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    uint32_t broadcast_seq;
  27
  28 }
  29
  30
  31 struct cv_data {
  32
  33    pthread_cond_t *cv;
  34
  35    uint32_t bc_seq
  36
  37 }
  38
  39
  40
  41 cleanup_handler(cv_data)
  42 {
  43   cv = cv_data->cv;
  44   lll_lock(cv->lock);
  45
  46   if (cv_data->bc_seq == cv->broadcast_seq) {
  47     ++cv->wakeup_seq;
  48     ++cv->woken_seq;
  49   }
  50
  51   /* make sure no signal gets lost.  */
  52   FUTEX_WAKE(cv->wakeup_seq, ALL);
  53
  54   lll_unlock(cv->lock);
  55 }
  56
  57
  58 cond_timedwait(cv, mutex, timeout):
  59 {
  60    lll_lock(cv->lock);
  61    mutex_unlock(mutex);
  62
  63    cleanup_push
  64
  65    ++cv->total_seq;
  66    val = seq =  cv->wakeup_seq;
  67    cv_data.bc = cv->broadcast_seq;
  68    cv_data.cv = cv;
  69
  70    while (1) {
  71
  72      lll_unlock(cv->lock);
  73
  74      enable_async(&cv_data);
  75
  76      ret = FUTEX_WAIT(cv->wakeup_seq, val, timeout);
  77
  78      restore_async
  79
  80      lll_lock(cv->lock);
  81
  82      if (bc != cv->broadcast_seq)
  83        goto bc_out;
  84
  85      val = cv->wakeup_seq;
  86
  87      if (val != seq && cv->woken_seq != val) {
  88        ret = 0;
  89        break;
  90      }
  91
  92      if (ret == TIMEDOUT) {
  93        ++cv->wakeup_seq;
  94        break;
  95      }
  96    }
  97
  98    ++cv->woken_seq;
  99
 100  bc_out:
 101    lll_unlock(cv->lock);
 102
 103    cleanup_pop
 104
 105    mutex_lock(mutex);
 106
 107    return ret;
 108 }
 109
 110 cond_signal(cv)
 111 {
 112    lll_lock(cv->lock);
 113
 114    if (cv->total_seq > cv->wakeup_seq) {
 115      ++cv->wakeup_seq;
 116      FUTEX_WAKE(cv->wakeup_seq, 1);
 117    }
 118
 119    lll_unlock(cv->lock);
 120 }
 121
 122 cond_broadcast(cv)
 123 {
 124    lll_lock(cv->lock);
 125
 126    if (cv->total_seq > cv->wakeup_seq) {
 127      cv->wakeup_seq = cv->total_seq;
 128      cv->woken_seq = cv->total_seq;
 129      ++cv->broadcast_seq;
 130      FUTEX_WAKE(cv->wakeup_seq, ALL);
 131    }
 132
 133    lll_unlock(cv->lock);
 134 }