1: Reader Writer Locks pseudocode
    2: ==============================
    3: 
    4:         pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
    5:         pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
    6:         pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
    7: 
    8: struct pthread_rwlock_t {
    9: 
   10:    unsigned int lock:
   11:          - internal mutex
   12: 
   13:    unsigned int writers_preferred;
   14:          - locking mode: 0 recursive, readers preferred
   15:                          1 nonrecursive, writers preferred
   16: 
   17:    unsigned int readers;
   18:          - number of read-only references various threads have
   19: 
   20:    pthread_t writer;
   21:          - descriptor of the writer or 0
   22: 
   23:    unsigned int readers_wakeup;
   24:          - 'all readers should wake up' futex.
   25: 
   26:    unsigned int writer_wakeup;
   27:          - 'one writer should wake up' futex.
   28: 
   29:    unsigned int nr_readers_queued;
   30:          - number of readers queued up.
   31: 
   32:    unsigned int nr_writers_queued;
   33:          - number of writers queued up.
   34: }
   35: 
   36: pthread_rwlock_rdlock(pthread_rwlock_t *rwlock)
   37: {
   38:   lll_lock(rwlock->lock);
   39:   for (;;) {
   40:     if (!rwlock->writer && (!rwlock->nr_writers_queued ||
   41:                                         !rwlock->writers_preferred))
   42:         break;
   43: 
   44:     rwlock->nr_readers_queued++;
   45:     val = rwlock->readers_wakeup;
   46:     lll_unlock(rwlock->lock);
   47: 
   48:     futex_wait(&rwlock->readers_wakeup, val)
   49: 
   50:     lll_lock(rwlock->lock);
   51:     rwlock->nr_readers_queued--;
   52:   }
   53:   rwlock->readers++;
   54:   lll_unlock(rwlock->lock);
   55: }
   56: 
   57: pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock)
   58: {
   59:   int result = EBUSY;
   60:   lll_lock(rwlock->lock);
   61:   if (!rwlock->writer && (!rwlock->nr_writers_queued ||
   62:                                         !rwlock->writers_preferred))
   63:     rwlock->readers++;
   64:   lll_unlock(rwlock->lock);
   65:   return result;
   66: }
   67: 
   68: pthread_rwlock_wrlock(pthread_rwlock_t *rwlock)
   69: {
   70:   lll_lock(rwlock->lock);
   71:   for (;;) {
   72:     if (!rwlock->writer && !rwlock->readers)
   73:        break;
   74: 
   75:     rwlock->nr_writers_queued++;
   76:     val = rwlock->writer_wakeup;
   77:     lll_unlock(rwlock->lock);
   78: 
   79:     futex_wait(&rwlock->writer_wakeup, val);
   80: 
   81:     lll_lock(rwlock->lock);
   82:     rwlock->nr_writers_queued--;
   83:   }
   84:   rwlock->writer = pthread_self();
   85:   lll_unlock(rwlock->lock);
   86: }
   87: 
   88: pthread_rwlock_unlock(pthread_rwlock_t *rwlock)
   89: {
   90:   lll_lock(rwlock->lock);
   91: 
   92:   if (rwlock->writer)
   93:     rwlock->writer = 0;
   94:   else
   95:     rwlock->readers--;
   96: 
   97:   if (!rwlock->readers) {
   98:     if (rwlock->nr_writers_queued) {
   99:       ++rwlock->writer_wakeup;
  100:       lll_unlock(rwlock->lock);
  101:       futex_wake(&rwlock->writer_wakeup, 1);
  102:       return;
  103:     } else
  104:       if (rwlock->nr_readers_queued) {
  105:         ++rwlock->readers_wakeup;
  106:         lll_unlock(rwlock->lock);
  107:         futex_wake(&rwlock->readers_wakeup, MAX_INT);
  108:         return;
  109:       }
  110:   }
  111: 
  112:   lll_unlock(rwlock->lock);
  113: }