clockevents: prevent multiple init/shutdown

While chasing the C1E/HPET bugreports I went through the clock events
code inch by inch and found that the broadcast device can be initialized
and shutdown multiple times. Multiple shutdowns are not critical, but
useless waste of time. Multiple initializations are simply broken. Another
CPU might have the device in use already after the first initialization and
the second init could just render it unusable again.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/time/tick-broadcast.c

@@ -210,7 +210,7 @@ static void tick_do_broadcast_on_off(void *why)
 	struct clock_event_device *bc, *dev;
 	struct tick_device *td;
 	unsigned long flags, *reason = why;
-	int cpu;
+	int cpu, bc_stopped;
 
 	spin_lock_irqsave(&tick_broadcast_lock, flags);
 
@@ -228,6 +228,8 @@ static void tick_do_broadcast_on_off(void *why)
 	if (!tick_device_is_functional(dev))
 		goto out;
 
+	bc_stopped = cpus_empty(tick_broadcast_mask);
+
 	switch (*reason) {
 	case CLOCK_EVT_NOTIFY_BROADCAST_ON:
 	case CLOCK_EVT_NOTIFY_BROADCAST_FORCE:
@@ -250,9 +252,10 @@ static void tick_do_broadcast_on_off(void *why)
 		break;
 	}
 
-	if (cpus_empty(tick_broadcast_mask))
+	if (cpus_empty(tick_broadcast_mask)) {
+		if (!bc_stopped)
 			clockevents_set_mode(bc, CLOCK_EVT_MODE_SHUTDOWN);
-	else {
+	} else if (bc_stopped) {
 		if (tick_broadcast_device.mode == TICKDEV_MODE_PERIODIC)
 			tick_broadcast_start_periodic(bc);
 		else
@@ -501,9 +504,12 @@ static void tick_broadcast_clear_oneshot(int cpu)
  */
 void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
 {
+	/* Set it up only once ! */
+	if (bc->event_handler != tick_handle_oneshot_broadcast) {
 		bc->event_handler = tick_handle_oneshot_broadcast;
 		clockevents_set_mode(bc, CLOCK_EVT_MODE_ONESHOT);
 		bc->next_event.tv64 = KTIME_MAX;
+	}
 }
 
 /*