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=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 E0FA9C41604 for ; Tue, 6 Oct 2020 14:43:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7B7F7208B8 for ; Tue, 6 Oct 2020 14:43:17 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="HqePj0Zb" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726123AbgJFOnR (ORCPT ); Tue, 6 Oct 2020 10:43:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37750 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725996AbgJFOnR (ORCPT ); Tue, 6 Oct 2020 10:43:17 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CA9A1C061755; Tue, 6 Oct 2020 07:43:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=RpDXxvgo6zmfmRh0+gX7Xwh2qKG+UvHfx0rgKu5yNLg=; b=HqePj0ZbufhT1RBncg0jCOIRk6 3838R4M31vzqnWTqtLXN7yNAhYbu/X6yehZDnZlyI5+xGwg+cZgWNs6KxMRn1W+EfoKRKxGepxhe3 KmHq8HJzNMt0OwSPH4G4ClSpxwd+F/xxgtULmeXxxZXrsZvQM7hJuo2hEymOdiqI+4HHKM7e370ll jZF6c0Ld8IYcEdzokFKPkgVqBb9BkVHhfYdTDIpJrBFZV0Y5BAinavkcWfpZ1gT+RZE71JACQ4qma 2vsMUgnTi5Xb+7GMJPu6l/f9sTu5LGPdn4lQSqJG1+Pz+cqSc1G2MGfE+MAmVTtD9RcO5rFNdy3It n0pmWeCw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kPoBE-0006nb-Nk; Tue, 06 Oct 2020 14:43:05 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 4922B3006D0; Tue, 6 Oct 2020 16:43:02 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 3333228527D6C; Tue, 6 Oct 2020 16:43:02 +0200 (CEST) Date: Tue, 6 Oct 2020 16:43:02 +0200 From: Peter Zijlstra To: "stern@rowland.harvard.edu" Cc: David Laight , "linux-toolchains@vger.kernel.org" , Will Deacon , Paul McKenney , "linux-kernel@vger.kernel.org" , "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" , "dlustig@nvidia.com" , "joel@joelfernandes.org" , "torvalds@linux-foundation.org" Subject: Re: Control Dependencies vs C Compilers Message-ID: <20201006144302.GY2628@hirez.programming.kicks-ass.net> References: <20201006114710.GQ2628@hirez.programming.kicks-ass.net> <3dfe7daed3c44f46a6989b6513ad7bb0@AcuMS.aculab.com> <20201006133115.GT2628@hirez.programming.kicks-ass.net> <20201006142324.GB416765@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201006142324.GB416765@rowland.harvard.edu> Precedence: bulk List-ID: X-Mailing-List: linux-toolchains@vger.kernel.org On Tue, Oct 06, 2020 at 10:23:24AM -0400, stern@rowland.harvard.edu wrote: > On Tue, Oct 06, 2020 at 03:31:15PM +0200, Peter Zijlstra wrote: > > Only if we get the compiler people on board and have them provide means > > are we guaranteed safe from the optimizer. Otherwise we'll just keep > > playing whack-a-mole with fancy new optimization techniques. And given > > how horridly painful it is to debug memory ordering problems, I feel it > > is best to make sure we're not going to have to more than necessary. > > Given that you would have to add a compiler annotation, isn't it just as > easy to use READ_ONCE and WRITE_ONCE? > > Or are you worried that even with READ_ONCE and WRITE_ONCE, the compiler > might still somehow defeat the control dependency? Yes. Also, not all instances actually have the WRITE_ONCE() on. The one in the perf ringbuffer for example uses local_read() for the load (which is basically READ_ONCE()), but doesn't have WRITE_ONCE() on the inside. Also, afaiu WRITE_ONCE() also doesn't stop the compiler from lifting it if it thinks it can get away with it -- memory-barriers.txt has examples. And then there's the case where the common branch has a store, I know ARM64 ARM isn't clear on that, but I'm thinking any actual hardware will still have to respect it, and it's a matter of 'fixing' the ARM. Mostly I just want the compiler people to say they'll guarantee the behaviour if we do 'X'. If 'X' happens to be 'any dynamic branch headed by a volatile load' that's fine by me. If they want a new keyword or attribute, that's also fine. But I want to have the compiler people tell me what they want and guarantee they'll not come and wreck things.