Returns a new barrier.
make-barrier returns a new barrier with count count.
callback is called by barrier-wait just before it unblocks barrier. It is called with a single argument, barrier, while holding an internal lock in barrier that will prevent other operations on barrier from running. The callback is guaranteed to happen before barrier-wait allows any of the other threads to continue.
Because the callback is called inside a lock, you should ensure that it is relatively short to prevent congestion if another thread tries to access barrier. Allocating a few objects is reasonable. If there is a more expensive operation that has to be done by only one of the threads, it can be done by the thread that returned
:unblocked from barrier-wait. The advantage of using the callback is that it is called before any of the waiting threads pass the barrier.
LispWorks User Guide and Reference Manual - 20 Sep 2017