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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham 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 0ACE2C433E0 for ; Mon, 6 Jul 2020 16:08:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E4E4A20722 for ; Mon, 6 Jul 2020 16:08:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729542AbgGFQI1 (ORCPT ); Mon, 6 Jul 2020 12:08:27 -0400 Received: from foss.arm.com ([217.140.110.172]:52620 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729367AbgGFQI0 (ORCPT ); Mon, 6 Jul 2020 12:08:26 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 89E3E1FB; Mon, 6 Jul 2020 09:08:25 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C6AF23F68F; Mon, 6 Jul 2020 09:08:22 -0700 (PDT) Date: Mon, 6 Jul 2020 17:08:20 +0100 From: Dave Martin To: Will Deacon Cc: linux-kernel@vger.kernel.org, Mark Rutland , "Michael S. Tsirkin" , Peter Zijlstra , Catalin Marinas , Jason Wang , virtualization@lists.linux-foundation.org, Arnd Bergmann , Alan Stern , Sami Tolvanen , Matt Turner , kernel-team@android.com, Marco Elver , Kees Cook , "Paul E. McKenney" , Boqun Feng , Josh Triplett , Ivan Kokshaysky , linux-arm-kernel@lists.infradead.org, Richard Henderson , Nick Desaulniers , linux-alpha@vger.kernel.org Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y Message-ID: <20200706160820.GC10992@arm.com> References: <20200630173734.14057-1-will@kernel.org> <20200630173734.14057-19-will@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200630173734.14057-19-will@kernel.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 30, 2020 at 06:37:34PM +0100, Will Deacon wrote: > When building with LTO, there is an increased risk of the compiler > converting an address dependency headed by a READ_ONCE() invocation > into a control dependency and consequently allowing for harmful > reordering by the CPU. > > Ensure that such transformations are harmless by overriding the generic > READ_ONCE() definition with one that provides acquire semantics when > building with LTO. > > Signed-off-by: Will Deacon > --- > arch/arm64/include/asm/rwonce.h | 63 +++++++++++++++++++++++++++++++ > arch/arm64/kernel/vdso/Makefile | 2 +- > arch/arm64/kernel/vdso32/Makefile | 2 +- > 3 files changed, 65 insertions(+), 2 deletions(-) > create mode 100644 arch/arm64/include/asm/rwonce.h > > diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h > new file mode 100644 > index 000000000000..515e360b01a1 > --- /dev/null > +++ b/arch/arm64/include/asm/rwonce.h > @@ -0,0 +1,63 @@ > +/* SPDX-License-Identifier: GPL-2.0 */ > +/* > + * Copyright (C) 2020 Google LLC. > + */ > +#ifndef __ASM_RWONCE_H > +#define __ASM_RWONCE_H > + > +#ifdef CONFIG_CLANG_LTO > + > +#include > +#include > + > +#ifndef BUILD_VDSO > + > +#ifdef CONFIG_AS_HAS_LDAPR > +#define __LOAD_RCPC(sfx, regs...) \ > + ALTERNATIVE( \ > + "ldar" #sfx "\t" #regs, \ ^ Should this be here? It seems that READ_ONCE() will actually read twice... even if that doesn't actually conflict with the required semantics of READ_ONCE(), it looks odd. Making a direct link between LTO and the memory model also seems highly spurious (as discussed in the other subthread) so can we have a comment explaining the reasoning? > + ".arch_extension rcpc\n" \ > + "ldapr" #sfx "\t" #regs, \ > + ARM64_HAS_LDAPR) > +#else > +#define __LOAD_RCPC(sfx, regs...) "ldar" #sfx "\t" #regs > +#endif /* CONFIG_AS_HAS_LDAPR */ [...] Cheers ---Dave