nptl/DESIGN-condvar.txt

   1 Conditional Variable pseudocode.
   2 ================================
   3
   4        int pthread_cond_timedwait (pthread_cond_t *cv, pthread_mutex_t *mutex);
   5        int pthread_cond_signal    (pthread_cond_t *cv);
   6        int pthread_cond_broadcast (pthread_cond_t *cv);
   7
   8 struct pthread_cond_t {
   9
  10    unsigned int cond_lock;
  11
  12          internal mutex
  13
  14    uint64_t total_seq;
  15
  16      Total number of threads using the conditional variable.
  17
  18    uint64_t wakeup_seq;
  19
  20      sequence number for next wakeup.
  21
  22    uint64_t woken_seq;
  23
  24      sequence number of last woken thread.
  25
  26 }
  27
  28
  29
  30 cleanup_handler(cv)
  31 {
  32   lll_lock(cv->lock);
  33
  34   ++cv->wakeup_seq;
  35   ++cv->woken_seq;
  36
  37   /* make sure no signal gets lost.  */
  38   FUTEX_WAKE(cv->wakeup_seq, ALL);
  39
  40   lll_unlock(cv->lock);
  41 }
  42
  43
  44 cond_timedwait(cv, mutex, timeout):
  45 {
  46    lll_lock(cv->lock);
  47    mutex_unlock(mutex);
  48
  49    cleanup_push
  50
  51    ++cv->total_seq;
  52    val = seq =  cv->wakeup_seq;
  53
  54    while (1) {
  55
  56      lll_unlock(cv->lock);
  57
  58      enable_async
  59
  60      ret = FUTEX_WAIT(cv->wakeup_seq, val, timeout);
  61
  62      restore_async
  63
  64      lll_lock(cv->lock);
  65
  66      val = cv->wakeup_seq;
  67
  68      if (val > seq && cv->woken_seq < val) {
  69        ret = 0;
  70        break;
  71      }
  72
  73      if (ret == TIMEDOUT) {
  74        ++cv->wakeup_seq;
  75        break;
  76      }
  77    }
  78
  79    ++cv->woken_seq;
  80
  81    lll_unlock(cv->lock);
  82
  83    cleanup_pop
  84
  85    mutex_lock(mutex);
  86
  87    return ret;
  88 }
  89
  90 cond_signal(cv)
  91 {
  92    lll_lock(cv->lock);
  93
  94    if (cv->total_seq > cv->wakeup_seq) {
  95      ++cv->wakeup_seq;
  96      FUTEX_WAKE(cv->wakeup_seq, 1);
  97    }
  98
  99    lll_unlock(cv->lock);
 100 }
 101
 102 cond_broadcast(cv)
 103 {
 104    lll_lock(cv->lock);
 105
 106    if (cv->total_seq > cv->wakeup_seq) {
 107      cv->wakeup_seq = cv->total_seq;
 108      FUTEX_WAKE(cv->wakeup_seq, ALL);
 109    }
 110
 111    lll_unlock(cv->lock);
 112 }