[STUMP] A shell command causes stumpwm freezed

[z_axis]

I am using stumpwm built with threaded clisp-2.49.  There is a user  
defined #'sh function as below:
(defun sh (cmd)
    (let ((str (ext:run-shell-command cmd :output :stream)))
        (loop for line = (read-line str nil)
            until (null line)
            do (print line))))

When run (sh "ping 192.168.1.1"),  The stumpwm will not do anything.  I  
know `ping 192.168.1.1` will not terminate unless user break it.

What is an elegant way to handle such a problem ?

Best Regards!