linux-kernel - Re: [PATCH 2/5] cgroup: Fix an RCU warning in alloc_css

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Date:	Thu, 22 Apr 2010 12:55:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	"containers@...ts.osdl.org" <containers@...ts.osdl.org>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KameZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 2/5] cgroup: Fix an RCU warning in alloc_css_id()

On Thu, Apr 22, 2010 at 05:30:00PM +0800, Li Zefan wrote:
> With CONFIG_PROVE_RCU=y, a warning can be triggered:
> 
>   # mount -t cgroup -o memory xxx /mnt
>   # mkdir /mnt/0
> 
> ...
> kernel/cgroup.c:4442 invoked rcu_dereference_check() without protection!
> ...
> 
> This is a false-positive. It's safe to directly access parent_css->id.

Also queued for 2.6.34, thank you!

							Thanx, Paul

> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> ---
>  kernel/cgroup.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 4ca928d..3a53c77 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -4561,13 +4561,13 @@ static int alloc_css_id(struct cgroup_subsys *ss, struct cgroup *parent,
>  {
>  	int subsys_id, i, depth = 0;
>  	struct cgroup_subsys_state *parent_css, *child_css;
> -	struct css_id *child_id, *parent_id = NULL;
> +	struct css_id *child_id, *parent_id;
> 
>  	subsys_id = ss->subsys_id;
>  	parent_css = parent->subsys[subsys_id];
>  	child_css = child->subsys[subsys_id];
> -	depth = css_depth(parent_css) + 1;
>  	parent_id = parent_css->id;
> +	depth = parent_id->depth;
> 
>  	child_id = get_new_cssid(ss, depth);
>  	if (IS_ERR(child_id))
> -- 
> 1.6.3
> 
> --
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