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.
>
> Make sure this won't happen.
>
> Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
OK, after clarification from Tejun, that this is helpful for other
controllers as well I think it is better than a memcg specific handling.
Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
> ---
> 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
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