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=-8.4 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL 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 C8EF4C433E1 for ; Tue, 7 Jul 2020 23:01:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A66B52065D for ; Tue, 7 Jul 2020 23:01:45 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="q9nHFQUG" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728607AbgGGXBn (ORCPT ); Tue, 7 Jul 2020 19:01:43 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47518 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728001AbgGGXBl (ORCPT ); Tue, 7 Jul 2020 19:01:41 -0400 Received: from mail-pj1-x1042.google.com (mail-pj1-x1042.google.com [IPv6:2607:f8b0:4864:20::1042]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C4A33C061755 for ; Tue, 7 Jul 2020 16:01:41 -0700 (PDT) Received: by mail-pj1-x1042.google.com with SMTP id o22so320272pjw.2 for ; Tue, 07 Jul 2020 16:01:41 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=Eir+/5D/9eAWW8HAsQF0UHQBLwiWBu8OPWiNvlXrgfo=; b=q9nHFQUGJSQa6+kpaDcgq/H1Me1dz6qWOflJ5qyN8UvbG2KFHaOFsiG0yh3griaGPL zpnFvEKrEBU6SBmCmwnkaiFfGBgE0BfLwr0eTl2JCp/Po7z+8CEmwFIoMC+7xoDrOrRF Peb0k+dE/YvzErpB5rct6JRCG3QiM6DuLuM2j5pxxlqqHQl5MlKR6lNvdR/reBAj+1Be S5+7mSAGbM84lAoSQaE6ExV4twhGu0RrHpWOQdKgGC3bTAmqkuoiThYE+owAp8819Png 2ZtcJDPTpIxlBOEhEyIzU8C7I4ASBxyJ4z2jd+jwdkycWq9tZVcYQ5iQ21gf4m6zXiEC 4ENA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Eir+/5D/9eAWW8HAsQF0UHQBLwiWBu8OPWiNvlXrgfo=; b=Lf7CSxv79IafPGUrYGaAPybxrNUIy0IjxPJ6TV+ry7/9kVRXkC/GsEqS3ubY8HIfqn s6NE7mScUJYKx+jqbsrBsGl/znHVfkarjORREhNqklRFV1fv0oiCoc6guGIP6G/x3G9w YYFtQOgmXs7aQWKX0DkreJJB5vUXaNe6+sye0x/4WksN0OjpqfNpLEhHqHHL2REBiQuJ +q5yi4Qk8mrjORgX2i/GzCxmujw/n7Bsbmg26DDzPOzLQlyU/AXzBK4XbEiNZzctSJKE ONEJ1vwfJWSjgv54UsGUIaUwAWRs1iDmDBzjrhEsZGAKUZgiI3eMI55N7l5e9z6TvX6h /0LQ== X-Gm-Message-State: AOAM531E/kYhzi4FPGMJup6OTOccWR2ps/qimnUghvBHO1vWa5h3Tiqx l0YdfYnQdLoDnLi57xu0t1A7bieLAaNdKr4iGmryqA== X-Google-Smtp-Source: ABdhPJwBfW8b+5wb0y3A65hpMEhCu/FpkdAhiiZU0fRVHBXtqfSUAse8Mqe3LAWxXGm5JXrx3EH2RrhSOLaypQu9ewE= X-Received: by 2002:a17:902:a50c:: with SMTP id s12mr33346815plq.119.1594162900874; Tue, 07 Jul 2020 16:01:40 -0700 (PDT) MIME-Version: 1.0 References: <20200630173734.14057-1-will@kernel.org> <20200630173734.14057-19-will@kernel.org> <20200701170722.4rte5ssnmrn2uqzg@bakewell.cambridge.arm.com> <20200702072301.GA15963@willie-the-truck> <20200706160023.GB10992@arm.com> <20200706163455.GV9247@paulmck-ThinkPad-P72> <20200706170556.GE10992@arm.com> <20200706173628.GZ9247@paulmck-ThinkPad-P72> <20200707102915.GI10992@arm.com> <20200707225122.GJ9247@paulmck-ThinkPad-P72> In-Reply-To: <20200707225122.GJ9247@paulmck-ThinkPad-P72> From: Nick Desaulniers Date: Tue, 7 Jul 2020 16:01:28 -0700 Message-ID: Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y To: "Paul E. McKenney" , Dave Martin , Peter Zijlstra , Will Deacon , Sami Tolvanen , Marco Elver Cc: Mark Rutland , "Michael S. Tsirkin" , Catalin Marinas , Jason Wang , virtualization@lists.linux-foundation.org, Alan Stern , Matt Turner , kernel-team , Kees Cook , Arnd Bergmann , Boqun Feng , Josh Triplett , Ivan Kokshaysky , Linux ARM , Richard Henderson , LKML , linux-alpha@vger.kernel.org, Steven Rostedt Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I'm trying to put together a Micro Conference for Linux Plumbers conference focused on "make LLVM slightly less shitty." Do you all plan on attending the conference? Would it be worthwhile to hold a session focused on discussing this (LTO and memory models) be worthwhile? On Tue, Jul 7, 2020 at 3:51 PM Paul E. McKenney wrote: > > On Tue, Jul 07, 2020 at 11:29:15AM +0100, Dave Martin wrote: > > On Mon, Jul 06, 2020 at 10:36:28AM -0700, Paul E. McKenney wrote: > > > On Mon, Jul 06, 2020 at 06:05:57PM +0100, Dave Martin wrote: > > [ . . . ] > > > > > The underlying problem here seems to be that the necessary ordering > > > > rule is not part of what passes for the C memory model prior to C11. > > > > If we want to control the data flow, don't we have to wrap the entire > > > > dereference in a macro? > > > > > > Yes, exactly. Because we are relying on things that are not guaranteed > > > by the C memory model, we need to pay attention to the implementations. > > > As I have said elsewhere, the price of control dependencies is eternal > > > vigilance. > > > > > > And this also applies, to a lesser extent, to address and data > > > dependencies, which are also not well supported by the C standard. > > > > > > There is one important case in which the C memory model -does- support > > > control dependencies, and that is when the dependent write is a normal > > > C-language write that is not involved in a data race. In that case, > > > if the compiler broke the control dependency, it might have introduced > > > a data race, which it is forbidden to do. However, this rule can also > > > be broken when the compiler knows too much, as it might be able to prove > > > that breaking the dependency won't introduce a data race. In that case, > > > according to the standard, it is free to break the dependency. > > > > Which only matters because the C abstract machine may not match reality. > > > > LTO has no bearing on the abstract machine though. > > > > If specific compiler options etc. can be added to inhibit the > > problematic optimisations, that would be ideal. I guess that can't > > happen overnight though. > > Sadly, I must agree. > > > > > > > > We likely won't realise if/when this goes wrong, other than impossible to > > > > > > > debug, subtle breakage that crops up seemingly randomly. Ideally, we'd be > > > > > > > able to detect this sort of thing happening at build time, and perhaps > > > > > > > even prevent it with compiler options or annotations, but none of that is > > > > > > > close to being available and I'm keen to progress the LTO patches in the > > > > > > > meantime because they are a requirement for CFI. > > > > > > > > > > > > My concern was not so much why LTO makes things dangerous, as why !LTO > > > > > > makes things safe... > > > > > > > > > > Because ignorant compilers are safe compilers! ;-) > > > > > > > > AFAICT ignorance is no gurantee of ordering in general -- the compiler > > > > is free to speculatively invent knowledge any place that the language > > > > spec allows it to. !LTO doesn't stop this happening. > > > > > > Agreed, according to the standard, the compiler has great freedom. > > > > > > We have two choices: (1) Restrict ourselves to live within the confines of > > > the standard or (2) Pay continued close attention to the implementation. > > > We have made different choices at different times, but for many ordering > > > situations we have gone with door #2. > > > > > > Me, I have been working to get the standard to better support our > > > use case. This is at best slow going. But don't take my word for it, > > > ask Will. > > > > I can believe it. They want to enable optimisations rather than prevent > > them... > > Right in one! ;-) > > > > > Hopefully some of the knowledge I invented in my reply is valid... > > > > > > It is. It is just that there are multiple valid strategies, and the > > > Linux kernel is currently taking a mixed-strategy approach. > > > > Ack. The hope that there is a correct way to fix everything dies > > hard ;) > > Either that, or one slowly degrades ones definition of "correct". :-/ > > > Life was cosier before I started trying to reason about language specs. > > Same here! > > Thanx, Paul -- Thanks, ~Nick Desaulniers