While working on a fix for 8157373, the guarantee

  guarantee(num_workers <= active_workers(),
            "Trying to execute task %s with %u workers which is more than the amount of active workers %u.",
            task->name(), num_workers, active_workers());

in

WorkGang::run_task(AbstractGangTask* task, uint num_workers)

fails because _active_workers is not updated after call to calc_parallel_marking_threads().