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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 7DDE1C76188 for ; Fri, 19 Jul 2019 08:42:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 55B362184E for ; Fri, 19 Jul 2019 08:42:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726916AbfGSImU (ORCPT ); Fri, 19 Jul 2019 04:42:20 -0400 Received: from foss.arm.com ([217.140.110.172]:40254 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725794AbfGSImT (ORCPT ); Fri, 19 Jul 2019 04:42:19 -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 BCF95337; Fri, 19 Jul 2019 01:42:18 -0700 (PDT) Received: from [10.1.36.145] (c02yf087jhd5.cambridge.arm.com [10.1.36.145]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1E19E3F71A; Fri, 19 Jul 2019 01:42:16 -0700 (PDT) Subject: Re: [PATCH 3/3] arm64: debug: Remove rcu_read_lock from debug exception To: Masami Hiramatsu Cc: Mark Rutland , "Paul E. McKenney" , Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Naresh Kamboju , Dan Rue , Matt Hart , Anders Roxell , Daniel Diaz References: <156342860634.8565.14804606041960884732.stgit@devnote2> <156342863822.8565.7624877983728871995.stgit@devnote2> <20190718062215.GG14271@linux.ibm.com> <20190718092022.GA3625@blommer> <20190718233133.146065f668da6297e57e52ef@kernel.org> From: James Morse Message-ID: <536ba068-50de-963e-c3a7-0440da56943a@arm.com> Date: Fri, 19 Jul 2019 09:42:05 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190718233133.146065f668da6297e57e52ef@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 7/18/19 3:31 PM, Masami Hiramatsu wrote: > On Thu, 18 Jul 2019 10:20:23 +0100 > Mark Rutland wrote: > >> On Wed, Jul 17, 2019 at 11:22:15PM -0700, Paul E. McKenney wrote: >>> On Thu, Jul 18, 2019 at 02:43:58PM +0900, Masami Hiramatsu wrote: >>>> Remove rcu_read_lock()/rcu_read_unlock() from debug exception >>>> handlers since the software breakpoint can be hit on idle task. >> >> Why precisely do we need to elide these? Are we seeing warnings today? > > Yes, unfortunately, or fortunately. Naresh reported that warns when > ftracetest ran. I confirmed that happens if I probe on default_idle_call too. > > /sys/kernel/debug/tracing # echo p default_idle_call >> kprobe_events > /sys/kernel/debug/tracing # echo 1 > events/kprobes/enable > /sys/kernel/debug/tracing # [ 135.122237] > [ 135.125035] ============================= > [ 135.125310] WARNING: suspicious RCU usage > [ 135.132224] Call trace: > [ 135.132491] dump_backtrace+0x0/0x140 > [ 135.132806] show_stack+0x24/0x30 > [ 135.133133] dump_stack+0xc4/0x10c > [ 135.133726] lockdep_rcu_suspicious+0xf8/0x108 > [ 135.134171] call_break_hook+0x170/0x178 > [ 135.134486] brk_handler+0x28/0x68 > [ 135.134792] do_debug_exception+0x90/0x150 > [ 135.135051] el1_dbg+0x18/0x8c > [ 135.135260] default_idle_call+0x0/0x44 > [ 135.135516] cpu_startup_entry+0x2c/0x30 > [ 135.135815] rest_init+0x1b0/0x280 > [ 135.136044] arch_call_rest_init+0x14/0x1c > [ 135.136305] start_kernel+0x4d4/0x500 >>> The exception entry and exit use irq_enter() and irq_exit(), in this >>> case, correct? Otherwise RCU will be ignoring this CPU. >> >> This is missing today, which sounds like the underlying bug. > > Agreed. I'm not so familier with how debug exception is handled on arm64, > would it be a kind of NMI or IRQ? Debug exceptions can interrupt both SError (think: machine check) and pseudo-NMI, which both in turn interrupt interrupt-masked code. So they are a kind of NMI. But, be careful not to call 'nmi_enter()' twice, see do_serror() for how we work around this... > Anyway, it seems that normal irqs are also not calling irq_enter/exit > except for arch/arm64/kernel/smp.c drivers/irqchip/irq-gic.c:gic_handle_irq() either calls handle_domain_irq() or handle_IPI(). The enter/exit calls live in those functions. Thanks, James