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753453AbXHIWYw (ORCPT ); Thu, 9 Aug 2007 18:24:52 -0400 In-Reply-To: <46BB74B9.4070702@nortel.com> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org >> Anyway, what's the supposed advantage of *(volatile *) vs. using >> a real volatile object? That you can access that same object in >> a non-volatile way? > > That's my understanding. That way accesses where you don't care about > volatility may be optimised. But those accesses might be done non-atomically then (for example, if the compiler knows it only needs to write one byte, it might not bother writing the rest), so that's no good if you want to read the thing atomically too. > For instance, in cases where there are already other things > controlling visibility (as are needed for atomic increment, for > example) you don't need to make the access itself volatile. Hrm, you mean within a lock or similar? You'll get the same semantics as volatile anyway there. Segher