Re: [RFC][PATCH 5/7] cgroup: make sure parent won't be destroyed before its children

Michal Hocko
2013-04-04T11:37:50Z

On Wed 03-04-13 17:13:08, Li Zefan wrote:
> Suppose we rmdir a cgroup and there're still css refs, this cgroup won't
> be freed. Then we rmdir the parent cgroup, and the parent is freed due
> to css ref draining to 0. Now it would be a disaster if the child cgroup
> tries to access its parent.

Hmm, I am not sure what is the correct layer for this to handle - cgroup
core or memcg. But we have enforced that in mem_cgroup_css_online where
we take an additional reference to the memcg.

Handling it in the memcg code would have an advantage of limiting an
additional reference only to use_hierarchy cases which is sufficient
as we never touch the parent otherwise (parent_mem_cgroup).

So I think this patch should just take css reference to parent during
online and drop it from mem_cgroup_css_free.

If there are more contollers that need this then it should be handled by
the cgroup core of course.

> Make sure this won't happen.
>
> Signed-off-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
> ---
> kernel/cgroup.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index fa54b92..78204bc 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -888,6 +888,13 @@ static void cgroup_free_fn(struct work_struct *work)
> mutex_unlock(&cgroup_mutex);
>
> /*
> + * We get a ref to the parent's dentry, and put the ref when
> + * this cgroup is being freed, so it's guaranteed that the
> + * parent won't be destroyed before its children.
> + */
> + dput(cgrp->parent->dentry);
> +
> + /*
> * Drop the active superblock reference that we took when we
> * created the cgroup
> */
> @@ -4171,6 +4178,9 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
> for_each_subsys(root, ss)
> dget(dentry);
>
> + /* hold a ref to the parent's dentry */
> + dget(parent->dentry);
> +
> /* creation succeeded, notify subsystems */
> for_each_subsys(root, ss) {
> err = online_css(ss, cgrp);
> --
> 1.8.0.2

--
Michal Hocko
SUSE Labs