acpi: bus: check once more for an empty list after locking it

List could have become empty after the unlocked check that was made earlier,
so check again inside the lock.

Should fix https://bugzilla.redhat.com/show_bug.cgi?id=427765Signed-off-by: NChuck Ebbert <cebbert@redhat.com>
Cc: <stable@kernel.org>
Cc: Len Brown <lenb@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/acpi/bus.c

@@ -373,10 +373,11 @@ int acpi_bus_receive_event(struct acpi_bus_event *event)
 	}
 
 	spin_lock_irqsave(&acpi_bus_event_lock, flags);
-	entry =
-	    list_entry(acpi_bus_event_list.next, struct acpi_bus_event, node);
-	if (entry)
+	if (!list_empty(&acpi_bus_event_list)) {
+		entry = list_entry(acpi_bus_event_list.next,
+				   struct acpi_bus_event, node);
 		list_del(&entry->node);
+	}
 	spin_unlock_irqrestore(&acpi_bus_event_lock, flags);
 
 	if (!entry)