diff --git a/ChangeLog b/ChangeLog
index 12362d271b..85abbc133a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Nov 20 09:36:55 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* signal.c (install_sighandler): added comments why we need
+	  rb_disable_interrupt().
+
 Tue Nov 20 09:31:33 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* signal.c (rb_disable_interrupt, rb_enable_interrupt): removed
diff --git a/signal.c b/signal.c
index a56152e9e4..3f4a90b522 100644
--- a/signal.c
+++ b/signal.c
@@ -986,8 +986,10 @@ install_sighandler(int signum, sighandler_t handler)
 {
     sighandler_t old;
 
+    /* At this time, there is no subthread. Then sigmask guarantee atomics. */
     rb_disable_interrupt();
     old = ruby_signal(signum, handler);
+    /* signal handler should be inherited during exec. */
     if (old != SIG_DFL) {
 	ruby_signal(signum, old);
     }