long keyctl_keyring_link(key_serial_t id, key_serial_t ringid)
{
	key_ref_t keyring_ref, key_ref;
	long ret;

	keyring_ref = lookup_user_key(ringid, KEY_LOOKUP_CREATE, KEY_NEED_WRITE);
	if (IS_ERR(keyring_ref)) {
		ret = PTR_ERR(keyring_ref);
		goto error;
	}

	key_ref = lookup_user_key(id, KEY_LOOKUP_CREATE, KEY_NEED_LINK);
	if (IS_ERR(key_ref)) {
		ret = PTR_ERR(key_ref);
		goto error2;
	}

	ret = key_link(key_ref_to_ptr(keyring_ref), key_ref_to_ptr(key_ref));

	key_ref_put(key_ref);
error2:
	key_ref_put(keyring_ref);
error:
	return ret;
}

The product contains a concurrent code sequence that 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 operating concurrently.