Re: [PATCH 3/3] eventfd: add internal reference counting to fix notifier race conditions

Davide Libenzi
2009-06-22T19:54:27Z

On Mon, 22 Jun 2009, Gregory Haskins wrote:

> Davide Libenzi wrote:
> > On Mon, 22 Jun 2009, Michael S. Tsirkin wrote:
> >
> >
> >> On Mon, Jun 22, 2009 at 11:03:22AM -0700, Davide Libenzi wrote:
> >>
> >>> In your case of kernel-to-kernel scenario, why would you need eventfd at
> >>> all, if userspace role in that model is simply to create it?
> >>>
> >> That's not 100% true. We have a mode where userspace is the producer
> >> and/or consumer (migration mode) and we switch between that and
> >> direct kernel-to-kernel communication.
> >>
> >
> > Then you'd need to ask yourself how to handle your complex case inside the
> > KVM code, so that other eventfd users are not affected by the extra fat
> > needed to handle your scenarios. Thing that seem to be continuosly tried.
> > A file* based kernel-to-kernel interface is rather wrong IMO.
> >
>
> Well, I will point out that the interface in question is
> eventfd_signal(struct file *), and you were the one that invented it
> afaict. Can't help it if we like it :)

Yes, I did. The case for eventfd was dual. First, it was a communication
link between userspace and kernel, for things like KAIO. Second, it was a
faster and smaller userspace replacement for things people used pipes
before.
In all those cases, at least one reference of the file* is alive in
userspace.
Even with the above case in mind, today I'd still use the eventfd_ctx as
internal kernel API accessory.

- Davide