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=-6.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS 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 E87F1C47082 for ; Mon, 7 Jun 2021 23:26:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C833560FEE for ; Mon, 7 Jun 2021 23:26:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230233AbhFGX2C (ORCPT ); Mon, 7 Jun 2021 19:28:02 -0400 Received: from mail.kernel.org ([198.145.29.99]:35260 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230183AbhFGX2C (ORCPT ); Mon, 7 Jun 2021 19:28:02 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id B38F8610E5; Mon, 7 Jun 2021 23:26:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1623108370; bh=327icG9dhrui9P/iN7jHzr+RvZa+0wmUKM11g81/s6k=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=WnkTh7CT/0S6iUf+GUbl1FyCBt+1dLdnURyMrDhPzKLnAipbbm8xt3B2FF6hS3bDb 6KvKXeGQBjx4lvM94WHyQnHEwdnl/8BMbFrnIuRzQzpqJbH6L94Ves10qwTg5RsowX ZchtG99dC2c5g/hZdbe4YpyVNqd+FfdPvEqIZnXxZMXHusi/KromhpVxm3i4ZH2BMH LuERZ6gZ5nfWzupfdvjd5WvQwff2bktswcTSCGeuibkMU5zpI3QSJqJjmq+F6Q2HPK QWp6Vz+zxgHNbRK/3ckyEmHHjXq7Mz57bNtabE+jJuoWgIrHH0jC/hSrdbkV31EEyb bAvjnszEhX9/g== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 764CB5C084C; Mon, 7 Jun 2021 16:26:10 -0700 (PDT) Date: Mon, 7 Jun 2021 16:26:10 -0700 From: "Paul E. McKenney" To: Segher Boessenkool 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: <20210607232610.GX4397@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.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> <20210607224037.GQ18427@gate.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210607224037.GQ18427@gate.crashing.org> Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Mon, Jun 07, 2021 at 05:40:37PM -0500, Segher Boessenkool wrote: > 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! :-) When it is the cheapest means of providing the needed ordering, I really do want a branch. ;-) And a branch would implement Alan's "control dependency" above. Thanx, Paul