[PATCH] watchdog/core: Fix AA deadlock due to watchdog holding cpu_hotplug_lock and wait for wq

[Luo Gengkun]

We found an AA deadlock problem as shown belowed:

TaskA				TaskB				WatchDog			system_wq

...
css_killed_work_fn:
P(cgroup_mutex)
...
								...
								__lockup_detector_reconfigure:
								P(cpu_hotplug_lock.read)
								...
				...
				cpu_up:
				percpu_down_write:
				P(cpu_hotplug_lock.write)
												...
												cgroup_bpf_release:
												P(cgroup_mutex)
								smp_call_on_cpu:
								Wait system_wq

cpuset_css_offline:
P(cpu_hotplug_lock.read)

WatchDog is waitting for system_wq, who is waitting for cgroup_mutex, to finish
the jobs, but the owner of the cgroup_mutex is waitting for cpu_hotplug_lock.
The key point is the cpu_hotplug_lock, cause the system_wq may be waitting other
lock. It seems unhealthy to hold a lock when waitting system_wq, because we
never know what jobs are system_wq doing. So I fix this by replace cpu_read_lock/unlock
with cpu_hotplug_disable/enable to prevent cpu offline/online.

Fixes: e31d6883f21c ("watchdog/core, powerpc: Lock cpus across reconfiguration")

Signed-off-by: Luo Gengkun <luogengkun@xxxxxxxxxxxxxxx>
---
 kernel/watchdog.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 51915b44ac73..6ac6fb8d3be0 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -867,7 +867,7 @@ int lockup_detector_offline_cpu(unsigned int cpu)
 
 static void __lockup_detector_reconfigure(void)
 {
-	cpus_read_lock();
+	cpu_hotplug_disable();
 	watchdog_hardlockup_stop();
 
 	softlockup_stop_all();
@@ -877,7 +877,7 @@ static void __lockup_detector_reconfigure(void)
 		softlockup_start_all();
 
 	watchdog_hardlockup_start();
-	cpus_read_unlock();
+	cpu_hotplug_enable();
 	/*
 	 * Must be called outside the cpus locked section to prevent
 	 * recursive locking in the perf code.
@@ -916,11 +916,11 @@ static __init void lockup_detector_setup(void)
 #else /* CONFIG_SOFTLOCKUP_DETECTOR */
 static void __lockup_detector_reconfigure(void)
 {
-	cpus_read_lock();
+	cpu_hotplug_disable();
 	watchdog_hardlockup_stop();
 	lockup_detector_update_enable();
 	watchdog_hardlockup_start();
-	cpus_read_unlock();
+	cpu_hotplug_enable();
 }
 void lockup_detector_reconfigure(void)
 {
-- 
2.34.1