USB: serial: ftdi_sio: Fix atomicity violation in get_serial_info()

Our static checker found a bug where set_serial_info() uses a mutex, but
get_serial_info() does not. Fortunately, the impact of this is relatively
minor. It doesn't cause a crash or any other serious issues. However, if a
race condition occurs between set_serial_info() and get_serial_info(),
there is a chance that the data returned by get_serial_info() will be
inconsistent.

Fixes: 3ae36bed3a93 ("fdti_sio: switch to ->[sg]et_serial()")
Signed-off-by: Qiu-ji Chen <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>

drivers/usb/serial/ftdi_sio.c

···
 	struct usb_serial_port *port = tty->driver_data;
 	struct ftdi_private *priv = usb_get_serial_port_data(port);
 
+	mutex_lock(&priv->cfg_lock);
 	ss->flags = priv->flags;
 	ss->baud_base = priv->baud_base;
 	ss->custom_divisor = priv->custom_divisor;
+	mutex_unlock(&priv->cfg_lock);
 }
 
 static int set_serial_info(struct tty_struct *tty, struct serial_struct *ss)