__radix_tree_lookup
void *__radix_tree_lookup(const struct radix_tree_root *, unsigned long index,
entry = __radix_tree_lookup(&idr->idr_rt, id, &node, &slot);
entry = __radix_tree_lookup(root, index, &node, &slot);
if (!__radix_tree_lookup(root, index, NULL, &slot))
return __radix_tree_lookup(root, index, NULL, NULL);