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=-4.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=unavailable 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 8624BC10F00 for ; Wed, 6 Mar 2019 11:08:58 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5EFE02064A for ; Wed, 6 Mar 2019 11:08:58 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730030AbfCFLI5 (ORCPT ); Wed, 6 Mar 2019 06:08:57 -0500 Received: from foss.arm.com ([217.140.101.70]:59258 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726689AbfCFLI4 (ORCPT ); Wed, 6 Mar 2019 06:08:56 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 2BC5B80D; Wed, 6 Mar 2019 03:08:56 -0800 (PST) Received: from [10.1.196.50] (e108454-lin.cambridge.arm.com [10.1.196.50]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E73713F71D; Wed, 6 Mar 2019 03:08:53 -0800 (PST) Subject: Re: [PATCH] tty/sysrq: Convert show_lock to raw_spinlock_t To: Steven Rostedt Cc: Sebastian Andrzej Siewior , "linux-kernel@vger.kernel.org" , "jslaby@suse.com" , "gregkh@linuxfoundation.org" , "linux-rt-users@vger.kernel.org" , Thomas Gleixner References: <20190304172053.17340-1-julien.grall@arm.com> <545fb96e-0f07-758e-5c79-a725e74e3387@arm.com> <20190304220152.nozgclpu2jx3zgvy@linutronix.de> <75159b6e-0733-e300-e4f5-d519ca7f6b13@arm.com> <20190304181521.06e42396@gandalf.local.home> From: Julien Grall Message-ID: <1ba3a2cf-36ff-bce0-8d2d-6e8ed8a81056@arm.com> Date: Wed, 6 Mar 2019 11:08:52 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <20190304181521.06e42396@gandalf.local.home> 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 Steve, On 04/03/2019 23:15, Steven Rostedt wrote: > On Mon, 4 Mar 2019 22:25:41 +0000 > Julien Grall wrote: >> On 04/03/2019 22:01, Sebastian Andrzej Siewior wrote: >>> On 2019-03-04 17:21:57 [+0000], Julien Grall wrote: >>>> (CC correctly linux-rt-users) >>>> >>>> On 04/03/2019 17:20, Julien Grall wrote: >>>>> At the moment show_lock is implemented using spin_lock_t and called from >>>>> an interrupt context on Arm64. The following backtrace was triggered by: >>>>> >>>>> 42sh# echo l > /proc/sysrq-trigger >>>>> >>>>> [ 4432.073756] sysrq: SysRq : Show backtrace of all active CPUs >>>>> [ 4432.403422] BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:974 >>>>> [ 4432.403424] sysrq: CPU6: >>>>> [ 4432.403426] in_atomic(): 1, irqs_disabled(): 128, pid: 2410, name: kworker/u16:2 >>>>> >>>>> [...] >>>>> >>>>> [ 4432.403581] Call trace: >>>>> [ 4432.403584] dump_backtrace+0x0/0x148 >>>>> [ 4432.403586] show_stack+0x14/0x20 >>>>> [ 4432.403588] dump_stack+0x9c/0xd4 >>>>> [ 4432.403592] ___might_sleep+0x1cc/0x298 >>>>> [ 4432.403595] rt_spin_lock+0x5c/0x70 >>>>> [ 4432.403596] showacpu+0x34/0x68 >>>>> [ 4432.403599] flush_smp_call_function_queue+0xd4/0x278 >>>>> [ 4432.403602] generic_smp_call_function_single_interrupt+0x10/0x18 >>>>> [ 4432.403605] handle_IPI+0x26c/0x668 >>>>> [ 4432.403607] gic_handle_irq+0x9c/0xa0 >>>>> [ 4432.403609] el1_irq+0xb4/0x13c >>>>> >>>>> With RT-patches, spin_lock can now sleep and therefore cannot be used from >>>>> interrupt context. Use a raw_spin_lock instead to prevent the lock to >>>>> sleep. >>>>> >>>>> Signed-off-by: Julien Grall >>> >>> I don't have to look at this properly but this looks is wrong. >> >> May I ask why does it look wrong? On Arm64, this code is called from an >> IRQ disabled context (see the check in flush_smp_call_function_queue). > > I'll answer. > > It's going to call printk, and depending on consoles and such, it may > not print anything (a printk without atomic consoles shows nothing). > > That said, perhaps we need to do something like the "safe printk", > where it only loads it into the log buffers and doesn't do the actual > prints. > > Hmm, do we do that now? I need to look at the latest printk code in RT. Thank you for the explanation. I am not entirely familiar with printk, so I may have overlooked something. Looking at the printk code (see vprintk_emit), the message will be loaded in the log buffer. The log buffer will be printed out by klogd if vprintk_emit were called from a context where IRQs or preemption is disabled. So I think the printk should still happen from atomic context but it will get delayed until we are out from the atomic context. Cheers, -- Julien Grall