Paul Menage wrote:
> On Fri, Nov 21, 2008 at 12:49 AM, Lai Jiangshan <laijs@xxxxxxxxxxxxxx> 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() ?
>
I don't think it's a good idea to add rcu_read_lock() in task_cgroup() or
task_subsys_state(). The returned pointer of the 2 functions should be
protected by rcu_read_lock or cgroup_lock, so the caller should still
hold the lock by itself.
An example of this is:
static int mem_cgroup_charge_common(...)
{
...
rcu_read_lock();
mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
if (unlikely(!mem)) {
rcu_read_unlock();
return 0;
}
/*
* For every charge from the cgroup, increment reference count
*/
css_get(&mem->css);
rcu_read_unlock();
...
}
> Paul
>
>> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
>> ---
>> 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,
>>
>>
>>
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