static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
{
	struct hlist_head *hashent = ucounts_hashentry(ns, uid);
	struct ucounts *ucounts, *new;

	spin_lock_irq(&ucounts_lock);
	ucounts = find_ucounts(ns, uid, hashent);
	if (!ucounts) {
		spin_unlock_irq(&ucounts_lock);

		new = kzalloc(sizeof(*new), GFP_KERNEL);
		if (!new)
			return NULL;

		new->ns = ns;
		new->uid = uid;
		new->count = 0;

		spin_lock_irq(&ucounts_lock);
		ucounts = find_ucounts(ns, uid, hashent);
		if (ucounts) {
			kfree(new);
		} else {
			hlist_add_head(&new->node, hashent);
			ucounts = new;
		}
	}
	if (ucounts->count == INT_MAX)
		ucounts = NULL;
	else
		ucounts->count += 1;
	spin_unlock_irq(&ucounts_lock);
	return ucounts;
}
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.