linux-kernel - Re: [PATCH 1/3] cgroup: don't destroy the default root

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Date:	Tue, 3 Jun 2014 08:57:28 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Li Zefan <lizefan@...wei.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH 1/3] cgroup: don't destroy the default root

Hello, Li.

On Tue, Jun 03, 2014 at 12:04:38PM +0800, Li Zefan wrote:
>  static void cgroup_get(struct cgroup *cgrp)
>  {
>  	WARN_ON_ONCE(cgroup_is_dead(cgrp));
> -	css_get(&cgrp->self);
> +	if (!(cgrp->self.flags & CSS_NO_REF))
> +		css_get(&cgrp->self);

Hmmm?  The same condition is tested by css_get().  Why should it be
tested again here?

>  static void cgroup_put(struct cgroup *cgrp)
>  {
> -	css_put(&cgrp->self);
> +	if (!(cgrp->self.flags & CSS_NO_REF))
> +		css_put(&cgrp->self);

Ditto.

> @@ -1781,10 +1783,12 @@ static void cgroup_kill_sb(struct super_block *sb)
>  	 * This prevents new mounts by disabling percpu_ref_tryget_live().
>  	 * cgroup_mount() may wait for @root's release.
>  	 */
> -	if (css_has_online_children(&root->cgrp.self))
> +	if (css_has_online_children(&root->cgrp.self)) {
>  		cgroup_put(&root->cgrp);
> -	else
> -		percpu_ref_kill(&root->cgrp.self.refcnt);
> +	} else {
> +		if (root != &cgrp_dfl_root)
> +			percpu_ref_kill(&root->cgrp.self.refcnt);
> +	}

As conceptually percpu_ref_kill() just puts the base ref and the
dfl_root's refcnt never reaches zero, it won't actually trigger.
Hmmm.... wouldn't the above leak a ref each time the default hierarchy
is unmounted tho?  Shouldn't it be like the following?

	if (root == &cgrp_dfl_root || css_has_online_children(...))
		cgroup_put(&root->cgrp);
	else
		percpu_ref_kill(...);

Thanks.

-- 
tejun
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