diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 5e46c2a7..7c434c3 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -338,6 +338,7 @@
  *
  * The cpumask field in the smp_hotplug_thread must not be updated directly
  * by the client, but only by calling this function.
+ * This function can only be called on a registered smp_hotplug_thread.
  */
 int smpboot_update_cpumask_percpu_thread(struct smp_hotplug_thread *plug_thread,
 					 const struct cpumask *new)