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

[Michal Hocko <mhocko@suse.cz>]

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@huawei.com>

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@suse.cz>

> ---
>  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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