kernel/watchdog.c: avoid race between lockup detector suspend/resume and CPU hotplug

The lockup detector suspend/resume interface that was introduced by
commit 8c073d27 ("watchdog: introduce watchdog_suspend() and
watchdog_resume()") 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>
Acked-by: Don Zickus <dzickus@redhat.com>
Reviewed-by: Aaron Tomlin <atomlin@redhat.com>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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)