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

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Date:	Fri, 05 Apr 2013 14:58:37 +0900
From:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Li Zefan <lizefan@...wei.com>
CC:	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Glauber Costa <glommer@...allels.com>,
	Michal Hocko <mhocko@...e.cz>,
	Johannes Weiner <hannes@...xchg.org>
Subject: Re: [RFC][PATCH 5/7] cgroup: make sure parent won't be destroyed
 before its children

(2013/04/03 18:13), 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@...wei.com>
> ---
>   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);
> 

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>



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