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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DB3B4C83F1E for ; Wed, 30 Aug 2023 18:38:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236765AbjH3Sic (ORCPT ); Wed, 30 Aug 2023 14:38:32 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51308 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242239AbjH3Hev (ORCPT ); Wed, 30 Aug 2023 03:34:51 -0400 Received: from madras.collabora.co.uk (madras.collabora.co.uk [46.235.227.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8F845CC9; Wed, 30 Aug 2023 00:34:48 -0700 (PDT) Received: from [192.168.100.7] (unknown [39.34.186.40]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: usama.anjum) by madras.collabora.co.uk (Postfix) with ESMTPSA id EE30866071DF; Wed, 30 Aug 2023 08:34:44 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1693380887; bh=FAqKMrDY1sgnEDen564Uz0JqloJwW0ZuxAgPpMdqgus=; h=Date:Cc:Subject:To:References:From:In-Reply-To:From; b=Qf2duL2p2Hicri0va/ik8ADYHPGFylNPxgGYRWTu8yc/Zo5SoYVrbSgjDerAoCDmc 9x0fbsqbD878DXxsTuaO9hDWCOozhx52nu+24C9dRmxGF6VVSOHx0vHcCCDL6U5kvQ UduiMAMHgQ4/9ECJWkPcMIP57EdIs0IoGbdvROxzsB9qylzYJFMk+APngqouGeref0 Y15ecksvXyNGrQq04GPDk1XarcKT2hgsONwXloHlCEfyrrHWqAoULqLW9THdzHT92Q RqHwn5ohgMmxR5mNBTy9cMgMRC5blzL5c4tRzW00EB2qg9rTJ0ygHQ1m65q87IWlmA pYxjXZF4Gms/A== Message-ID: Date: Wed, 30 Aug 2023 12:34:38 +0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: Muhammad Usama Anjum , kernel@collabora.com, stable@vger.kernel.org Subject: Re: [PATCH v3] tty/sysrq: replace smp_processor_id() with get_cpu() To: Greg Kroah-Hartman , linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org, Ingo Molnar , Jiri Slaby References: <20230822102606.2821311-1-usama.anjum@collabora.com> <2023082258-lethargic-hazily-5c7e@gregkh> Content-Language: en-US From: Muhammad Usama Anjum In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-serial@vger.kernel.org On 8/23/23 4:06 PM, Muhammad Usama Anjum wrote: > On 8/22/23 6:24 PM, Greg Kroah-Hartman wrote: >> On Tue, Aug 22, 2023 at 03:26:06PM +0500, Muhammad Usama Anjum wrote: >>> The smp_processor_id() shouldn't be called from preemptible code. >>> Instead use get_cpu() and put_cpu() which disables preemption in >>> addition to getting the processor id. This fixes the following bug: >>> >>> [ 119.143590] sysrq: Show backtrace of all active CPUs >>> [ 119.143902] BUG: using smp_processor_id() in preemptible [00000000] code: bash/873 >>> [ 119.144586] caller is debug_smp_processor_id+0x20/0x30 >>> [ 119.144827] CPU: 6 PID: 873 Comm: bash Not tainted 5.10.124-dirty #3 >>> [ 119.144861] Hardware name: QEMU QEMU Virtual Machine, BIOS 2023.05-1 07/22/2023 >>> [ 119.145053] Call trace: >>> [ 119.145093] dump_backtrace+0x0/0x1a0 >>> [ 119.145122] show_stack+0x18/0x70 >>> [ 119.145141] dump_stack+0xc4/0x11c >>> [ 119.145159] check_preemption_disabled+0x100/0x110 >>> [ 119.145175] debug_smp_processor_id+0x20/0x30 >>> [ 119.145195] sysrq_handle_showallcpus+0x20/0xc0 >>> [ 119.145211] __handle_sysrq+0x8c/0x1a0 >>> [ 119.145227] write_sysrq_trigger+0x94/0x12c >>> [ 119.145247] proc_reg_write+0xa8/0xe4 >>> [ 119.145266] vfs_write+0xec/0x280 >>> [ 119.145282] ksys_write+0x6c/0x100 >>> [ 119.145298] __arm64_sys_write+0x20/0x30 >>> [ 119.145315] el0_svc_common.constprop.0+0x78/0x1e4 >>> [ 119.145332] do_el0_svc+0x24/0x8c >>> [ 119.145348] el0_svc+0x10/0x20 >>> [ 119.145364] el0_sync_handler+0x134/0x140 >>> [ 119.145381] el0_sync+0x180/0x1c0 >>> >>> Cc: stable@vger.kernel.org >>> Fixes: 47cab6a722d4 ("debug lockups: Improve lockup detection, fix generic arch fallback")This commit had introduced the smp_processor_id() function in > sysrq_handle_showallcpus(). > >> >> How has this never shown up before now? What changed to cause this to >> now be triggered? This feels odd that no one has seen this in the past >> 20+ years :( > Not sure. Probably the combination of reproduction has happened now. The > following three conditions are needed for the warning to appear: > * Enable CONFIG_DEBUG_PREEMPT > * Arch which doesn't define arch_trigger_all_cpu_backtrace such as arm64 > * Trigger showallcpu's stack sysrqAny thoughts? > >> >> >>> Signed-off-by: Muhammad Usama Anjum >>> --- >>> Changes since v2: >>> - Add changelog and resend >>> >>> Changes since v1: >>> - Add "Cc: stable@vger.kernel.org" tag >>> --- >>> drivers/tty/sysrq.c | 3 ++- >>> 1 file changed, 2 insertions(+), 1 deletion(-) >>> >>> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c >>> index 23198e3f1461a..6b4a28bcf2f5f 100644 >>> --- a/drivers/tty/sysrq.c >>> +++ b/drivers/tty/sysrq.c >>> @@ -262,13 +262,14 @@ static void sysrq_handle_showallcpus(u8 key) >>> if (in_hardirq()) >>> regs = get_irq_regs(); >>> >>> - pr_info("CPU%d:\n", smp_processor_id()); >>> + pr_info("CPU%d:\n", get_cpu()); >>> if (regs) >>> show_regs(regs); >>> else >>> show_stack(NULL, NULL, KERN_INFO); >>> >>> schedule_work(&sysrq_showallcpus); >>> + put_cpu(); >> >> Why are you putting the cpu _after_ you schedule the work? > The sysrq_showallcpus work prints stack traces on all CPUs other than the > current CPU. So we are re-enabling preemption after scheduling work from > current CPU. So that it doesn't get changed. >> >> thanks, >> >> greg k-h > -- BR, Muhammad Usama Anjum