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 25051C47082 for ; Mon, 7 Jun 2021 17:56:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0722560E0B for ; Mon, 7 Jun 2021 17:56:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231200AbhFGR6O (ORCPT ); Mon, 7 Jun 2021 13:58:14 -0400 Received: from gate.crashing.org ([63.228.1.57]:37131 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230291AbhFGR6M (ORCPT ); Mon, 7 Jun 2021 13:58:12 -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 157Hq28m015783; Mon, 7 Jun 2021 12:52:02 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 157Hq13C015762; Mon, 7 Jun 2021 12:52:01 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 7 Jun 2021 12:52:00 -0500 From: Segher Boessenkool To: Alexander Monakov Cc: Linus Torvalds , Jakub Jelinek , 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: <20210607175200.GG18427@gate.crashing.org> References: <20210605145739.GB1712909@rowland.harvard.edu> <20210606001418.GH4397@paulmck-ThinkPad-P17-Gen-1> <20210606012903.GA1723421@rowland.harvard.edu> <20210606185922.GF7746@tucnak> 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 Mon, Jun 07, 2021 at 11:01:39AM +0300, Alexander Monakov wrote: > Uhh... I was not talking about some (non-existent) "optimizing linker". > LTO works by relaunching the compiler from the linker and letting it > consume multiple translation units (which are fully preprocessed by that > point). So the very thing you wanted to avoid -- such barriers appearing > in close proximity where they can be deduplicated -- may arise after a > little bit of cross-unit inlining. > > My main point here is that using __COUNTER__ that way (making things > "unique" for the compiler) does not work in general when LTO enters the > picture. As long as that is remembered, I'm happy. Yup. Exactly the same issue as using this in any function that may end up inlined. > > In the case of "volatile_if()", we actually would like to have not a > > memory clobber, but a "memory read". IOW, it would be a barrier for > > any writes taking place, but reads can move around it. > > > > I don't know of any way to express that to the compiler. We've used > > hacks for it before (in gcc, BLKmode reads turn into that kind of > > barrier in practice, so you can do something like make the memory > > input to the asm be a big array). But that turned out to be fairly > > unreliable, so now we use memory clobbers even if we just mean "reads > > random memory". > > So the barrier which is a compiler barrier but not a machine barrier is > __atomic_signal_fence(model), but internally GCC will not treat it smarter > than an asm-with-memory-clobber today. It will do nothing for relaxed ordering, and do blockage for everything else. Can it do anything weaker than that? Segher