This exclusive barrier allows to synchronise one threads with a specific one, so a master thread will wait for a donor thread to reach the barrier or a donor thread will wait for a master thread to reach the barrier.
The idea is assisted with a normal definition of barrier. Let’s consider that there are multiple donor threads that must synchronise with a master thread. Only one thread will be allowed to pass by wait_donor() until this one synchronises with the master thread, when this happens, wait_donor() will allow to pass another thread and the process will be repeated.
For this example, I am using boost as a barrier to assist the synchronisation, but any other implementation should work as well. The general instance of a barrier has a size of two, that’s how the a general barrier will allow to pass master thread and one donor thread.
Imagine a boatman (ie. Charon), Charon will pass a soul from side A of the river to side B (underworld), but the boat has capacity for only two passengers. Charon will have to carry one soul at a time, and since nobody leaves the underworld, charon will come to side B alone to carry another soul, and so on and so forth.
Here is an ilustration:
