linux-kernel - Re: [PATCH RESEND] cgroups: fix incorrect using rcu_dereference() in cgroup_subsys

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Message-ID: <20110104212357.GU2026@linux.vnet.ibm.com>
Date:	Tue, 4 Jan 2011 13:23:57 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Paul Menage <menage@...gle.com>, Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND] cgroups: fix incorrect using rcu_dereference()
 in cgroup_subsys_state()

On Tue, Jan 04, 2011 at 04:18:06PM +0800, Lai Jiangshan wrote:
> From: Li Zefan <lizf@...fujitsu.com>
> Date: Mon, 25 Aug 2008 11:05:28 +0800
> (Original) Subject: [PATCH] cgroup: fix wrong rcu_dereference()
> 
> It is tsk->cgroups which is protected by RCU, not ->subsys[subsys_id].
> 
> laijs: updated it(the surrounding code have been changed since these two years).

This looks plausible to me, assuming that the cgroups guys are OK with
it.

One requested change: could you please delete the rcu_read_lock_held()?
This is now implied by rcu_dereference_check().  With that:

Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index ed4ba11..a798814 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -535,10 +535,11 @@ static inline struct cgroup_subsys_state *cgroup_subsys_state(
>   * cgroup_subsys::attach() methods.
>   */
>  #define task_subsys_state_check(task, subsys_id, __c)			\
> -	rcu_dereference_check(task->cgroups->subsys[subsys_id],		\
> +	rcu_dereference_check(task->cgroups,				\
>  			      rcu_read_lock_held() ||			\
>  			      lockdep_is_held(&task->alloc_lock) ||	\
> -			      cgroup_lock_is_held() || (__c))
> +			      cgroup_lock_is_held() ||			\
> +			      (__c))->subsys[subsys_id]
> 
>  static inline struct cgroup_subsys_state *
>  task_subsys_state(struct task_struct *task, int subsys_id)
--
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