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    SubjectRe: [PATCH 3/3] eventfd: add internal reference counting to fix notifier race conditions
    On Mon, 22 Jun 2009, Gregory Haskins wrote:

    > Davide Libenzi wrote:
    > > On Sun, 21 Jun 2009, Gregory Haskins wrote:
    > >
    > >
    > >> This looks great, Davide. I am fairly certain I can now solve the races
    > >> and even implement Michael's DEASSIGN feature with this patch in place.
    > >> I will actually fire it up tomorrow when I am back in the office and
    > >> give it a spin, but I do not spy any more races via visual inspection.
    > >>
    > >> Kind Regards,
    > >> -Greg
    > >>
    > >> PS: I was wrong with my previous statement about requiring an embeddable
    > >> object (eventfd_notifier for me, eventfd_pollcb for you). I think you
    > >> can technically solve this issue minimally by merely locking the POLLHUP
    > >> and exposing the kref. However, I think that leads to an more awkward
    > >> interface (e.g. we already have eventfd_fget() plus we add a new one
    > >> like eventfd_refget(), which might confuse users), so I prefer what you
    > >> did here. Just thought I would throw that out there in case you would
    > >> prefer to change even fewer lines.
    > >>
    > >
    > > I actually ended up exposing the eventfd context anyway, since IMO is a
    > > better option instead of holding references to the eventfd file (that
    > > makes VFS people uneasy).
    > >
    > I liked "version - 1" better ;)
    > I think ultimately we still want to hold the fget() for
    > eventfd_signal(), as it is the producer side. Without it, we have no
    > way of knowing when the last producer goes away if they happen to be an
    > in-kernel user.

    No you don't. Holding a file* reference does not give you any assurance
    whatsoever that the last consumer did not go away. The f_count value will
    simply be 1 (just you).
    If a producer side want to take proper action when the last consumer
    leaves the building, it needs to register a pollcb hook and handle
    Exposing the opaque eventfd context, and basing the eventfd operations on
    that one (instead of the live referenced file*) is a cleaner interface.
    Can you please base your IRQfd bits on top of that, so that we can give
    this IRQfd thread a closure?

    - Davide

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