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watchdog: fix watchdog may detect false positive of softlockup

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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 <luogengkun@huaweicloud.com>
Signed-off-by: Nysal Jan K.A. <nysal@linux.ibm.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: "Nysal Jan K.A." <nysal@linux.ibm.com>
Cc: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Luo Gengkun
2025-04-21 03:50:21 +00:00
committed by Greg Kroah-Hartman
parent 5180561aff
commit 442e80dcf6
1 changed files with 27 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

41
kernel/watchdog.c
View File

@@ -40,6 +40,7 @@ int __read_mostly watchdog_user_enabled = 1;
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;
@@ -627,12 +628,20 @@ int lockup_detector_offline_cpu(unsigned int cpu)
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)
@@ -650,7 +659,7 @@ static void __lockup_detector_reconfigure(void)
void lockup_detector_reconfigure(void)
{
mutex_lock(&watchdog_mutex);
__lockup_detector_reconfigure();
__lockup_detector_reconfigure(false);
mutex_unlock(&watchdog_mutex);
}
@@ -670,27 +679,29 @@ static __init void lockup_detector_setup(void)
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 */
@@ -726,11 +737,11 @@ 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);
}
/*
@@ -763,7 +774,7 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
old = READ_ONCE(*param);
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;
@@ -812,11 +823,13 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
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;
@@ -837,7 +850,7 @@ int proc_watchdog_cpumask(struct ctl_table *table, int write,
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;
@@ -857,7 +870,7 @@ static struct ctl_table watchdog_sysctls[] = {
},
{
.procname = "watchdog_thresh",
.data = &watchdog_thresh,
.data = &watchdog_thresh_next,
.maxlen = sizeof(int),
.mode = 0644,
.proc_handler = proc_watchdog_thresh,