diff --git a/ChangeLog b/ChangeLog
index 22dd1fafec..810442acf9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Mar  6 22:56:14 2013  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* thread_pthread.c (ubf_select): add to small comments why we
+	  need to call rb_thread_wakeup_timer_thread().
+
 Wed Mar  6 21:42:24 2013  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* thread_pthread.c (rb_thread_create_timer_thread): factor out
diff --git a/thread_pthread.c b/thread_pthread.c
index c986f223c0..381647a7a7 100644
--- a/thread_pthread.c
+++ b/thread_pthread.c
@@ -1098,8 +1098,16 @@ ubf_select(void *ptr)
 {
     rb_thread_t *th = (rb_thread_t *)ptr;
     add_signal_thread_list(th);
+
+    /*
+     * ubf_select_each() doesn't guarantee to wake up the target thread.
+     * Therefore, we need to activate timer thread when called from
+     * Thread#kill etc.
+     * In the other hands, we shouldn't call rb_thread_wakeup_timer_thread()
+     * if running on timer thread because it may make endless wakeups.
+     */
     if (pthread_self() != timer_thread_id)
-	rb_thread_wakeup_timer_thread(); /* activate timer thread */
+	rb_thread_wakeup_timer_thread();
     ubf_select_each(th);
 }