1 files changed, 7 insertions, 2 deletions

diff --git a/drivers/usb/serial/keyspan_pda.c b/drivers/usb/serial/keyspan_pda.c
index 6ce292ef1c47..b650fb4754b4 100644
--- a/drivers/usb/serial/keyspan_pda.c
+++ b/drivers/usb/serial/keyspan_pda.c
@@ -636,14 +636,19 @@ static int keyspan_pda_write_room (struct usb_serial_port *port)
 static int keyspan_pda_chars_in_buffer (struct usb_serial_port *port)
 {
         struct keyspan_pda_private *priv;
+        unsigned long flags;
+        int ret = 0;
 
         priv = usb_get_serial_port_data(port);
 
         /* when throttled, return at least WAKEUP_CHARS to tell select() (via
            n_tty.c:normal_poll() ) that we're not writeable. */
+
+        spin_lock_irqsave(&port->lock, flags);
         if (port->write_urb_busy || priv->tx_throttled)
-                return 256;
-        return 0;
+                ret = 256;
+        spin_unlock_irqrestore(&port->lock, flags);
+        return ret;
 }