linux-kernel - Re: [PATCH 04/14] device_cgroup: remove direct access to cgroup->children

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Date:	Wed, 14 May 2014 14:52:54 +0200
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	lizefan@...wei.com, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, hannes@...xchg.org,
	Aristeu Rozanski <aris@...hat.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>
Subject: Re: [PATCH 04/14] device_cgroup: remove direct access to
 cgroup->children

Quoting Tejun Heo (tj@...nel.org):
> Currently, devcg::has_children() directly tests cgroup->children for
> list emptiness.  The field is not a published field and scheduled to
> go away.  In addition, the test isn't strictly correct as devcg should
> only care about children which are visible to userland.
> 
> This patch converts has_children() to use css_next_child() instead.
> The subtle incorrectness is noted and will be dealt with later.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Aristeu Rozanski <aris@...hat.com>
> Cc: Serge Hallyn <serge.hallyn@...ntu.com>

Acked-by: Serge Hallyn <serge.hallyn@...onical.com>

> ---
>  security/device_cgroup.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> index 3116015..75b4b18 100644
> --- a/security/device_cgroup.c
> +++ b/security/device_cgroup.c
> @@ -477,9 +477,17 @@ static int propagate_exception(struct dev_cgroup *devcg_root,
>  
>  static inline bool has_children(struct dev_cgroup *devcgroup)
>  {
> -	struct cgroup *cgrp = devcgroup->css.cgroup;
> +	bool ret;
>  
> -	return !list_empty(&cgrp->children);
> +	/*
> +	 * FIXME: There may be lingering offline csses and this function
> +	 * may return %true when there isn't any userland-visible child
> +	 * which is incorrect for our purposes.
> +	 */
> +	rcu_read_lock();
> +	ret = css_next_child(NULL, &devcgroup->css);
> +	rcu_read_unlock();
> +	return ret;
>  }
>  
>  /*
> -- 
> 1.9.0
> 
> --
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