[PATCH] USB: One potential problem in gadget/serial.c

It looks like that the gs_serial module maybe sleep with spinlock in gs_close.
Sometimes, system hang when I remove the gs_serial module.

From: Fengwei Yin <xaityyy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/serial.c

@@ -890,10 +890,12 @@ static void gs_close(struct tty_struct *tty, struct file *file)
 	/* wait for write buffer to drain, or */
 	/* at most GS_CLOSE_TIMEOUT seconds */
 	if (gs_buf_data_avail(port->port_write_buf) > 0) {
+		spin_unlock_irqrestore(&port->port_lock, flags);
 		wait_cond_interruptible_timeout(port->port_write_wait,
 		port->port_dev == NULL
 		|| gs_buf_data_avail(port->port_write_buf) == 0,
 		&port->port_lock, flags, GS_CLOSE_TIMEOUT * HZ);
+		spin_lock_irqsave(&port->port_lock, flags);
 	}
 
 	/* free disconnected port on final close */