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 625C8E784AF for ; Mon, 9 Oct 2023 16:01:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1377095AbjJIQBo (ORCPT ); Mon, 9 Oct 2023 12:01:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36666 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230326AbjJIQBo (ORCPT ); Mon, 9 Oct 2023 12:01:44 -0400 Received: from madras.collabora.co.uk (madras.collabora.co.uk [IPv6:2a00:1098:0:82:1000:25:2eeb:e5ab]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3383591; Mon, 9 Oct 2023 09:01:42 -0700 (PDT) Received: from [192.168.100.7] (unknown [39.34.187.171]) (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 A44B2660716E; Mon, 9 Oct 2023 17:01:36 +0100 (BST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1696867300; bh=JyCJjEdKDLAlTbnUu+YzmviHWlf4l4V/ZaDd4AoNfZ8=; h=Date:Cc:Subject:To:References:From:In-Reply-To:From; b=arLoE8vSMK/pwr6KnXQ+vQrYtq9qdcjIYiv7IEBKOikws56wuAIQVlTpi6EtrzhWu vRT33AmD8Cu+60weu9K6XF8Yro/M1RfKLQYWcPEH8sVUo4h5jUbZK98Ra5j1Kk/gAj +gWiNC6i7E+a6tfbo6+cCNAfNXq9OhPm6M3/1gwKayAfPBYAjoBtXuRYJftPOcq4TW o0flrKM6W11fUTtaKtOEhN8L8D2fMJIB3jUeuzWyRUFFd+T/7Q5NKdpSZzV6Q2WiJN WFhUYgyCq2y7c5CKsVlOpi14BRUJdgNuOy68ho6nJj8LT8vyGfw96J+jn0gSfymVEn 89Id3+cpBkw1w== Message-ID: Date: Mon, 9 Oct 2023 21:01:33 +0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: Muhammad Usama Anjum , Ingo Molnar , kernel@collabora.com, stable@vger.kernel.org, linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org Subject: Re: [PATCH v3] tty/sysrq: replace smp_processor_id() with get_cpu() To: Greg Kroah-Hartman , Jiri Slaby References: <20230822102606.2821311-1-usama.anjum@collabora.com> <2023091835-quill-congress-b691@gregkh> <2023091957-roundish-epilepsy-9686@gregkh> Content-Language: en-US From: Muhammad Usama Anjum In-Reply-To: <2023091957-roundish-epilepsy-9686@gregkh> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On 9/19/23 12:13 PM, Greg Kroah-Hartman wrote: > On Tue, Sep 19, 2023 at 07:52:42AM +0200, Jiri Slaby wrote: >> On 18. 09. 23, 10:10, 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") >>>> 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()); >>> >>> Why not call put_cpu() right here? >>> >>>> if (regs) >>>> show_regs(regs); >>>> else >>>> show_stack(NULL, NULL, KERN_INFO); >>>> schedule_work(&sysrq_showallcpus); >>>> + put_cpu(); >>> >>> Why wait so long here after you have scheduled work? Please drop the >>> cpu reference right away, you don't need to hold it for this length of >>> time, right? >> >> As I understand it, this way, schedule_work() will queue the work on the >> "gotten" (current) CPU. So sysrq_showregs_othercpus() will really dump other >> than the "gotten" cpu. > > Ok, that makes a bit more sense, but that's not what the code does > today, have people seen the regs dumped from the wrong cpu in the past? > >> If that is the case, it indeed should have been described in the commit log. Thanks for review. I'll add the explanation in the commit log and send again. > > Agreed. > > thanks for the review, > > greg k-h -- BR, Muhammad Usama Anjum