diff --git a/ChangeLog b/ChangeLog
index 7245d49220..55c06e1c1c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun Dec 26 20:28:34 2010  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* process.c (before_exec): add small comment.
+
 Sun Dec 26 20:52:21 2010  Tanaka Akira  <akr@fsij.org>
 
 	* ext/socket/mkconstants.rb: define INET_ADDRSTRLEN as 16 if not
diff --git a/process.c b/process.c
index 396ebbe924..2391fb7295 100644
--- a/process.c
+++ b/process.c
@@ -995,6 +995,10 @@ void rb_thread_reset_timer_thread(void);
 
 static int forked_child = 0;
 
+/*
+ * On old MacOS X, exec() may return ENOTSUPP if the process have multiple threads.
+ * Therefore we have to kill internal threads at once. [ruby-core: 10583]
+ */
 #define before_exec() \
     (rb_enable_interrupt(), (void)(forked_child ? 0 : (rb_thread_stop_timer_thread(), 1)))
 #define after_exec() \