Barriers pseudocode
===================

    int pthread_barrier_wait(barrier_t * barrier);

struct barrier_t {

   unsigned int lock:
         - internal mutex

   unsigned int left;
         - current barrier count, # of threads still needed.

   unsigned int init_count;
         - number of threads needed for the barrier to continue.

   unsigned int curr_event;
         - generation count
}

pthread_barrier_wait(barrier_t *barrier)
{
  unsigned int event;

  lll_lock(barrier->lock);
  if (!--barrier->left) {
    barrier->left = barrier->init_count;   
    barrier->curr_event++;
    futex_wake(&barrier->curr_event, INT_MAX)
    lll_unlock(barrier->lock);

    return BARRIER_SERIAL_THREAD;
  }

  event = barrier->curr_event;
  for (;;) {
    lll_unlock(barrier->lock);

    futex_wait(&barrier->curr_event, event)

    lll_lock(barrier->lock);
    if (event != barrier->curr_event)
      break;
  }
  lll_unlock(barrier->lock);

  return 0;
}