Suspend one thread and resume another in one system call ?

[Nikita V. Youshchenko]

Hello.

Consider a situation when:
- there is a set of threads,
- only one of those should be allowed to work at a time,
- there are "switch moments" when one thread should suspend, and other 
(known) resume.

"Classic" solutions like have each thread sleeping on a semaphore, and 
making switch operation "sem_post(next_sem); sem_wait(my_sem)" cause two 
syscalls per switch.

Could anyone suggest a solution with only one kernel entry per switch - 
running thread enters kernel and is de-scheduled, new thread resumes and 
leaves kernel?

Nikita

P.S.
Since there are other threads in the same application working in the 
parallel with the mentioned "exclusive set", user-space solutions like 
libpth are not interesting.
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