src/thread/pthread_once.c

   1 #include "pthread_impl.h"
   2
   3 static void undo(void *control)
   4 {
   5         /* Wake all waiters, since the waiter status is lost when
   6          * resetting control to the initial state. */
   7         if (a_swap(control, 0) == 3)
   8                 __wake(control, -1, 1);
   9 }
  10
  11 hidden int __pthread_once_full(pthread_once_t *control, void (*init)(void))
  12 {
  13         /* Try to enter initializing state. Four possibilities:
  14          *  0 - we're the first or the other cancelled; run init
  15          *  1 - another thread is running init; wait
  16          *  2 - another thread finished running init; just return
  17          *  3 - another thread is running init, waiters present; wait */
  18
  19         for (;;) switch (a_cas(control, 0, 1)) {
  20         case 0:
  21                 pthread_cleanup_push(undo, control);
  22                 init();
  23                 pthread_cleanup_pop(0);
  24
  25                 if (a_swap(control, 2) == 3)
  26                         __wake(control, -1, 1);
  27                 return 0;
  28         case 1:
  29                 /* If this fails, so will __wait. */
  30                 a_cas(control, 1, 3);
  31         case 3:
  32                 __wait(control, 0, 3, 1);
  33                 continue;
  34         case 2:
  35                 return 0;
  36         }
  37 }
  38
  39 int __pthread_once(pthread_once_t *control, void (*init)(void))
  40 {
  41         /* Return immediately if init finished before, but ensure that
  42          * effects of the init routine are visible to the caller. */
  43         if (*(volatile int *)control == 2) {
  44                 a_barrier();
  45                 return 0;
  46         }
  47         return __pthread_once_full(control, init);
  48 }
  49
  50 weak_alias(__pthread_once, pthread_once);