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 E69B8C76188 for ; Fri, 19 Jul 2019 10:00:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BB8F62173B for ; Fri, 19 Jul 2019 10:00:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726916AbfGSKAJ (ORCPT ); Fri, 19 Jul 2019 06:00:09 -0400 Received: from foss.arm.com ([217.140.110.172]:41202 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725794AbfGSKAI (ORCPT ); Fri, 19 Jul 2019 06:00:08 -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 DAF05337; Fri, 19 Jul 2019 03:00:05 -0700 (PDT) Received: from lakrids.cambridge.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 4AC3F3F59C; Fri, 19 Jul 2019 03:00:04 -0700 (PDT) Date: Fri, 19 Jul 2019 10:59:59 +0100 From: Mark Rutland To: Masami Hiramatsu Cc: "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 , James Morse Subject: Re: [PATCH 3/3] arm64: debug: Remove rcu_read_lock from debug exception Message-ID: <20190719095958.GA19605@lakrids.cambridge.arm.com> References: <156342860634.8565.14804606041960884732.stgit@devnote2> <156342863822.8565.7624877983728871995.stgit@devnote2> <20190718062215.GG14271@linux.ibm.com> <20190718092022.GA3625@blommer> <20190718233133.146065f668da6297e57e52ef@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190718233133.146065f668da6297e57e52ef@kernel.org> User-Agent: Mutt/1.11.1+11 (2f07cb52) (2018-12-01) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 18, 2019 at 11:31:33PM +0900, 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 > [ 135.125581] 5.2.0-08445-g9187c508bdc7 #20 Not tainted > [ 135.125904] ----------------------------- > [ 135.126205] include/linux/rcupdate.h:594 rcu_read_lock() used illegally while idle! > [ 135.126839] > [ 135.126839] other info that might help us debug this: > [ 135.126839] > [ 135.127410] > [ 135.127410] RCU used illegally from idle CPU! > [ 135.127410] rcu_scheduler_active = 2, debug_locks = 1 > [ 135.128114] RCU used illegally from extended quiescent state! > [ 135.128555] 1 lock held by swapper/0/0: > [ 135.128944] #0: (____ptrval____) (rcu_read_lock){....}, at: call_break_hook+0x0/0x178 > [ 135.130499] > [ 135.130499] stack backtrace: > [ 135.131192] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0-08445-g9187c508bdc7 #20 > [ 135.131841] Hardware name: linux,dummy-virt (DT) > [ 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 > [ 135.136597] > > > > 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? They're more like faults, in that they're synchronous exceptions. Given that, I think using irq_enter() / irq_exit() would be surprising here, but perhaps they're misnamed. What do other architectures do here? Having a kprobe on the critical path to idle doesn't sound specific to arm64, but perhaps it is (and we should rule it out). Thanks, Mark.