Re: [PATCH 1/8] cgroup: implement cgroup_is_ancestor()

Tejun Heo
2013-04-08T21:36:53Z

On Mon, Apr 08, 2013 at 08:03:44PM +0200, Michal Hocko wrote:
> __mem_cgroup_same_or_subtree relies on css_is_ancestor if hierarchy is
> enabled for ages. This, however, is not correct because use_hierarchy
> doesn't need to be true all the way up the cgroup hierarchy. Consider
> the following example:
> root (use_hierarchy=0)
> \
> A (use_hierarchy=0)
> \
> B (use_hierarchy=1)
> \
> C (use_hierarchy=1)
>
> __mem_cgroup_same_or_subtree(A, C) would return true even though C is
> not from the same hierarchy subtree. The bug shouldn't be critical but
> at least dump_tasks might print unrelated tasks (via
> task_in_mem_cgroup).

Huh? Isn't that avoided by the !root_memcg->use_hierarchy test?

> @@ -1470,9 +1470,12 @@ bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
> {
> if (root_memcg == memcg)
> return true;
> - if (!root_memcg->use_hierarchy || !memcg)
^^^^^^^^^^^^^^^^^^^^^^^^^^
> + if (!memcg)
> return false;
> - return css_is_ancestor(&memcg->css, &root_memcg->css);
> + while ((memcg = parent_mem_cgroup(memcg)))
> + if (memcg == root_memcg)
> + return true;
> + return false;
> }
>
> static bool mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
> --
> 1.7.10.4
>
> --
> Michal Hocko
> SUSE Labs

--
tejun