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

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

Matt Helsley wrote:
> On Tue, 2006-06-20 at 15:28 +1000, Peter Williams wrote:
>> Matt Helsley wrote:
>>> This introduces a second, per-task, blocking notifier chain. The per-task
>>> chain offers watchers the chance to register with a specific task nstead of
>>> all tasks. It also allows the watcher to associate a block of data with the task
>>> by wrapping the notifier block using containerof().
>>>
>>> Both the global, all-tasks chain and the per-task chain are called from the samefunction. The two types of chains share the same set of notification
>>> values, however registration functions and the registered notifier blocks must
>>> be separate.
>>>
>>> These notifiers are only safe if notifier blocks are registered with the current
>>> task while in the context of the current task. This ensures that there are no
>>> races between registration, unregistration, and notification.
>>>
>>> Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
>>> Cc: Jes Sorensen <jes@sgi.com>
>>> Cc: Chandra S. Seetharaman <sekharan@us.ibm.com>
>>> Cc: Alan Stern <stern@rowland.harvard.edu>
>> [bits deleted]
>>
>>> Index: linux-2.6.17-rc6-mm2/kernel/sys.c
>>> ===================================================================
>>> --- linux-2.6.17-rc6-mm2.orig/kernel/sys.c
>>> +++ linux-2.6.17-rc6-mm2/kernel/sys.c
>>> @@ -450,13 +450,41 @@ int unregister_task_watcher(struct notif
>>>  	return blocking_notifier_chain_unregister(&task_watchers, nb);
>>>  }
>>>  
>>>  EXPORT_SYMBOL_GPL(unregister_task_watcher);
>>>  
>>> +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/:

Yes, sorry.

> 
> 	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.

OK.  But it causes a compiler warning:

/home/peterw/KERNELS/CpuCaps/TW-2.6.17-rc6-mm2/kernel/sys.c: In function 
‘notify_per_task_watchers’:
/home/peterw/KERNELS/CpuCaps/TW-2.6.17-rc6-mm2/kernel/sys.c:464: 
warning: control reaches end of non-void function

> 
> 	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.

OK.  I think I can safely ignore init.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

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