Antoine:  I agree, the semaphore is the quick and robust solution.

sbt: I see your point.  Still, I think we still may have a flaw:  The statement that (owned-timeouts) is never an under-estimate isn't true on modern architectures, I think.  The order of the atomic decrement operations in the code means nothing and cannot be depended on to guarantee such a claim:  The thread doing the reading may see the individual updates in any order, and so the estimate may be an over- or an underestimate.

It would fix this and simplify things a lot to take the special case for timeout==0 out of the code.