How to create one non-preemptible and another preemptible thread in the user space?

Hello All,

I want one of the user threads to act as a non-preemptible thread. As soon as an
interrupt comes, the user thread is woken up. But, the problem is that it is being preempted
before it goes back to a sleep.
Another thread should execute only when the other thread is not executing.
One trial made was to make the non-preemptible thread as FIFO scheduled and
the pre-emptible thread as Round - Robin scheduled. But this didnot yield the desired
results. Is there anything that I have missed ?
How is it possible to implement such a situation where one thread is
non-preemptible and the other has to be always preemptible(only by the other thread) ?

Thanks.
Vinay.