glibc/nptl/DESIGN-rwlock.txt

Reader Writer Locks pseudocode
==============================

	pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
	pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
	pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);

struct pthread_rwlock_t {

   unsigned int lock:
         - internal mutex

   unsigned int writers_preferred;
         - locking mode: 0 recursive, readers preferred
                         1 nonrecursive, writers preferred

   unsigned int readers;
         - number of read-only references various threads have

   pthread_t writer;
         - descriptor of the writer or 0

   unsigned int readers_wakeup;
         - 'all readers should wake up' futex.

   unsigned int writer_wakeup;
         - 'one writer should wake up' futex.

   unsigned int nr_readers_queued;
         - number of readers queued up.

   unsigned int nr_writers_queued;
         - number of writers queued up.
}

pthread_rwlock_rdlock(pthread_rwlock_t *rwlock)
{
  lll_lock(rwlock->lock);
  for (;;) {
    if (!rwlock->writer && (!rwlock->nr_writers_queued ||
					!rwlock->writers_preferred))
        break;

    rwlock->nr_readers_queued++;
    val = rwlock->readers_wakeup;
    lll_unlock(rwlock->lock);

    futex_wait(&rwlock->readers_wakeup, val)

    lll_lock(rwlock->lock);
  }
  rwlock->readers++;
  lll_unlock(rwlock->lock);
}

pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock)
{
  int result = EBUSY;
  lll_lock(rwlock->lock);
  if (!rwlock->writer && (!rwlock->nr_writers_queued ||
					!rwlock->writers_preferred))
    rwlock->readers++;
  lll_unlock(rwlock->lock);
  return result;
}

pthread_rwlock_wrlock(pthread_rwlock_t *rwlock)
{
  lll_lock(rwlock->lock);
  for (;;) {
    if (!rwlock->writer && !rwlock->readers)
       break;

    rwlock->nr_writers_queued++;
    val = rwlock->writer_wakeup;
    lll_unlock(rwlock->lock);

    futex_wait(&rwlock->writer_wakeup, val);

    lll_lock(rwlock->lock);
    rwlock->nr_writers_queued--;
  }
  rwlock->writer = pthread_self();
  lll_unlock(rwlock->lock);
}

pthread_rwlock_unlock(pthread_rwlock_t *rwlock)
{
  lll_lock(rwlock->lock);

  if (rwlock->writer)
    rwlock->writer = 0;
  else
    rwlock->readers--;

  if (!rwlock->readers) {
    if (rwlock->nr_writers_queued) {
      ++rwlock->writer_wakeup;
      lll_unlock(rwlock->lock);
      futex_wake(&rwlock->writer_wakeup, 1);
      return;
    } else
      if (rwlock->nr_readers_queued) {
        ++rwlock->readers_wakeup;
        lll_unlock(rwlock->lock);
        futex_wake(&rwlock->readers_wakeup, MAX_INT);
        return;
      }
  }

  lll_unlock(rwlock->lock);
}