softlockup: ensure the task has been switched out once

When we check if a task has been switched out since the last scan, we might
have a race condition on the following scenario:

- the task is freshly created and scheduled

- it puts its state to TASK_UNINTERRUPTIBLE and is not yet switched out

- check_hung_task() scans this task and will report a false positive because
  t->nvcsw + t->nivcsw == t->last_switch_count == 0

Add a check for such cases.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/hung_task.c

@@ -72,7 +72,13 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 {
 	unsigned long switch_count = t->nvcsw + t->nivcsw;
 
-	if (t->flags & PF_FROZEN)
+	/*
+	 * Ensure the task is not frozen.
+	 * Also, when a freshly created task is scheduled once, changes
+	 * its state to TASK_UNINTERRUPTIBLE without having ever been
+	 * switched out once, it musn't be checked.
+	 */
+	if (unlikely(t->flags & PF_FROZEN || !switch_count))
 		return;
 
 	if (switch_count != t->last_switch_count) {