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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 DFF3AC4740A for ; Tue, 10 Sep 2019 13:24:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B05D521479 for ; Tue, 10 Sep 2019 13:24:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1568121863; bh=pnVHhUoA0bQkP4vAFyAiTnaV2vHizoN7pMNixvUz4Xo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=hoP1/+pgEFWNlbSlp8AuuggC0ROBZ8Fm7WPikmeOd7sTkFghE/KOpq5AYW4XwTIcO d/SCQQfW60ZCJMHis7RFUu8Na9Ci7yJ3UDSVy2TmozmCQZfMBQ+ksXtQDeO8GW3gwl +VAOvBrMXYDtVJSUfK0GxdnIvMQ1ppPodh7ufgik= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732159AbfIJNYW (ORCPT ); Tue, 10 Sep 2019 09:24:22 -0400 Received: from mail.kernel.org ([198.145.29.99]:48902 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726491AbfIJNYW (ORCPT ); Tue, 10 Sep 2019 09:24:22 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 2A11F2067B; Tue, 10 Sep 2019 13:24:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1568121861; bh=pnVHhUoA0bQkP4vAFyAiTnaV2vHizoN7pMNixvUz4Xo=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=QXJGMdSYwg11CHswIYBqj4ts9Z2dVSTLWmih/qgJkikZh0AkTYGXL36ue2YP1eo/3 WlvCFhAvWd67NqgUkVLu5zZwv/MYLkiz9FQ6f8zpJDob8YMGyJD1W2ZtI1eTJPyN2i 4J8W+Jn7BKTUvXG3S2Lqz+MxeTJ7EK20sA1gd468= Date: Tue, 10 Sep 2019 14:24:15 +0100 From: Will Deacon To: Arnd Bergmann Cc: Catalin Marinas , Andrew Murray , Mark Rutland , Linux ARM , "linux-kernel@vger.kernel.org" , clang-built-linux , Masahiro Yamada , Borislav Petkov , Ingo Molnar Subject: Re: [PATCH] arm64: fix unreachable code issue with cmpxchg Message-ID: <20190910132415.4j2ygxhuanihvzhx@willie-the-truck> References: <20190909202153.144970-1-arnd@arndb.de> <20190910074606.45k5m2pkztlpj4nj@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 10, 2019 at 10:04:24AM +0200, Arnd Bergmann wrote: > On Tue, Sep 10, 2019 at 9:46 AM Will Deacon wrote: > > > > On Mon, Sep 09, 2019 at 10:21:35PM +0200, Arnd Bergmann wrote: > > > On arm64 build with clang, sometimes the __cmpxchg_mb is not inlined > > > when CONFIG_OPTIMIZE_INLINING is set. > > > > Hmm. Given that CONFIG_OPTIMIZE_INLINING has also been shown to break > > assignment of local 'register' variables on GCC, perhaps we should just > > disable that option for arm64 (at least) since we don't have any toolchains > > that seem to like it very much! I'd certainly prefer that over playing > > whack-a-mole with __always_inline. > > Right, but I can also see good reasons to keep going: > > - In theory, CONFIG_OPTIMIZE_INLINING is the right thing to do -- the compilers > also make some particularly bad decisions around inlining when each inline > turns into an __always_inline, as has been the case in Linux for a long time. > I think in most cases, we get better object code with CONFIG_OPTIMIZE_INLINING > and in the cases where this is worse, it may be better to fix the compiler. > The new "asm_inline" macro should also help with that. Sure, in theory, but it looks like there isn't a single arm64 compiler out there which gets it right. > - The x86 folks have apparently whacked most of the moles already, see this > commit from 2008 > > commit 3f9b5cc018566ad9562df0648395649aebdbc5e0 > Author: Ingo Molnar > Date: Fri Jul 18 16:30:05 2008 +0200 > > x86: re-enable OPTIMIZE_INLINING > > re-enable OPTIMIZE_INLINING more widely. Jeff Dike fixed the remaining > outstanding issue in this commit: > > | commit 4f81c5350b44bcc501ab6f8a089b16d064b4d2f6 > | Author: Jeff Dike > | Date: Mon Jul 7 13:36:56 2008 -0400 > | > | [UML] fix gcc ICEs and unresolved externs > [...] > | This patch reintroduces unit-at-a-time for gcc >= 4.0, > bringing back the > | possibility of Uli's crash. If that happens, we'll debug it. > > it's still default-off and thus opt-in. This appears to be fixing an ICE, whereas the issue reported recently for arm64 gcc was silent miscompilation of atomics in some cases. Unfortunately, I can't seem to find the thread :/ Mark, you were on that one too, right? > - The inlining decisions of gcc and clang are already very different, and > the bugs we are finding around that are much more common than > the difference between CONFIG_OPTIMIZE_INLINING=y/n on a > given compiler. Sorry, not sure that you're getting at here. Anyway, the second version of your patch looks fine, but I would still prefer to go the extra mile and disable CONFIG_OPTIMIZE_INLINING altogether given that I don't think it's a safe option to enable for us. Will