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 C226CCD3445 for ; Tue, 19 Sep 2023 07:13:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231649AbjISHNz (ORCPT ); Tue, 19 Sep 2023 03:13:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38698 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231629AbjISHNx (ORCPT ); Tue, 19 Sep 2023 03:13:53 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 52FC1100; Tue, 19 Sep 2023 00:13:47 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 610B9C433C7; Tue, 19 Sep 2023 07:13:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1695107627; bh=wb2+b/UJ6JMib1m6bJ2CWIMdJn37CvkgZAjJlUHbdC0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=qxnshcEyD68LkG5eo3qqao2eZ+huPFgvwUpTVO3ynzNkjnoJpxVyHCYOcdydW7p0O 87qmYg7A3g5neW5Nr9XfYXFxo1sMZtEx+nm1bDgGHjs4IaMxxduy57CmeQ7xjNLw93 YUgjW99YSrDTRekU5aIBGOxmleLPzPrXHv/xgfFg= Date: Tue, 19 Sep 2023 09:13:40 +0200 From: Greg Kroah-Hartman To: Jiri Slaby 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() Message-ID: <2023091957-roundish-epilepsy-9686@gregkh> References: <20230822102606.2821311-1-usama.anjum@collabora.com> <2023091835-quill-congress-b691@gregkh> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org 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. Agreed. thanks for the review, greg k-h