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 901E4C47095 for ; Mon, 7 Jun 2021 22:45:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6FCE861090 for ; Mon, 7 Jun 2021 22:45:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230386AbhFGWqu (ORCPT ); Mon, 7 Jun 2021 18:46:50 -0400 Received: from gate.crashing.org ([63.228.1.57]:45316 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229997AbhFGWqt (ORCPT ); Mon, 7 Jun 2021 18:46:49 -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 157MedpN032068; Mon, 7 Jun 2021 17:40:39 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 157MebTI032067; Mon, 7 Jun 2021 17:40:37 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 7 Jun 2021 17:40:37 -0500 From: Segher Boessenkool To: "Paul E. McKenney" Cc: Alan Stern , Linus Torvalds , Peter Zijlstra , Will Deacon , Andrea Parri , Boqun Feng , Nick Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Linux Kernel Mailing List , linux-toolchains@vger.kernel.org, linux-arch Subject: Re: [RFC] LKMM: Add volatile_if() Message-ID: <20210607224037.GQ18427@gate.crashing.org> References: <20210606195242.GA18427@gate.crashing.org> <20210606202616.GC18427@gate.crashing.org> <20210606233729.GN4397@paulmck-ThinkPad-P17-Gen-1> <20210607141242.GD18427@gate.crashing.org> <20210607152712.GR4397@paulmck-ThinkPad-P17-Gen-1> <20210607182335.GI18427@gate.crashing.org> <20210607195144.GC1779688@rowland.harvard.edu> <20210607201633.GW4397@paulmck-ThinkPad-P17-Gen-1> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210607201633.GW4397@paulmck-ThinkPad-P17-Gen-1> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 07, 2021 at 01:16:33PM -0700, Paul E. McKenney wrote: > On Mon, Jun 07, 2021 at 03:51:44PM -0400, Alan Stern wrote: > > On Mon, Jun 07, 2021 at 01:23:35PM -0500, Segher Boessenkool wrote: > > > On Mon, Jun 07, 2021 at 08:27:12AM -0700, Paul E. McKenney wrote: > > > > > > > > The barrier() thing can work - all we need to do is to simply make it > > > > > > > > impossible for gcc to validly create anything but a conditional > > > > > > > > branch. > > > > > > > > > What would you suggest as a way of instructing the compiler to emit the > > > > > > conditional branch that we are looking for? > > > > > > > > > > You write it in the assembler code. > > > > > > > > > > Yes, it sucks. But it is the only way to get a branch if you really > > > > > want one. Now, you do not really need one here anyway, so there may be > > > > > some other way to satisfy the actual requirements. > > > > > > > > Hmmm... What do you see Peter asking for that is different than what > > > > I am asking for? ;-) > > > > > > I don't know what you are referring to, sorry? > > > > > > I know what you asked for: literally some way to tell the compiler to > > > emit a conditional branch. If that is what you want, the only way to > > > make sure that is what you get is by writing exactly that in assembler. > > > > That's not necessarily it. > > > > People would be happy to have an easy way of telling the compiler that > > all writes in the "if" branch of an if statement must be ordered after > > any reads that the condition depends on. Or maybe all writes in either > > the "if" branch or the "else" branch. And maybe not all reads that the > > condition depends on, but just the reads appearing syntactically in the > > condition. Or maybe even just the volatile reads appearing in the > > condition. Nobody has said exactly. > > > > The exact method used for doing this doesn't matter. It could be > > accomplished by treating those reads as load-acquires. Or it could be > > done by ensuring that the object code contains a dependency (control or > > data) from the reads to the writes. Or it could be done by treating > > the writes as store-releases. But we do want the execution-time > > penalty to be small. > > > > In short, we want to guarantee somehow that the conditional writes are > > not re-ordered before the reads in the condition. (But note that > > "conditional writes" includes identical writes occurring in both > > branches.) > > What Alan said! ;-) Okay, I'll think about that. But you wrote: > > > > > > What would you suggest as a way of instructing the compiler to emit the > > > > > > conditional branch that we are looking for? ... and that is what I answered. I am sorry if you do not like being taken literally, but that is how I read technical remarks: as literally what they say. If you say you want a branch, I take it you want a branch! :-) Segher