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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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