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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 60803C388F2 for ; Sat, 7 Nov 2020 02:37:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0CDA42078B for ; Sat, 7 Nov 2020 02:37:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728467AbgKGCcf (ORCPT ); Fri, 6 Nov 2020 21:32:35 -0500 Received: from netrider.rowland.org ([192.131.102.5]:59771 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1728111AbgKGCcf (ORCPT ); Fri, 6 Nov 2020 21:32:35 -0500 Received: (qmail 65132 invoked by uid 1000); 6 Nov 2020 21:32:14 -0500 Date: Fri, 6 Nov 2020 21:32:14 -0500 From: Alan Stern To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, parri.andrea@gmail.com, will@kernel.org, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com Subject: Re: [PATCH memory-model 5/8] tools/memory-model: Add a glossary of LKMM terms Message-ID: <20201107023214.GA64998@rowland.harvard.edu> References: <20201105215953.GA15309@paulmck-ThinkPad-P72> <20201105220017.15410-5-paulmck@kernel.org> <20201106165930.GC47039@rowland.harvard.edu> <20201106180445.GX3249@paulmck-ThinkPad-P72> <20201106192351.GA53131@rowland.harvard.edu> <20201106195912.GA3249@paulmck-ThinkPad-P72> <20201106204008.GA55521@rowland.harvard.edu> <20201106210413.GB3249@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201106210413.GB3249@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Fri, Nov 06, 2020 at 01:04:13PM -0800, Paul E. McKenney wrote: > On Fri, Nov 06, 2020 at 03:40:08PM -0500, Alan Stern wrote: > > Is it really true that data dependencies are so easily destroyed? I > > would expect that a true "semantic" dependency (i.e., one where the > > value written really does vary according to the value read) would be > > rather hard to second guess. > > The usual optimizations apply, for but one example: > > r1 = READ_ONCE(x); > WRITE_ONCE(y, (r1 + 1) % MAX_ELEMENTS); > > If MAX_ELEMENTS is 1, so long, data dependency! Sure, but if MAX_ELEMENTS is 1 then the value written will always be 0 no matter what value r1 has, so it isn't a semantic dependency. Presumably a semantic data dependency would be much more robust. I wonder if it's worth pointing out this distinction to the reader. > With pointers, the compiler has fewer optimization opportunities, > but there are still cases where it can break the dependency. > Or transform it to a control dependency. Transforming a data dependency into a control dependency wouldn't make any important difference; the hardware would still provide the desired ordering. Alan