USB: serial: grab disconnect mutex in chars_in_buffer

Grab disconnect mutex in chars_in_buffer before checking disconnected
flag or calling driver specific function.

This allows subdrivers to query any hardware buffer status without
having to handle the locking themselves.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/usb-serial.c

@@ -361,15 +361,21 @@ static int serial_write_room(struct tty_struct *tty)
 static int serial_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
+	struct usb_serial *serial = port->serial;
+	int count = 0;
 
 	dev_dbg(tty->dev, "%s - port %d\n", __func__, port->number);
 
+	mutex_lock(&serial->disc_mutex);
 	/* if the device was unplugged then any remaining characters
 	   fell out of the connector ;) */
-	if (port->serial->disconnected)
-		return 0;
-	/* pass on to the driver specific version of this function */
-	return port->serial->type->chars_in_buffer(tty);
+	if (serial->disconnected)
+		count = 0;
+	else
+		count = serial->type->chars_in_buffer(tty);
+	mutex_unlock(&serial->disc_mutex);
+
+	return count;
 }
 
 static void serial_throttle(struct tty_struct *tty)