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=-1.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 2FF6DC76195 for ; Thu, 18 Jul 2019 14:31:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 06ECE217F4 for ; Thu, 18 Jul 2019 14:31:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1563460316; bh=n0QuHuPmfGNDd/UjBj7iq8FnefrdHa1lz8nf16gnbDo=; h=Date:From:To:Cc:Subject:In-Reply-To:References:List-ID:From; b=VbNIPeqLOTk7TMs/tynvYdzAgQzIguIlAVt5n15wPuhPMYkkmNGu397Q54cqDePrL Ayvtz2CMxhgTUinZspdVTNy1M6v9oSHawJtPMYdVTsjk5ZPdzl5zOmPdURz1xNw0BP vP8mG7wrj5HHQ57+p4ovJcYKP7eZ2XPZqfKzi4aQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390887AbfGRObz (ORCPT ); Thu, 18 Jul 2019 10:31:55 -0400 Received: from mail.kernel.org ([198.145.29.99]:57530 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2390758AbfGRObh (ORCPT ); Thu, 18 Jul 2019 10:31:37 -0400 Received: from devnote2 (ae027047.dynamic.ppp.asahi-net.or.jp [14.3.27.47]) (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 D2D48217F4; Thu, 18 Jul 2019 14:31:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1563460296; bh=n0QuHuPmfGNDd/UjBj7iq8FnefrdHa1lz8nf16gnbDo=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=SAx8gDCjJJ1wdu+rR633sjpFn5TnWHaw0ry7jDsMkQ0OJxT3DG0n4HrAafwjAmlSv c874iRbW+O2MqL45QNl0N8V73NSlzxPS3wZtU+mLqoFvncxRhIigcgHEv16ZuA03Ug OlOMEGUQ5iEUFMYfXhfniBGHtbYlo1bWZjLa1SCk= Date: Thu, 18 Jul 2019 23:31:33 +0900 From: Masami Hiramatsu To: Mark Rutland Cc: "Paul E. McKenney" , Masami Hiramatsu , 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 Subject: Re: [PATCH 3/3] arm64: debug: Remove rcu_read_lock from debug exception Message-Id: <20190718233133.146065f668da6297e57e52ef@kernel.org> In-Reply-To: <20190718092022.GA3625@blommer> References: <156342860634.8565.14804606041960884732.stgit@devnote2> <156342863822.8565.7624877983728871995.stgit@devnote2> <20190718062215.GG14271@linux.ibm.com> <20190718092022.GA3625@blommer> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Anyway, it seems that normal irqs are also not calling irq_enter/exit except for arch/arm64/kernel/smp.c. We need to fix that too for avoiding unexpected RCU issues. Thank you, -- Masami Hiramatsu