From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760206AbZBLUPS (ORCPT ); Thu, 12 Feb 2009 15:15:18 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757483AbZBLUPD (ORCPT ); Thu, 12 Feb 2009 15:15:03 -0500 Received: from smtp1.linux-foundation.org ([140.211.169.13]:56811 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757158AbZBLUPC (ORCPT ); Thu, 12 Feb 2009 15:15:02 -0500 Date: Thu, 12 Feb 2009 12:13:29 -0800 (PST) From: Linus Torvalds X-X-Sender: torvalds@localhost.localdomain To: "Paul E. McKenney" cc: Mathieu Desnoyers , ltt-dev@lists.casi.polymtl.ca, linux-kernel@vger.kernel.org, Bryan Wu , uclinux-dist-devel@blackfin.uclinux.org Subject: Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost) In-Reply-To: <20090212200249.GG6759@linux.vnet.ibm.com> Message-ID: References: <20090211185203.GA29852@Krystal> <20090211200903.GG6694@linux.vnet.ibm.com> <20090211214258.GA32407@Krystal> <20090212003549.GU6694@linux.vnet.ibm.com> <20090212023308.GA21157@linux.vnet.ibm.com> <20090212040824.GA12346@Krystal> <20090212050120.GA8317@linux.vnet.ibm.com> <20090212070539.GA15896@Krystal> <20090212164621.GC6759@linux.vnet.ibm.com> <20090212192941.GC2047@Krystal> <20090212200249.GG6759@linux.vnet.ibm.com> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 12 Feb 2009, Paul E. McKenney wrote: > > Actually the best way to do this would be: > > while (ACCESS_ONCE(sig_done) < 1) > continue; > > If ACCESS_ONCE() needs to be made architecture-specific to make this > really work on Blackfin, we should make that change. I really wouldn't want to mix up compiler barriers and cache barriers this way. I think "cpu_relax()" is likely the right thing to piggy-back on for broken cache-coherency. > And, now that you mention it, I have heard rumors that other CPU > families can violate cache coherence in some circumstances. I personally suspect that the BF pseudo-SMP code is just broken, and that it likely has tons of subtle bugs and races - because we _do_ depend on cache coherency at least for accessing objects next to each other. I just never personally felt like I had the energy to care deeply enough. But I draw the line at making ACCESS_ONCE() imply anything but a compiler optimization issue. Linus