Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752292AbbGMWPP (ORCPT ); Mon, 13 Jul 2015 18:15:15 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:47310 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751093AbbGMWPN (ORCPT ); Mon, 13 Jul 2015 18:15:13 -0400 Date: Tue, 14 Jul 2015 00:15:03 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Peter Hurley , Will Deacon , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Benjamin Herrenschmidt Subject: Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() Message-ID: <20150713221503.GD19282@twins.programming.kicks-ass.net> References: <1436789704-10086-1-git-send-email-will.deacon@arm.com> <20150713131143.GY19282@twins.programming.kicks-ass.net> <20150713140915.GD2632@arm.com> <20150713142109.GE2632@arm.com> <20150713155447.GB19282@twins.programming.kicks-ass.net> <20150713182332.GW3717@linux.vnet.ibm.com> <55A41481.7000702@hurleysoftware.com> <20150713201642.GY3717@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150713201642.GY3717@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1586 Lines: 53 On Mon, Jul 13, 2015 at 01:16:42PM -0700, Paul E. McKenney wrote: > On Mon, Jul 13, 2015 at 03:41:53PM -0400, Peter Hurley wrote: > > > Does that answer the question, or am I missing the point? > > > > Yes, it shows that smp_mb__after_unlock_lock() has no purpose, since it > > is defined only for PowerPC and your test above just showed that for > > the sequence The only purpose is to provide transitivity, but the documentation fails to explicitly call that out. > > > > store a > > UNLOCK M > > LOCK N > > store b > > > > a and b is always observed as an ordered pair {a,b}. > > Not quite. > > This is instead the sequence that is of concern: > > store a > unlock M > lock N > load b So its late and that table didn't parse, but that should be ordered too. The load of b should not be able to escape the lock N. If only because LWSYNC is a valid RMB and any LOCK implementation must load the lock state to observe it unlocked. > > Additionally, the assertion in Documentation/memory_barriers.txt that > > the sequence above can be reordered as > > > > LOCK N > > store b > > store a > > UNLOCK M > > > > is not true on any existing arch in Linux. > > It was at one time and might be again. What would be required to make this true? I'm having a hard time seeing how things can get reordered like that. -- 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/