walk_tg_tree_from的图解

/*
 * 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;
}