diff --git a/ChangeLog b/ChangeLog
index 970a90e74d..e9a31f29d6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sat Dec 25 15:08:08 2010  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
+
+	* ext/pty/pty.c (chfunc): Added rb_thread_atfork_before_exec().
+	  We must reinitialize GVL at new process creation. Otherwise
+	  we may meet an insane deadlock. [Bug#4121][ruby-dev:42686]
+
 Sat Dec 25 14:27:09 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* io.c (rb_io_extract_encoding_option): accept Encoding object as
diff --git a/ext/pty/pty.c b/ext/pty/pty.c
index b2b5caa2af..76543dbaec 100644
--- a/ext/pty/pty.c
+++ b/ext/pty/pty.c
@@ -156,6 +156,8 @@ chfunc(void *data, char *errbuf, size_t errbuf_len)
 	return -1; \
     } while (0)
 
+    rb_thread_atfork_before_exec();
+
     /*
      * Set free from process group and controlling terminal
      */