kthread: Simplify kthread_park() completion

Oleg explains the reason we could hit park+park is that
smpboot_update_cpumask_percpu_thread()'s

  for_each_cpu_and(cpu, &tmp, cpu_online_mask)
	smpboot_park_kthread();

turns into:

  for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)and)
	smpboot_park_kthread();

on UP, ignoring the mask. But since we just completely removed that
function, this is no longer relevant.

So revert commit:

  b1f5b37 ("kthread: Allow kthread_park() on a parked kthread")

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/kthread.c

@@ -190,7 +190,7 @@ static void __kthread_parkme(struct kthread *self)
 		if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
 			break;
 
-		complete_all(&self->parked);
+		complete(&self->parked);
 		schedule();
 	}
 	__set_current_state(TASK_RUNNING);
@@ -465,7 +465,6 @@ void kthread_unpark(struct task_struct *k)
 	if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags))
 		__kthread_bind(k, kthread->cpu, TASK_PARKED);
 
-	reinit_completion(&kthread->parked);
 	clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
 	/*
 	 * __kthread_parkme() will either see !SHOULD_PARK or get the wakeup.
@@ -493,6 +492,9 @@ int kthread_park(struct task_struct *k)
 	if (WARN_ON(k->flags & PF_EXITING))
 		return -ENOSYS;
 
+	if (WARN_ON_ONCE(test_bit(KTHREAD_SHOULD_PARK, &kthread->flags)))
+		return -EBUSY;
+
 	set_bit(KTHREAD_SHOULD_PARK, &kthread->flags);
 	if (k != current) {
 		wake_up_process(k);