walk_tg_tree_from的图解

在遍历task_group的时候，需要会调用到walk_tg_tree_from函数，从函数注释看，这个函数的流程是：

以from为根节点，当进入一个节点时调用down回调函数，当离开一个节点时调用up函数。这个函数
采用的是深度遍历。

下面用一张图来说明：

/*
 * Iterate task_group tree rooted at *from, calling @down when first entering a
 * node and @up when leaving it for the final time.
 *
 * Caller must hold rcu_lock or sufficient equivalent.
 */
int walk_tg_tree_from(struct task_group *from,
			     tg_visitor down, tg_visitor up, void *data)
{
	struct task_group *parent, *child;
	int ret;

	parent = from;

down:
	ret = (*down)(parent, data);
	if (ret)
		goto out;
	list_for_each_entry_rcu(child, &parent->children, siblings) {
		parent = child;
		goto down;

up:
		continue;
	}
	ret = (*up)(parent, data);
	if (ret || parent == from)
		goto out;

	child = parent;
	parent = parent->parent;
	if (parent)
		goto up;
out:
	return ret;
}