From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 06FD2C5CFE7 for ; Tue, 10 Jul 2018 20:24:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9AA2D208E5 for ; Tue, 10 Jul 2018 20:24:36 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9AA2D208E5 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=rowland.harvard.edu Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732801AbeGJUZP (ORCPT ); Tue, 10 Jul 2018 16:25:15 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:34854 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1732246AbeGJUZO (ORCPT ); Tue, 10 Jul 2018 16:25:14 -0400 Received: (qmail 6900 invoked by uid 2102); 10 Jul 2018 16:24:34 -0400 Received: from localhost (sendmail-bs@127.0.0.1) by localhost with SMTP; 10 Jul 2018 16:24:34 -0400 Date: Tue, 10 Jul 2018 16:24:34 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: "Paul E. McKenney" cc: LKMM Maintainers -- Akira Yokosawa , Andrea Parri , Boqun Feng , Daniel Lustig , David Howells , Jade Alglave , Luc Maranget , Nicholas Piggin , Peter Zijlstra , Will Deacon , Kernel development list Subject: Re: [PATCH v3] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire In-Reply-To: <20180710195853.GC3593@linux.vnet.ibm.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 10 Jul 2018, Paul E. McKenney wrote: > On Tue, Jul 10, 2018 at 02:18:13PM -0400, Alan Stern wrote: > > More than one kernel developer has expressed the opinion that the LKMM > > should enforce ordering of writes by locking. In other words, given > > the following code: > > > > WRITE_ONCE(x, 1); > > spin_unlock(&s): > > spin_lock(&s); > > WRITE_ONCE(y, 1); > > > > the stores to x and y should be propagated in order to all other CPUs, > > even though those other CPUs might not access the lock s. In terms of > > the memory model, this means expanding the cumul-fence relation. > > > > Locks should also provide read-read (and read-write) ordering in a > > similar way. Given: > > > > READ_ONCE(x); > > spin_unlock(&s); > > spin_lock(&s); > > READ_ONCE(y); // or WRITE_ONCE(y, 1); > > > > the load of x should be executed before the load of (or store to) y. > > The LKMM already provides this ordering, but it provides it even in > > the case where the two accesses are separated by a release/acquire > > pair of fences rather than unlock/lock. This would prevent > > architectures from using weakly ordered implementations of release and > > acquire, which seems like an unnecessary restriction. The patch > > therefore removes the ordering requirement from the LKMM for that > > case. > > > > All the architectures supported by the Linux kernel (including RISC-V) > > do provide this ordering for locks, albeit for varying reasons. > > Therefore this patch changes the model in accordance with the > > developers' wishes. > > > > Signed-off-by: Alan Stern > > It now applies, thank you very much! > > Is this something that you are comfortable pushing into the upcoming > merge window, or should I hold off until the next one? Given the concerns that Andrea raised, and given that neither Peter, Will, nor Daniel has commented on v.3 of the patch, I think we should hold off for a little while. Alan