Re: [Lse-tech] [PATCH 09/11] Task watchers: Add support for per-task watchers

[Peter Williams <pwil3058@bigpond.net.au>]

Peter Williams wrote:
> Peter Williams wrote:
>> Andrew Morton wrote:
>>> On Tue, 20 Jun 2006 18:20:48 -0700
>>> Matt Helsley <matthltc@us.ibm.com> wrote:
>>>
>>>>>>>> +static inline int notify_per_task_watchers(unsigned int val,
>>>>>>>> +                       struct task_struct *task)
>>>>>>>> +{
>>>>>>>> +    if (get_watch_event(val) != WATCH_TASK_INIT)
>>>>>>>> +        return raw_notifier_call_chain(&task->notify, val, task);
>>>>>>>> +    RAW_INIT_NOTIFIER_HEAD(&task->notify);
>>>>>>>> +    if (task->real_parent)
>>>>>>>> +        return raw_notifier_call_chain(&task->real_parent->notify,
>>>>>>>> +                              val, task);
>>>>>>>> +}
>>>>>>> It's possible for this task to exit without returning a result.
>>>>>> Assuming you meant s/task/function/:
>>>>>>
>>>>>>     In the common case this will return a result because most 
>>>>>> tasks have a
>>>>>> real parent. The only exception should be the init task. However, the
>>>>>> init task does not "fork" from another task so this function will 
>>>>>> never
>>>>>> get called with WATCH_TASK_INIT and the init task.
>>>>>>
>>>>>>     This means that if one wants to use per-task watchers to 
>>>>>> associate data
>>>>>> and a function call with *every* task, special care will need to be
>>>>>> taken to register with the init task.
>>>>> no......
>>>>     I've been looking through the source and, from what I can see, 
>>>> the end
>>>> of the function is not reachable. I think I need to add:
>>>>
>>>> notify_watchers(WATCH_TASK_INIT, &init_task);
>>>>
>>>> to make this into an applicable warning.
>>>
>>> If the end of the function isn't reachable then the
>>> `if (task->real_parent)' can simply be removed.
>>
>> Perhaps with a comment to say that it's safe (and why) to dereference 
>> task->real_parent to help code reviewers?
> 
> Now that I understand this code better, I'm at a loss to understand why 
> you're calling the parent with the notifier.  I can't think of any 
> circumstances where this would be useful.

I spoke to soon and can now see how I can use this functionality.  If I 
register my per task watcher with init_task, this functionality will 
allow me to do away with the normal watcher.

Sorry for doubting,
Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce