The patch titled Subject: watchdog: fix watchdog may detect false positive of softlockup has been added to the -mm mm-nonmm-unstable branch. Its filename is watchdog-fix-watchdog-may-detect-false-positive-of-softlockup.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-nonmm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Luo Gengkun luogengkun@huaweicloud.com Subject: watchdog: fix watchdog may detect false positive of softlockup Date: Mon, 21 Apr 2025 03:50:21 +0000

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.

Link: https://lkml.kernel.org/r/20250421035021.3507649-1-luogengkun@huaweicloud.co... Signed-off-by: Luo Gengkun luogengkun@huaweicloud.com Cc: Doug Anderson dianders@chromium.org Cc: Joel Granados joel.granados@kernel.org Cc: Song Liu song@kernel.org Cc: Thomas Gleinxer tglx@linutronix.de Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

kernel/watchdog.c | 37 ++++++++++++++++++++++++------------- 1 file changed, 24 insertions(+), 13 deletions(-)

--- a/kernel/watchdog.c~watchdog-fix-watchdog-may-detect-false-positive-of-softlockup +++ a/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); }

@@ -908,7 +917,7 @@ 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); } @@ -924,11 +933,11 @@ static void __lockup_detector_reconfigur } 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 +955,11 @@ 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 +993,7 @@ 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 +1044,13 @@ 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 +1071,7 @@ 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 +1091,7 @@ 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, _

Patches currently in -mm which might be from luogengkun@huaweicloud.com are

watchdog-fix-watchdog-may-detect-false-positive-of-softlockup.patch