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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 3CCA2C433E0 for ; Wed, 1 Jul 2020 09:11:28 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 8EC9F20775 for ; Wed, 1 Jul 2020 09:11:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="EBZVsbt8" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8EC9F20775 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19193-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 19760 invoked by uid 550); 1 Jul 2020 09:11:18 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 19727 invoked from network); 1 Jul 2020 09:11:17 -0000 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==; 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> 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.