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=-5.3 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 42494C47096 for ; Sun, 6 Jun 2021 11:41:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 15E2561220 for ; Sun, 6 Jun 2021 11:41:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229878AbhFFLnQ (ORCPT ); Sun, 6 Jun 2021 07:43:16 -0400 Received: from gate.crashing.org ([63.228.1.57]:59079 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229465AbhFFLnP (ORCPT ); Sun, 6 Jun 2021 07:43:15 -0400 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 156Baslq023488; Sun, 6 Jun 2021 06:36:54 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 156BapHt023485; Sun, 6 Jun 2021 06:36:51 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Sun, 6 Jun 2021 06:36:51 -0500 From: Segher Boessenkool To: "Paul E. McKenney" Cc: Peter Zijlstra , Linus Torvalds , will@kernel.org, stern@rowland.harvard.edu, parri.andrea@gmail.com, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, linux-kernel@vger.kernel.org, linux-toolchains@vger.kernel.org, linux-arch@vger.kernel.org Subject: Re: [RFC] LKMM: Add volatile_if() Message-ID: <20210606113651.GR18427@gate.crashing.org> References: <20210604153518.GD18427@gate.crashing.org> <20210604164047.GH18427@gate.crashing.org> <20210604185526.GW4397@paulmck-ThinkPad-P17-Gen-1> <20210604195301.GM18427@gate.crashing.org> <20210604204042.GZ4397@paulmck-ThinkPad-P17-Gen-1> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210604204042.GZ4397@paulmck-ThinkPad-P17-Gen-1> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-toolchains@vger.kernel.org On Fri, Jun 04, 2021 at 01:40:42PM -0700, Paul E. McKenney wrote: > On Fri, Jun 04, 2021 at 02:53:01PM -0500, Segher Boessenkool wrote: > > On Fri, Jun 04, 2021 at 11:55:26AM -0700, Paul E. McKenney wrote: > > > On Fri, Jun 04, 2021 at 11:40:47AM -0500, Segher Boessenkool wrote: > > > > My point is that you ask compiler developers to paint themselves into a > > > > corner if you ask them to change such fundamental C syntax. > > > > > > Once we have some experience with a language extension, the official > > > syntax for a standardized version of that extension can be bikeshedded. > > > Committees being what they are, what we use in the meantime will > > > definitely not be what is chosen, so there is not a whole lot of point > > > in worrying about the exact syntax in the meantime. ;-) > > > > I am only saying that it is unlikely any compiler that is used in > > production will want to experiment with "volatile if". > > That unfortunately matches my experience over quite a few years. But if > something can be implemented using existing extensions, the conversations > often get easier. Especially given many more people are now familiar > with concurrency. This was about the syntax "volatile if", not about the concept, let's call that "volatile_if". And no, it was not me who brought this up :-) > > > Which is exactly why these conversations are often difficult. There is > > > a tension between pushing the as-if rule as far as possible within the > > > compiler on the one hand and allowing developers to write code that does > > > what is needed on the other. ;-) > > > > There is a tension between what users expect from the compiler and what > > actually is promised. The compiler is not pushing the as-if rule any > > further than it always has: it just becomes better at optimising over > > time. The as-if rule is and always has been absolute. > > Heh! The fact that the compiler has become better at optimizing > over time is exactly what has been pushing the as-if rule further. > > The underlying problem is that it is often impossible to write large > applications (such as the Linux kernel) completely within the confines of > the standard. Thus, most large applications, and especially concurrent > applications, are vulnerable to either the compiler becoming better > at optimizing or compilers pushing the as-if rule, however you want to > say it. Oh definitely. But there is nothing the compiler can do about most cases of undefined behaviour: it cannot detect it, and there is no way it *can* be handled sanely. Take for example dereferencing a pointer that does not point to an object. Segher