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, URIBL_BLOCKED,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 85CBFC4743E for ; Sun, 6 Jun 2021 19:57:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5FCA3613AD for ; Sun, 6 Jun 2021 19:57:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229813AbhFFT65 (ORCPT ); Sun, 6 Jun 2021 15:58:57 -0400 Received: from gate.crashing.org ([63.228.1.57]:47652 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229772AbhFFT64 (ORCPT ); Sun, 6 Jun 2021 15:58:56 -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 156JqiJX012123; Sun, 6 Jun 2021 14:52:44 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 156JqgQT012119; Sun, 6 Jun 2021 14:52:42 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Sun, 6 Jun 2021 14:52:42 -0500 From: Segher Boessenkool To: Linus Torvalds Cc: Alan Stern , "Paul E. McKenney" , 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: <20210606195242.GA18427@gate.crashing.org> References: <20210604214010.GD4397@paulmck-ThinkPad-P17-Gen-1> <20210605145739.GB1712909@rowland.harvard.edu> <20210606001418.GH4397@paulmck-ThinkPad-P17-Gen-1> <20210606012903.GA1723421@rowland.harvard.edu> <20210606115336.GS18427@gate.crashing.org> <20210606184021.GY18427@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-arch@vger.kernel.org On Sun, Jun 06, 2021 at 11:48:32AM -0700, Linus Torvalds wrote: > On Sun, Jun 6, 2021 at 11:43 AM Segher Boessenkool > wrote: > > > > You truly should have written a branch in tthe asm if you truly wanted > > a branch instruction. > > That's exactly what I don't want to do, and what the original patch by > PeterZ did. Yes, I know. But it is literally the *only* way to *always* get a conditional branch: by writing one. > And to work well, it needs "asm goto", which is so recent that a lot > of compilers don't support it (thank God for clang dragging gcc > kicking and screaming to implement it at all - I'd asked for it over a > decade ago). GCC has had it since 2009. > So you get bad code generation in a lot of cases, which entirely > obviates the _point_ of this all - which is that we can avoid an > expensive operation (a memory barrier) by just doing clever code > generation. > > So if we can't get the clever code generation, it's all pretty much > moot, imnsho. Yes. Segher