Re: [PATCH 1/2] cgroups: enhance task_cgroup()

[Paul Menage <menage@google.com>]

On Fri, Nov 21, 2008 at 12:49 AM, Lai Jiangshan <laijs@cn.fujitsu.com> wrote:
>
> task_cgroup() calls cgroup_subsys_state().

No, it calls task_subsys_state()

> and we must use rcu_read_lock() to protect cgroup_subsys_state().
> so we must use rcu_read_lock() to protect task_cgroup().
>
> but it'll not so friendly to caller: the callers of task_cgroup() have
> held cgroup_lock(). it means that struct cgroup will not be freed.
>
> So this patch add rcu_read_lock() in task_cgroup() to enhance task_cgroup().
> And we do NOT NEED FIX task_cgroup()'s callers, and cgroup_lock()
> can protect task_cgroup().

Is there a reason to add an implicit rcu_read_lock() in task_cgroup()
and not directly in task_subsys_state() ?

Paul

>
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> ---
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 1164963..22901ff 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -359,6 +360,10 @@
>  static inline struct cgroup* task_cgroup(struct task_struct *task,
>                                               int subsys_id)
>  {
> -       return task_subsys_state(task, subsys_id)->cgroup;
> +       struct cgroup *ret;
> +       rcu_read_lock();
> +       ret = task_subsys_state(task, subsys_id)->cgroup;
> +       rcu_read_unlock();
> +       return ret;
>  }
>
>  int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *ss,
>
>
>