From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4006424BBEB; Mon, 23 Jun 2025 13:17:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750684627; cv=none; b=ScePmLajtqQm0/KSjkrjaybV5Y7qUKCwpjCXY5oZH59VEZYMs85LTp7w2Cx+wBjoHtAgdK95eS+6woISd/1SqM2qo4/UrjRzv32PRBb6ya9nzGiInoSd2iKiOqFWpi185TkBRL92Af3hjtYMYSzvf0i9PbeLttg1dqHgBKHoldI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750684627; c=relaxed/simple; bh=MZjDvTeHK3fuFvKRKFQEMk8YIb/Srzmo1OaFhmZJCuM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=HSQaSHyrW7X6Agsgm3AvvDA3elN40en0EQEyodDZx3sukBkrL/g+u9YMLnzF3w0IQMeBCgT2TsgBJmyZwMa7xF55WwTrIawDR46v/E3V01W6cFAvhxYtg/iDmJWCEWVVbVRU1q3qSY3Rub6RQT5/DGKaGiizmBLSa6FMYCYJDxY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=ycyK9PK7; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="ycyK9PK7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9DE76C4CEEA; Mon, 23 Jun 2025 13:17:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1750684627; bh=MZjDvTeHK3fuFvKRKFQEMk8YIb/Srzmo1OaFhmZJCuM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=ycyK9PK7Wxr1BhjNQFf8bB7/pycOaWNfYN99QrPOx8HGLx1mOmRUWB+JsjmhBiqWe u0mVZlm9qTW7luaeEcvxmA1NN9iAYgTfRNolSAvRAMLh6KQk2dG5UcqLndlwWslhRk ctRNLxazT+DtY5Wq6UcLFYNJT7RowhltNrkiiorQ= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Luo Gengkun , "Nysal Jan K.A." , Doug Anderson , Joel Granados , Song Liu , Thomas Gleinxer , Venkat Rao Bagalkote , Andrew Morton Subject: [PATCH 6.15 139/592] watchdog: fix watchdog may detect false positive of softlockup Date: Mon, 23 Jun 2025 15:01:37 +0200 Message-ID: <20250623130703.583877077@linuxfoundation.org> X-Mailer: git-send-email 2.50.0 In-Reply-To: <20250623130700.210182694@linuxfoundation.org> References: <20250623130700.210182694@linuxfoundation.org> User-Agent: quilt/0.68 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.15-stable review patch. If anyone has any objections, please let me know. ------------------ From: Luo Gengkun commit 7123dbbef88cfd9f09e8a7899b0911834600cfa3 upstream. When updating `watchdog_thresh`, there is a race condition between writing the new `watchdog_thresh` value and stopping the old watchdog timer. If the old timer triggers during this window, it may falsely detect a softlockup due to the old interval and the new `watchdog_thresh` value being used. The problem can be described as follow: # We asuume previous watchdog_thresh is 60, so the watchdog timer is # coming every 24s. echo 10 > /proc/sys/kernel/watchdog_thresh (User space) | +------>+ update watchdog_thresh (We are in kernel now) | | # using old interval and new `watchdog_thresh` +------>+ watchdog hrtimer (irq context: detect softlockup) | | +-------+ | | + softlockup_stop_all To fix this problem, introduce a shadow variable for `watchdog_thresh`. The update to the actual `watchdog_thresh` is delayed until after the old timer is stopped, preventing false positives. The following testcase may help to understand this problem. --------------------------------------------- echo RT_RUNTIME_SHARE > /sys/kernel/debug/sched/features echo -1 > /proc/sys/kernel/sched_rt_runtime_us echo 0 > /sys/kernel/debug/sched/fair_server/cpu3/runtime echo 60 > /proc/sys/kernel/watchdog_thresh taskset -c 3 chrt -r 99 /bin/bash -c "while true;do true; done" & echo 10 > /proc/sys/kernel/watchdog_thresh & --------------------------------------------- The test case above first removes the throttling restrictions for real-time tasks. It then sets watchdog_thresh to 60 and executes a real-time task ,a simple while(1) loop, on cpu3. Consequently, the final command gets blocked because the presence of this real-time thread prevents kworker:3 from being selected by the scheduler. This eventually triggers a softlockup detection on cpu3 due to watchdog_timer_fn operating with inconsistent variable - using both the old interval and the updated watchdog_thresh simultaneously. [nysal@linux.ibm.com: fix the SOFTLOCKUP_DETECTOR=n case] Link: https://lkml.kernel.org/r/20250502111120.282690-1-nysal@linux.ibm.com Link: https://lkml.kernel.org/r/20250421035021.3507649-1-luogengkun@huaweicloud.com Signed-off-by: Luo Gengkun Signed-off-by: Nysal Jan K.A. Cc: Doug Anderson Cc: Joel Granados Cc: Song Liu Cc: Thomas Gleinxer Cc: "Nysal Jan K.A." Cc: Venkat Rao Bagalkote Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman --- kernel/watchdog.c | 41 +++++++++++++++++++++++++++-------------- 1 file changed, 27 insertions(+), 14 deletions(-) --- a/kernel/watchdog.c +++ b/kernel/watchdog.c @@ -47,6 +47,7 @@ int __read_mostly watchdog_user_enabled static int __read_mostly watchdog_hardlockup_user_enabled = WATCHDOG_HARDLOCKUP_DEFAULT; static int __read_mostly watchdog_softlockup_user_enabled = 1; int __read_mostly watchdog_thresh = 10; +static int __read_mostly watchdog_thresh_next; static int __read_mostly watchdog_hardlockup_available; struct cpumask watchdog_cpumask __read_mostly; @@ -870,12 +871,20 @@ int lockup_detector_offline_cpu(unsigned return 0; } -static void __lockup_detector_reconfigure(void) +static void __lockup_detector_reconfigure(bool thresh_changed) { cpus_read_lock(); watchdog_hardlockup_stop(); softlockup_stop_all(); + /* + * To prevent watchdog_timer_fn from using the old interval and + * the new watchdog_thresh at the same time, which could lead to + * false softlockup reports, it is necessary to update the + * watchdog_thresh after the softlockup is completed. + */ + if (thresh_changed) + watchdog_thresh = READ_ONCE(watchdog_thresh_next); set_sample_period(); lockup_detector_update_enable(); if (watchdog_enabled && watchdog_thresh) @@ -888,7 +897,7 @@ static void __lockup_detector_reconfigur void lockup_detector_reconfigure(void) { mutex_lock(&watchdog_mutex); - __lockup_detector_reconfigure(); + __lockup_detector_reconfigure(false); mutex_unlock(&watchdog_mutex); } @@ -908,27 +917,29 @@ static __init void lockup_detector_setup return; mutex_lock(&watchdog_mutex); - __lockup_detector_reconfigure(); + __lockup_detector_reconfigure(false); softlockup_initialized = true; mutex_unlock(&watchdog_mutex); } #else /* CONFIG_SOFTLOCKUP_DETECTOR */ -static void __lockup_detector_reconfigure(void) +static void __lockup_detector_reconfigure(bool thresh_changed) { cpus_read_lock(); watchdog_hardlockup_stop(); + if (thresh_changed) + watchdog_thresh = READ_ONCE(watchdog_thresh_next); lockup_detector_update_enable(); watchdog_hardlockup_start(); cpus_read_unlock(); } void lockup_detector_reconfigure(void) { - __lockup_detector_reconfigure(); + __lockup_detector_reconfigure(false); } static inline void lockup_detector_setup(void) { - __lockup_detector_reconfigure(); + __lockup_detector_reconfigure(false); } #endif /* !CONFIG_SOFTLOCKUP_DETECTOR */ @@ -946,11 +957,11 @@ void lockup_detector_soft_poweroff(void) #ifdef CONFIG_SYSCTL /* Propagate any changes to the watchdog infrastructure */ -static void proc_watchdog_update(void) +static void proc_watchdog_update(bool thresh_changed) { /* Remove impossible cpus to keep sysctl output clean. */ cpumask_and(&watchdog_cpumask, &watchdog_cpumask, cpu_possible_mask); - __lockup_detector_reconfigure(); + __lockup_detector_reconfigure(thresh_changed); } /* @@ -984,7 +995,7 @@ static int proc_watchdog_common(int whic } else { err = proc_dointvec_minmax(table, write, buffer, lenp, ppos); if (!err && old != READ_ONCE(*param)) - proc_watchdog_update(); + proc_watchdog_update(false); } mutex_unlock(&watchdog_mutex); return err; @@ -1035,11 +1046,13 @@ static int proc_watchdog_thresh(const st mutex_lock(&watchdog_mutex); - old = READ_ONCE(watchdog_thresh); + watchdog_thresh_next = READ_ONCE(watchdog_thresh); + + old = watchdog_thresh_next; err = proc_dointvec_minmax(table, write, buffer, lenp, ppos); - if (!err && write && old != READ_ONCE(watchdog_thresh)) - proc_watchdog_update(); + if (!err && write && old != READ_ONCE(watchdog_thresh_next)) + proc_watchdog_update(true); mutex_unlock(&watchdog_mutex); return err; @@ -1060,7 +1073,7 @@ static int proc_watchdog_cpumask(const s err = proc_do_large_bitmap(table, write, buffer, lenp, ppos); if (!err && write) - proc_watchdog_update(); + proc_watchdog_update(false); mutex_unlock(&watchdog_mutex); return err; @@ -1080,7 +1093,7 @@ static const struct ctl_table watchdog_s }, { .procname = "watchdog_thresh", - .data = &watchdog_thresh, + .data = &watchdog_thresh_next, .maxlen = sizeof(int), .mode = 0644, .proc_handler = proc_watchdog_thresh,