From nobody Tue Feb 10 04:23:47 2026 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 2B0F2C7EE22 for ; Mon, 15 May 2023 12:30:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241836AbjEOMaq (ORCPT ); Mon, 15 May 2023 08:30:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54866 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240917AbjEOMam (ORCPT ); Mon, 15 May 2023 08:30:42 -0400 X-Greylist: delayed 642 seconds by postgrey-1.37 at lindbergh.monkeyblade.net; Mon, 15 May 2023 05:30:40 PDT Received: from mail-m2836.qiye.163.com (mail-m2836.qiye.163.com [103.74.28.36]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E8D0E19C for ; Mon, 15 May 2023 05:30:40 -0700 (PDT) Received: from ubuntu.localdomain (unknown [36.170.45.137]) by mail-m2836.qiye.163.com (Hmail) with ESMTPA id 796BFC0237; Mon, 15 May 2023 20:19:54 +0800 (CST) From: Yu Chen To: minyard@acm.org Cc: openipmi-developer@lists.sourceforge.net, linux-kernel@vger.kernel.org, chen.yu@easystack.cn Subject: [PATCH] watchdog: Avoid 100% CPU usage during reading watchdog when a task get signal Date: Mon, 15 May 2023 05:19:41 -0700 Message-Id: <20230515121941.59859-1-chen.yu@easystack.cn> X-Mailer: git-send-email 2.25.1 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-HM-Spam-Status: e1kfGhgUHx5ZQUpXWQgPGg8OCBgUHx5ZQUlOS1dZFg8aDwILHllBWSg2Ly tZV1koWUFJQjdXWS1ZQUlXWQ8JGhUIEh9ZQVkZHRkeVhpIQk0YS0xNTU5KSlUZERMWGhIXJBQOD1 lXWRgSC1lBWUhNVUpMS1VPTlVKSExZV1kWGg8SFR0UWUFZT0tIVUpKS0hKQ1VKS0tVS1kG X-HM-Tid: 0a881f5a58b1841ekuqw796bfc0237 X-HM-MType: 1 X-HM-Sender-Digest: e1kMHhlZQR0aFwgeV1kSHx4VD1lBWUc6MC46EBw6SzJPNxxIDz0MARYR NRUaCi5VSlVKTUNPSk5ISkJOSU9LVTMWGhIXVRgTHhVVAg47HhoIAggPGhgQVRgVRVlXWRILWUFZ SE1VSkxLVU9OVUpITFlXWQgBWUFIS0pKNwY+ Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" A simple reproducer demonstrating the problem: (use ipmi_watchdog.ko) In one terminal: $ cat /dev/watchdog ... In another terminal: $ ps -aux | grep cat 14755 pts/1 R+ 43:00 cat /dev/watchdog 51943 pts/2 S+ 0:00 grep --color=3Dauto cat $ kill -9 14755 $ $ cat /proc/14755/status | grep SigPnd SigPnd: 0000000000000100 $ $ top Tasks: 1049 total, 2 running, 1047 sleeping, 0 stopped, 0 zombie %Cpu(s): 0.0 us, 1.0 sy, 0.0 ni, 98.9 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st MiB Mem : 522594.8 total, 517241.4 free, 2922.1 used, 2431.2 buff/cache MiB Swap: 0.0 total, 0.0 free, 0.0 used. 516589.2 avail Mem PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 14755 root 20 0 215552 1024 576 R 100.0 0.0 0:15.12 cat 53417 root 20 0 224960 7040 3648 R 0.7 0.0 0:00.10 top 11 root 20 0 0 0 0 I 0.3 0.0 0:02.85 rcu_sched 1772 root 20 0 512256 387776 380800 S 0.3 0.1 0:32.05 python We can see that when the cat process gets the signal, the CPU usage is 100%, Since signal_pending is true, the pick_next_task function in schedule always returns itself, it retries schedule indefinitely. ipmi_read() will busyloop. Signed-off-by: Yu Chen --- drivers/char/ipmi/ipmi_watchdog.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/drivers/char/ipmi/ipmi_watchdog.c b/drivers/char/ipmi/ipmi_wat= chdog.c index 0d4a8dcac..173ed4266 100644 --- a/drivers/char/ipmi/ipmi_watchdog.c +++ b/drivers/char/ipmi/ipmi_watchdog.c @@ -803,6 +803,11 @@ static ssize_t ipmi_read(struct file *file, init_waitqueue_entry(&wait, current); add_wait_queue(&read_q, &wait); while (!data_to_read) { + if (signal_pending(current)) { + remove_wait_queue(&read_q, &wait); + rv =3D -ERESTARTSYS; + goto out; + } set_current_state(TASK_INTERRUPTIBLE); spin_unlock_irq(&ipmi_read_lock); schedule(); @@ -810,10 +815,6 @@ static ssize_t ipmi_read(struct file *file, } remove_wait_queue(&read_q, &wait); =20 - if (signal_pending(current)) { - rv =3D -ERESTARTSYS; - goto out; - } } data_to_read =3D 0; =20 --=20 2.27.0