USB: serial: fix unthrottle races

(quoted diff) +6 -0

@@ -484,6 +503,12 @@ void usb_serial_generic_unthrottle(struct tty_struct *tty)
        port->throttled = port->throttle_req = 0;
        spin_unlock_irq(&port->lock);
 
+       /*
+        * Matches the smp_mb__after_atomic() in
+        * usb_serial_generic_read_bulk_callback().
+        */
+       smp_mb();
+
        if (was_throttled)
                usb_serial_generic_submit_read_urbs(port, GFP_KERNEL);