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

[Matt Helsley <matthltc@us.ibm.com>]

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/:

	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.

> Peter

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