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=-4.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, TVD_SUBJ_WIPE_DEBT,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 B228FC433DB for ; Thu, 11 Feb 2021 14:45:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7AB2164E92 for ; Thu, 11 Feb 2021 14:45:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231395AbhBKOoS (ORCPT ); Thu, 11 Feb 2021 09:44:18 -0500 Received: from gate.crashing.org ([63.228.1.57]:53778 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230071AbhBKOij (ORCPT ); Thu, 11 Feb 2021 09:38:39 -0500 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 11BEUxQ2020284; Thu, 11 Feb 2021 08:30:59 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 11BEUxVp020283; Thu, 11 Feb 2021 08:30:59 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Thu, 11 Feb 2021 08:30:59 -0600 From: Segher Boessenkool To: Christophe Leroy Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , npiggin@gmail.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON() Message-ID: <20210211143059.GE28121@gate.crashing.org> References: <694c7195c81d1bcc781b3c14f452886683d6c524.1613029237.git.christophe.leroy@csgroup.eu> <20210211114910.GA28121@gate.crashing.org> <6126ca14-419a-9e15-7ffa-b295f26a552e@csgroup.eu> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6126ca14-419a-9e15-7ffa-b295f26a552e@csgroup.eu> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 11, 2021 at 03:09:43PM +0100, Christophe Leroy wrote: > Le 11/02/2021 à 12:49, Segher Boessenkool a écrit : > >On Thu, Feb 11, 2021 at 07:41:52AM +0000, Christophe Leroy wrote: > >>powerpc BUG_ON() is based on using twnei or tdnei instruction, > >>which obliges gcc to format the condition into a 0 or 1 value > >>in a register. > > > >Huh? Why is that? > > > >Will it work better if this used __builtin_trap? Or does the kernel only > >detect very specific forms of trap instructions? > > We already made a try with __builtin_trap() 1,5 year ago, see > https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20510ce03cc9463f1c9e743c1d93b939de501b53.1566219503.git.christophe.leroy@c-s.fr/ > > The main problems encountered are: > - It is only possible to use it for BUG_ON, not for WARN_ON because GCC > considers it as noreturn. Is there any workaround ? A trap is noreturn by definition: -- Built-in Function: void __builtin_trap (void) This function causes the program to exit abnormally. > - The kernel (With CONFIG_DEBUG_BUGVERBOSE) needs to be able to identify > the source file and line corresponding to the trap. How can that be done > with __builtin_trap() ? The DWARF debug info should be sufficient. Perhaps you can post-process some way? You can create a trap that falls through yourself (by having a trap-on condition with a condition that is always true, but make the compiler not see that). This isn't efficient though. Could you file a feature request (in bugzilla)? It is probably useful for generic code as well, but we could implement this for powerpc only if needed. Segher