static int __init ip6_tunnel_init(void)
{
	int  err;

	if (xfrm6_tunnel_register(&ip4ip6_handler, AF_INET)) {
		printk(KERN_ERR "ip6_tunnel init: can't register ip4ip6\n");
		err = -EAGAIN;
		goto out;
	}

	if (xfrm6_tunnel_register(&ip6ip6_handler, AF_INET6)) {
		printk(KERN_ERR "ip6_tunnel init: can't register ip6ip6\n");
		err = -EAGAIN;
		goto unreg_ip4ip6;
	}

	err = register_pernet_device(&ip6_tnl_net_ops);
	if (err < 0)
		goto err_pernet;
	return 0;
err_pernet:
	xfrm6_tunnel_deregister(&ip6ip6_handler, AF_INET6);
unreg_ip4ip6:
	xfrm6_tunnel_deregister(&ip4ip6_handler, AF_INET);
out:
	return err;
}
The program contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.