Re: kqueue disable on delivery...

On Sun, 17 Sep 2006, John-Mark Gurney wrote:

> I have implemented a couple additional features to kqueue.  These allow
> kqueue to be a multithreaded event delivery system that can guarantee
> that the event will only be active in one thread at any time.
>
> The first is EV_DOD, aka disable on delivery.  When the event will be
> delivered to userland, the knote is marked disabled so we don't
> have to go through the expense of reallocing the knote each time.
> (Reallocation of the knote is also lock intensive, and disabling is
> cheap.)

In my opinion, it's too implementation specific flag.

> Even though this means that the event will only ever be active in a
> thread at a time, (when you're done handling the event, you reenable
> it), removing the event from the queue outside the event handler (say
> a timeout handler for the connection) poses to be a problem.  If you
> simply close the socket, the event disappears, but then there is a
> race between another event being created with the same socket, and
> notification of the handler that you want the event to stop.
>
> In order to handle that situation, I have come up w/ EV_FORCEOS, aka
> FORCE ONE_SHOT.  EV_ONESHOT events have the advantage that once queued,
> they don't care if they have been activated or not, they will be returned
> the next round.  This means that the timeout handler can safely set
> EV_FORCEOS on the handler, and either if it's _DISABLED (handler running
> and will reenable it), or it's _ENABLED, it will get dispatched, allowing
> the handler to detect the EV_FORCEOS flag and teardown the connection.

I think it should be EVFILT_USER event, allowing to
EV_SET(&kev, fd, EVFILT_USER, 0, 0, 0, udata);
and the event should automatically sets the EV_ONESHOT flag internally.


Igor Sysoev
http://sysoev.ru/en/