That's not my point.
My point is that from a technical standpoint, I think giving user land
higher priorities than the kernel is _wrong_.
It gets you into all the priority inversion stuff, where you suddently
must not do simple system calls because the regular kernel locks are no
longer safe to use. That's a HUGE design mistake, and a classic one. Yes,
others have done it that way. A billion flies _can_ be wrong - I'd rather
eact lamb chops than shit.
- I think the microkernel approach is fundamentally broken. Karim claims
there is no priority inversion, but he must have his blinders on. Every
single spinlock in the kernel assumes that the kernel isn't preempted,
which means that user apps that can preempt the kernel cannot use them.
(Or RTAI just handles the priority inversion the way that it has been
handled in other places: by dropping the priority on the floor when
calling into the kernel. Whatever. It's still priority inversion, and
it's still broken).
It's worse than that. Something as simple as growing your stack a bit
too much will cause a hard kernel failure (or failure of the RT part,
assuming that the priority is dropped). Karim claims to give "user
land" hard-real-time abilities, but the fact is, it's not "user land"
any more. it's a limited shadow, and a _perversion_ of what user land
is supposed to be all about.
This is my _technical_ reason for saying that user-land hard realtime
sucks, and SHOULD NOT BE DONE. That way lies madness, and crap.
- My other argument is one of FUD against the patent. People claim that
the RTLinux patent stands in their way, and they are full of _crap_.
- The patent only covers a specific way of doing things, which as
far as I can tell isn't even an issue with RTAI. In short, the
RTLinux patent has about as much to do with "holding up
real-time development on Linux" as every other patent out there.
- Yes, if you go the RTLinux way, you either need to make your RT
kernel modules GPL'd, or you need to pay FSMlabs. Since I would
strongly suggest you make kernel modules GPL'd anyway, this just
isn't an issue. The fact that FSMlabs can get people to pay for
their patent is just another "tax on stupidity".
And "tax on stupidity" is fine by me. People who don't want to
use the GPL might as well pay for it, either by paying FSMlabs
or by paying somebody else. I don't care.
Have I made myself sufficiently clear by splitting up the issues into a
technical part and a FUD part?
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