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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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