diff --git a/drivers/usb/serial/ftdi_sio.c b/drivers/usb/serial/ftdi_sio.c
index 5f08702..5a8ae27 100644
--- a/drivers/usb/serial/ftdi_sio.c
+++ b/drivers/usb/serial/ftdi_sio.c
@@ -33,6 +33,7 @@
 #include <linux/errno.h>
 #include <linux/init.h>
 #include <linux/slab.h>
+#include <linux/smp_lock.h>
 #include <linux/tty.h>
 #include <linux/tty_driver.h>
 #include <linux/tty_flip.h>