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

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Paul Menage wrote:
> 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() ?

Yes.

The caller have held the cgroup_lock() when it calls task_cgroup().
After we add an implicit rcu_read_lock() in task_cgroup(),
we don't need rcu_read_lock()/task_lock() for using task_cgroup().

For cgroup_exit() will change tsk->cgroups, if we don't
add an implicit rcu_read_lock() in task_cgroup(), we have to fix 7
places which using task_cgroup().


task_subsys_state() is different, it is used in fast path,
If we add an implicit rcu_read_lock() in task_subsys_state(),
we still need rcu_read_lock()/task_lock() for using it,
so it's redundant rcu_read_lock(), and slower the fast path a little.


Lai.

> 
> Paul
>