Re: [KVM PATCH 2/3] eventfd: add a notifier mechanism

Davide Libenzi
2009-04-24T16:17:26Z

On Fri, 24 Apr 2009, Gregory Haskins wrote:

> Davide Libenzi wrote:
> > On Thu, 23 Apr 2009, Gregory Haskins wrote:
> >
> >
> >>> Take a look at init_waitqueue_func_entry(), in particula at that "func"
> >>> parameter. Then look at how __wake_up_common() does its thing.
> >>> You don't need to be "waiting" for our wakeup system to work. Callbacks
> >>> works just fine, otherwise things like epoll could not work at all.
> >>>
> >>>
> >> Yeah, I was looking at that this afternoon after you mentioned it. That
> >> makes sense.
> >>
> >> As of right now the wqh is embedded in the eventfd, accessible only by
> >> the .read() vtable entry. In order to do this as you suggest, I imagine
> >> I need to slightly modify the eventfd interface to allow waiters other
> >> than the embedded readers to join the wait-queue. How would you like to
> >> see that interface look?
> >>
> >
> > Actually, you need no changes in eventfd, since you can use its poll()
> > directly, by dropping a callback entry.
> > Take a look at how epoll does it in ep_insert()...
> >
> Ah! I just tried your suggestion and it was simple and it works. That
> is brilliant.
>
> I will post a follow-up v2 that drops patch 2/3. Thanks, Davide.

I assume you're in control of the file/fd lifetime, so that the file*
cannot vanish while you've your wait_queue dropped inside its
wait_queue_head, isn't it?

- Davide