Re: [patch] Minor PAGG attach/detach semantic change for 2.6.11

I fixed this in the RCU version of pnotify I'm working per the lse-tech 
community discussion - thanks for the reminder the other day (in a non-list 
email).

If the RCU version crashes and burns for some reason and we go back to
the non-CUu one, I'll need to make the fix there too.  The function now
looks like this.  I hope this is what you had in mind (untested as of
this moment).

/**
 * __pnotify_fork - Add kernel module subscriber to same subscribers as parent
 * @to_task: The child task that will inherit the parent's subscribers
 * @from_task: The parent task
 *
 * Used to attach a new task to the same subscribers the parent has in its
 * subscriber list.
 *
 * The "from" argument is the parent task.  The "to" argument is the child
 * task.
 *
 * See Documentation/pnotify.txt for details on
 * how to handle return codes from the attach function pointer.
 *
 * Locking: The to_task is currently in-construction, so we don't
 * need to worry about write-locks.  We do need to be sure the parent's
 * subscriber list, which we copy here, doesn't go away on us.  This is
 * done via RCU.
 *
 */
int
__pnotify_fork(struct task_struct *to_task, struct task_struct *from_task)
{
	struct pnotify_subscriber *from_subscriber;
	int ret;

	/* We need to be sure the parent's list we copy from doesn't disappear */
	rcu_read_lock();

	list_for_each_entry_rcu(from_subscriber, &from_task->pnotify_subscriber_list, entry) {
		struct pnotify_subscriber *to_subscriber = NULL;

		to_subscriber = pnotify_subscribe(to_task, from_subscriber->events);
		if (!to_subscriber) {
			ret=-ENOMEM;
			__pnotify_exit(to_task);
			rcu_read_unlock();
			return ret;
		}
		ret = to_subscriber->events->fork(to_task, to_subscriber,
		  from_subscriber->data);
	
		rcu_read_unlock(); /* no more to do with the parent's data */

		if (ret < 0) {
			/* Propagates to copy_process as a fork failure */
			/* No __pnotify_exit because there is one in the failure path
			 * for copy_process in fork.c */
			return ret; /* Fork failure */
		}
		else if (ret > 0) {
			/* Success, but fork function pointer in the pnotify_events structure
			 * doesn't want the kenrel module subscribed */
			/* Again, this is the in-construction-child so no write lock */
			pnotify_unsubscribe(to_subscriber);
		}
	}

	return 0;	/* success */
}