[PATCH 1/4] watchdog: avoid race between lockup detector suspend/resume and CPU hotplug

[Ulrich Obergfell <uobergfe@redhat.com>]

The lockup detector suspend/resume interface that was introduced by
commit 8c073d27d7ad293bf734cc8475689413afadab81 does not protect
itself against races with CPU hotplug. Hence, theoretically it is
possible that a new watchdog thread is started on a hotplugged CPU
while the lockup detector is suspended, and the thread could thus
interfere unexpectedly with the code that requested to suspend the
lockup detector. Avoid the race by calling

  get_online_cpus() in lockup_detector_suspend()
  put_online_cpus() in lockup_detector_resume()

Signed-off-by: Ulrich Obergfell <uobergfe@redhat.com>
---
 kernel/watchdog.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 0a23125..7357842 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -719,6 +719,7 @@ int lockup_detector_suspend(void)
 {
 	int ret = 0;
 
+	get_online_cpus();
 	mutex_lock(&watchdog_proc_mutex);
 	/*
 	 * Multiple suspend requests can be active in parallel (counted by
@@ -759,6 +760,7 @@ void lockup_detector_resume(void)
 		watchdog_unpark_threads();
 
 	mutex_unlock(&watchdog_proc_mutex);
+	put_online_cpus();
 }
 
 static int update_watchdog_all_cpus(void)
-- 
1.7.11.7