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    SubjectRe: upcalls from kernel code to user space daemons
    Bernd Petrovitsch wrote:
    > On Mon, 2004-06-14 at 23:57, Chris Friesen wrote:
    > > Oliver Neukum wrote:
    > >
    > > > > userspace daemon loops on ioctl()
    > > > > kernel portion of ioctl call goes to sleep until something to do
    > > > > when needed, fill in data and return to userspace
    > > > > userspace does stuff, then passes data back down via ioctl()
    > > > > ioctl() puts userspace back to sleep and continues on with other
    > work
    > > >
    > > > You could just as well implement an ordinary read()
    > >
    > > Not quite. The userspace is passing data down as well. I don't know
    > how you'd
    > > do that with read().
    > For this you use write().

    And you eat another syscall per userspace call. For the ioctl() case, you only
    need to issue a single call to ioctl(). You pass the result of the previous
    request down, and then when it returns you get the data for the next request.

    Although I have to admit it's not pretty, and the performance improvements may
    not be worth the obfuscation of the code.

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