condvar

Hi there,

to be able to efficiently implement a condvar mechanism (and similar 
things), it would be great to atomically release one semaphore while 
aquiring another.
Together with the new B_RELEASE_ALL flag, condvar could then be 
implemented very easily.

The functionality is very trivial to implement, that's why I've already 
done it - the question is on how we make it public.
There are basically 3 possibilities:
1) keep it private for R1
2) (the messy version) add another flag to acquire_sem_etc() that 
reinterpretes the "count" parameter as semaphore to be released - this 
has the advantage that no new system call is added; it's by no means 
backwards compatible, though (the app would run incorrectly on 
systems).
3) (the clean version) add a new pair of system calls: switch_sem() and 
switch_sem_etc() (other naming suggestions are welcome). Applications 
using these functions would not run on R5 or Zeta anymore.

For now, I did 3) here in my working copy, and I wanted to know what 
you think of it.

Bye,
   Axel.



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