USB: serial: digi_acceleport: add chars_in_buffer locking

Both the dp_write_urb_in_use flag and dp_out_buf_len counter should be
accessed while holding the driver port lock. Add the missing locking to
chars_in_buffer and clean up the implementation somewhat by using a
common exit path.
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/digi_acceleport.c

@@ -1044,17 +1044,19 @@ static unsigned int digi_chars_in_buffer(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
 	struct digi_port *priv = usb_get_serial_port_data(port);
+	unsigned long flags;
+	unsigned int chars;
 
-	if (priv->dp_write_urb_in_use) {
-		dev_dbg(&port->dev, "digi_chars_in_buffer: port=%d, chars=%d\n",
-			priv->dp_port_num, port->bulk_out_size - 2);
-		return port->bulk_out_size - 2;
-	} else {
-		dev_dbg(&port->dev, "digi_chars_in_buffer: port=%d, chars=%d\n",
-			priv->dp_port_num, priv->dp_out_buf_len);
-		return priv->dp_out_buf_len;
-	}
+	spin_lock_irqsave(&priv->dp_port_lock, flags);
+	if (priv->dp_write_urb_in_use)
+		chars = port->bulk_out_size - 2;
+	else
+		chars = priv->dp_out_buf_len;
+	spin_unlock_irqrestore(&priv->dp_port_lock, flags);
 
+	dev_dbg(&port->dev, "%s: port=%d, chars=%d\n", __func__,
+			priv->dp_port_num, chars);
+	return chars;
 }
 
 static void digi_dtr_rts(struct usb_serial_port *port, int on)