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=-0.8 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,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 54F3EC433E1 for ; Wed, 1 Jul 2020 09:11:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3479620775 for ; Wed, 1 Jul 2020 09:11:14 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="EBZVsbt8" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729572AbgGAJLH (ORCPT ); Wed, 1 Jul 2020 05:11:07 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57626 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729571AbgGAJLG (ORCPT ); Wed, 1 Jul 2020 05:11:06 -0400 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A8856C061755; Wed, 1 Jul 2020 02:11:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=6wLa3Z5Tnqc6pIqvd9Kuq9IIUcvVwIhLZPWeRnHbIeA=; b=EBZVsbt8QnQM1/nIz26KitisJv 1/eDkGMq6rQ6dFjtEOLryOxl9/2R75doGqasl9ZzuMjNGgYkxJYYMHUMNG4E6n09Gm//aQPvahyhB p+3C2xeyUEBqpAC1GAkbGnjo0IAVtK9oQpwC/WxxI98dOzpe9P7CUV+ThLIauqvkrm1FWM7+2x2jJ c9++4TT1vBaEWdzmoSBQ8OF24E3tQQ2du5JTeEtX0OsuBDeQLQKiztWWKJ979nB8hOz9pI+bYZH6S yALNVqqvO3OdagdOb2OGHjX96PwUbqjI5NeBGkEmxGN8jLNiuZipV96hvXOkCQKlrSu67dIaS6kh2 mX80E9Ew==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1jqYle-0002ST-A3; Wed, 01 Jul 2020 09:10:58 +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 33CD9305B23; Wed, 1 Jul 2020 11:10:54 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 170A4201CB82C; Wed, 1 Jul 2020 11:10:54 +0200 (CEST) Date: Wed, 1 Jul 2020 11:10:54 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Marco Elver , Nick Desaulniers , Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200701091054.GW4781@hirez.programming.kicks-ass.net> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624211540.GS4817@hirez.programming.kicks-ass.net> <20200625080313.GY4817@hirez.programming.kicks-ass.net> <20200625082433.GC117543@hirez.programming.kicks-ass.net> <20200625085745.GD117543@hirez.programming.kicks-ass.net> <20200630191931.GA884155@elver.google.com> <20200630201243.GD4817@hirez.programming.kicks-ass.net> <20200630203016.GI9247@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200630203016.GI9247@paulmck-ThinkPad-P72> Sender: linux-pci-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org On Tue, Jun 30, 2020 at 01:30:16PM -0700, Paul E. McKenney wrote: > On Tue, Jun 30, 2020 at 10:12:43PM +0200, Peter Zijlstra wrote: > > I'm not convinced C11 memory_order_consume would actually work for us, > > even if it would work. That is, given: > > > > https://lore.kernel.org/lkml/20150520005510.GA23559@linux.vnet.ibm.com/ > > > > only pointers can have consume, but like I pointed out, we have code > > that relies on dependent loads from integers. > > I agree that C11 memory_order_consume is not normally what we want, > given that it is universally promoted to memory_order_acquire. > > However, dependent loads from integers are, if anything, more difficult > to defend from the compiler than are control dependencies. This applies > doubly to integers that are used to index two-element arrays, in which > case you are just asking the compiler to destroy your dependent loads > by converting them into control dependencies. Yes, I'm aware. However, as you might know, I'm firmly in the 'C is a glorified assembler' camp (as I expect most actual OS people are, out of necessity if nothing else) and if I wanted a control dependency I would've bloody well written one. I think an optimizing compiler is awesome, but only in so far as that optimization is actually helpful -- and yes, I just stepped into a giant twilight zone there. That is, any optimization that has _any_ controversy should be controllable (like -fno-strict-overflow -fno-strict-aliasing) and I'd very much like the same here. In a larger context, I still think that eliminating speculative stores is both necessary and sufficient to avoid out-of-thin-air. So I'd also love to get some control on that.