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 7CC9CC48BCD for ; Fri, 4 Jun 2021 16:45:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6B284611C1 for ; Fri, 4 Jun 2021 16:45:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230331AbhFDQqu (ORCPT ); Fri, 4 Jun 2021 12:46:50 -0400 Received: from gate.crashing.org ([63.228.1.57]:35910 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229690AbhFDQqu (ORCPT ); Fri, 4 Jun 2021 12:46:50 -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 154GemFV026057; Fri, 4 Jun 2021 11:40:48 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 154GelkX026054; Fri, 4 Jun 2021 11:40:47 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Fri, 4 Jun 2021 11:40:47 -0500 From: Segher Boessenkool To: Peter Zijlstra Cc: Linus Torvalds , will@kernel.org, paulmck@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: <20210604164047.GH18427@gate.crashing.org> References: <20210604153518.GD18427@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 04, 2021 at 06:10:55PM +0200, Peter Zijlstra wrote: > On Fri, Jun 04, 2021 at 10:35:18AM -0500, Segher Boessenkool wrote: > > On Fri, Jun 04, 2021 at 01:44:37PM +0200, Peter Zijlstra wrote: > > > On naming (sorry Paul for forgetting that in the initial mail); while I > > > think using the volatile qualifier for the language feature (can we haz > > > plz, kthxbai) makes perfect sense, Paul felt that we might use a > > > 'better' name for the kernel use, ctrl_dep_if() was proposed. > > > > In standard C statements do not have qualifiers. Unless you can > > convince the ISO C committee to have them on "if", you will have a very > > hard time convincing any serious compiler to do this. > > While some people like talking to the Committee, I would much rather > explore language extensions with the compiler communities. Such > extensions can then make their way into the Committee once they show > their usefulness. My point is that you ask compiler developers to paint themselves into a corner if you ask them to change such fundamental C syntax. > If you have another proposal on how to express this; one you'd rather > see implemented, I'm all ears. I would love to see something that meshes well with the rest of C. But there is no 1-1 translation from C code to machine code (not in either direction), so anything that more or less depends on that will always be awkward. If you can actually express the dependency in your source code that will get us 95% to where we want to be. > Data dependencies, control dependencies and address dependencies, C > doesn't really like them, we rely on them. It would be awesome if we can > fix this. Yes. The problem is that C is a high-level language. All C semantics are expressed on a an "as-if" level, never as "do this, then that" -- well, of course that *is* what it says, it's an imperative language just like most, but that is just how you *think* about things on a conceptual level, there is nothing that says the machine code has to do the same thing in the same order as you wrote! Segher