nptl/DESIGN-barrier.txt

   1 Barriers pseudocode
   2 ===================
   3
   4     int pthread_barrier_wait(barrier_t *barrier);
   5
   6 struct barrier_t {
   7
   8    unsigned int lock:
   9          - internal mutex
  10
  11    unsigned int left;
  12          - current barrier count, # of threads still needed.
  13
  14    unsigned int init_count;
  15          - number of threads needed for the barrier to continue.
  16
  17    unsigned int curr_event;
  18          - generation count
  19 }
  20
  21 pthread_barrier_wait(barrier_t *barrier)
  22 {
  23   unsigned int event;
  24   result = 0;
  25
  26   lll_lock(barrier->lock);
  27   if (!--barrier->left) {
  28     barrier->curr_event++;
  29     futex_wake(&barrier->curr_event, INT_MAX)
  30
  31     result = BARRIER_SERIAL_THREAD;
  32   } else {
  33     event = barrier->curr_event;
  34     lll_unlock(barrier->lock);
  35     do {
  36       futex_wait(&barrier->curr_event, event)
  37     } while (event == barrier->curr_event);
  38   }
  39
  40   if (atomic_increment_val (barrier->left) == barrier->init_count)
  41     lll_unlock(barrier->lock);
  42
  43   return result;
  44 }