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

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Date:	Tue, 4 Jan 2011 17:31:47 -0800
From:	Paul Menage <menage@...gle.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 4, 2011 at 12:18 AM, Lai Jiangshan <laijs@...fujitsu.com> 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).
>
> 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]

Can you reformat this a bit? It took a fair bit of staring to realize
that (__c))->subsys[subsys_id] wasn't trying to dereference __c.

Maybe make it:

#define task_subsys_state_check(task, subsys_id, __c) ({ \
  struct css_set *_css = rcu_dereference_check(...); \
  _css->subsys[subsys_id]; \
})

Paul
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