Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754441AbYKGVFn (ORCPT ); Fri, 7 Nov 2008 16:05:43 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753825AbYKGVEx (ORCPT ); Fri, 7 Nov 2008 16:04:53 -0500 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.125]:47228 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753569AbYKGVEv (ORCPT ); Fri, 7 Nov 2008 16:04:51 -0500 Date: Fri, 7 Nov 2008 16:04:48 -0500 (EST) From: Steven Rostedt X-X-Sender: rostedt@gandalf.stny.rr.com To: "Paul E. McKenney" cc: Mathieu Desnoyers , Peter Zijlstra , David Howells , Linus Torvalds , akpm@linux-foundation.org, Ingo Molnar , linux-kernel@vger.kernel.org, Nicolas Pitre , Ralf Baechle , benh@kernel.crashing.org, paulus@samba.org, David Miller , Ingo Molnar , Thomas Gleixner , linux-arch@vger.kernel.org Subject: Re: [RFC patch 08/18] cnt32_to_63 should use smp_rmb() In-Reply-To: <20081107205411.GG6917@linux.vnet.ibm.com> Message-ID: References: <20081107053349.861709786@polymtl.ca> <20081107052336.652868737@polymtl.ca> <25257.1226055312@redhat.com> <20081107170902.GD22134@Krystal> <20081107191833.GA31809@Krystal> <1226086322.31966.67.camel@lappy.programming.kicks-ass.net> <20081107200243.GB32761@Krystal> <20081107204546.GA3324@Krystal> <20081107205411.GG6917@linux.vnet.ibm.com> User-Agent: Alpine 1.10 (DEB 962 2008-03-14) 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 Content-Length: 1215 Lines: 32 On Fri, 7 Nov 2008, Paul E. McKenney wrote: > > > Would that make more sense ? > > > > > > > Oh, actually, I got things reversed in this email : the readl(io_addr) > > must be done _after_ the __m_cnt_hi read. > > > > Therefore, two consecutive executions would look like : > > > > barrier(); /* Make sure the compiler does not reorder __m_cnt_hi and > > previous mmio read. */ > > read __m_cnt_hi > > smp_rmb(); /* Waits for every cached memory reads to complete */ > > If these are MMIO reads, then you need rmb() rather than smp_rmb(), > at least on architectures that can reorder writes (Power, Itanium, > and I believe also ARM, ...). The read is from a clock source. The only writes that are happening is by the clock itself. On a UP system, is a rmb still needed? That is, can you have two reads on the same CPU from the clock source that will produce a backwards clock? That to me sounds like the clock interface is broken. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/