um: time-travel: Fix IRQ handling in time_travel_handle_message()

As the comment here indicates, we need to do the polling in the
idle loop without blocking interrupts, since interrupts can be
vhost-user messages that we must process even while in our idle
loop.

I don't know why I explained one thing and implemented another,
but we have indeed observed random hangs due to this, depending
on the timing of the messages.

Fixes: 88ce6424 ("um: Implement time-travel=ext")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/time.c

@@ -70,13 +70,17 @@ static void time_travel_handle_message(struct um_timetravel_msg *msg,
 	 * read of the message and write of the ACK.
 	 */
 	if (mode != TTMH_READ) {
+		bool disabled = irqs_disabled();
+
+		BUG_ON(mode == TTMH_IDLE && !disabled);
+
+		if (disabled)
+			local_irq_enable();
 		while (os_poll(1, &time_travel_ext_fd) != 0) {
-			if (mode == TTMH_IDLE) {
-				BUG_ON(!irqs_disabled());
-				local_irq_enable();
-				local_irq_disable();
-			}
+			/* nothing */
 		}
+		if (disabled)
+			local_irq_disable();
 	}
 
 	ret = os_read_file(time_travel_ext_fd, msg, sizeof(*msg));