Re: [PATCH 1/2] cgroup: Delay the clearing of cgrp->kn->priv

[Tejun Heo]

Hello, Li.

On Tue, Sep 02, 2014 at 06:56:58PM +0800, Li Zefan wrote:
>     for ((; ;))
>     {
>         echo $$ > /cgroup/sub/cgroup.procs
>         ech $$ > /cgce 6f2e0c38c2108a74 ]---
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          copy & paste error?
...
> Reported-by: Toralf Förster <toralf.foerster@xxxxxx>
> Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
> ---
> 
> Toralf, Thanks for reporting the bug. I'm not able to repy to your email,
> because I was kicked out of the cgroup mailing list so didn't receive
> emails from mailing list for a week.
> 
> ---
>  kernel/cgroup.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 1c56924..e03fc62 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -4185,6 +4185,15 @@ static void css_release_work_fn(struct work_struct *work)
>  
>  	mutex_unlock(&cgroup_mutex);
>  
> +	/*
> +	 * There are two control paths which try to determine cgroup from
> +	 * dentry without going through kernfs - cgroupstats_build() and
> +	 * css_tryget_online_from_dir().  Those are supported by RCU
> +	 * protecting clearing of cgrp->kn->priv backpointer.
> +	 */
> +	if (!ss && cgroup_parent(cgrp))
> +		RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);

Can we move the above into the preceding else block?  I don't think
holding cgroup_mutex or not makes any difference here.  Also, why do
we need the cgroup_parent() check?  Do we deref root's kn->priv in the
destruction path?  If so, can you please note that in the comment?

Thanks.

-- 
tejun
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