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
  25   lll_lock(barrier->lock);
  26   if (!--barrier->left) {
  27     barrier->left = barrier->init_count;
  28     barrier->curr_event++;
  29     futex_wake(&barrier->curr_event, INT_MAX)
  30     lll_unlock(barrier->lock);
  31
  32     return BARRIER_SERIAL_THREAD;
  33   }
  34
  35   event = barrier->curr_event;
  36   for (;;) {
  37     lll_unlock(barrier->lock);
  38
  39     futex_wait(&barrier->curr_event, event)
  40
  41     lll_lock(barrier->lock);
  42     if (event != barrier->curr_event)
  43       break;
  44   }
  45   lll_unlock(barrier->lock);
  46
  47   return 0;
  48 }
  49