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   lll_unlock(cv->lock);
  38 }
  39
  40
  41 cond_timedwait(cv, mutex, timeout):
  42 {
  43    lll_lock(cv->lock);
  44    mutex_unlock(mutex);
  45
  46    cleanup_push
  47
  48    ++cv->total_seq;
  49    val = seq =  cv->wakeup_seq;
  50
  51    while (1) {
  52
  53      lll_unlock(cv->lock);
  54
  55      enable_async
  56
  57      ret = FUTEX_WAIT(cv->wakeup_seq, val, timeout);
  58
  59      restore_async
  60
  61      lll_lock(cv->lock);
  62
  63      val = cv->wakeup_seq;
  64
  65      if (cv->woken_seq >= seq && cv->woken_seq < val) {
  66        ret = 0;
  67        break;
  68      }
  69
  70      if (ret == TIMEDOUT) {
  71        ++cv->wakeup_seq;
  72        break;
  73      }
  74    }
  75
  76    ++cv->woken_seq;
  77
  78    lll_unlock(cv->lock);
  79
  80    cleanup_pop
  81
  82    mutex_lock(mutex);
  83
  84    return ret;
  85 }
  86
  87 cond_signal(cv)
  88 {
  89    lll_lock(cv->lock);
  90
  91    if (cv->total_seq > cv->wakeup_seq) {
  92      ++cv->wakeup_seq;
  93      FUTEX_WAKE(cv->wakeup_seq, 1);
  94    }
  95
  96    lll_unlock(cv->lock);
  97 }
  98
  99 cond_broadcast(cv)
 100 {
 101    lll_lock(cv->lock);
 102
 103    if (cv->total_seq > cv->wakeup_seq) {
 104      cv->wakeup_seq = cv->total_seq;
 105      FUTEX_WAKE(cv->wakeup_seq, ALL);
 106    }
 107
 108    lll_unlock(cv->lock);
 109 }