Re: [KVM-RFC PATCH 0/2] irqfd: use POLLHUP notification for close()

Michael S. Tsirkin
2009-06-03T06:41:03Z

On Tue, Jun 02, 2009 at 01:41:05PM -0400, Gregory Haskins wrote:
> > And having close not clean up the state unless you do an ioctl first is
> > very messy IMO - I don't think you'll find any such examples in kernel.
> >
> >
>
> I agree, and that is why I am advocating this POLLHUP solution. It was
> only this other way to begin with because the technology didn't exist
> until Davide showed me the light.
>
> Problem with your request is that I already looked into what is
> essentially a bi-directional reference problem (for a different reason)
> when I started the POLLHUP series. Its messy to do this in a way that
> doesn't negatively impact the fast path (introducing locking, etc) or
> make my head explode making sure it doesn't race. Afaict, we would need
> to solve this problem to do what you are proposing (patches welcome).
>
> If this hybrid decoupled-deassign + unified-close is indeed an important
> feature set, I suggest that we still consider this POLLHUP series for
> inclusion, and then someone can re-introduce DEASSIGN support in the
> future as a CAP bit extension. That way we at least get the desirable
> close() properties that we both seem in favor of, and get this advanced
> use case when we need it (and can figure out the locking design).
>

FWIW, I took a look and yes, it is non-trivial.
I concur, we can always add the deassign ioctl later.

--
MST