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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:

    > The general thesis is for decoupling of the two subsystems. In order to
    > do this, you need some form of polymorphism and an intermediate "handle"
    > mechanism which is userspace friendly. File-descriptors already fit
    > this role neatly, with the "int fd" being the handle, and the f_ops
    > being the polymorphic interface. Eventfd is of course, a subclass of
    > this concept in that it has these same general properties but with
    > signaling semantics (non-blocking collapsible events, etc).
    > Say, for example, you wanted disk IO completion events to generate an
    > interrupt into a guest. One way to do this would, of course, modify all
    > the disk-io code so it knows how to directly inject a KVM guest
    > interrupt. While this would work, someone would undoubtedly get flamed
    > for such a suggestion ;)
    > Another way to do it is to treat the AIO eventfd as the hook point.
    > IIUC AIO already knows how to be an eventfd producer. KVM, by virtue of
    > irqfd, already knows how to be an eventfd consumer. So now kvm can
    > consume AIO, or it can consume userspace events equally well, and
    > without modification. Neither side needs to know about the other per
    > se, other than the details on how to use the eventfd interface.
    > Don't get me wrong: We expect userspace to use all this stuff too. I
    > just expect that we will see all permutations of producer/consumer +
    > userspace/kernel combinations, so I want to retain that "all producers
    > have left" notification feature set. Today eventfd supports producers
    > or consumers in userspace, and producers in the kernel. This new work
    > we are doing adds consumer support in the kernel. Kernel to kernel is
    > just a natural extension of that.

    A file* is the VFS link between userspace and the kernel. Is not a magical
    polymorphic interface to be used for whatever kernel side reasons.
    Basing a kernel internal API over it is flawed.
    On top of that, a single reference count does not put you on cover about
    the possible combinations of producers and consumers. For that, you'd need
    a pipe-like reference handling logic, that is way far from the eventfd scope.
    So please stop making hypothetical cases about interface usages, and
    *when* we will have a real case, we'll see what the better handling for it
    will be.

    - Davide

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