QtConcurrent: why releaseThread and reserveThread cause deadlock?

QMutexLocker locker(&mutex);

taskQueue.append(task); // Place the task on the task queue
if (waitingThreads > 0) {
   // there are already running idle thread. They are waiting on the 'runnableReady' 
   // QWaitCondition. Wake one up them up.
   waitingThreads--;
   runnableReady.wakeOne();
} else if (runningThreadCount < maxThreadCount) {
   startNewThread(task);
}

void QThreadPoolThread::run()
{
  QMutexLocker locker(&manager->mutex);
  while (true) {
    /* ... */
    if (manager->taskQueue.isEmpty()) {
      // no pending task, wait for one.
      bool expired = !manager->runnableReady.wait(locker.mutex(), 
                                                  manager->expiryTimeout);
      if (expired) {
        manager->runningThreadCount--;
        return;
      } else {
        continue;
      }
    }
    QRunnable *r = manager->taskQueue.takeFirst();
    // run the task
    locker.unlock();
    r->run();
    locker.relock();
  }
}

int main() {
    QThreadPool* gtpool = QThreadPool::globalInstance();
    //gtpool->setExpiryTimeout(50); <-- don't set the expiry Timeout, use the default one.
    qDebug() << gtpool->maxThreadCount();

    int count = 0;
    for (;;) {

        QVector<int> vec;
        for (int i = 0; i < 40 ; i++) {
            vec.push_back(i);
        }
        // launch a task with nested map
        Task1 task; // Task1 will have nested concurrent map
        QFuture<void> f = QtConcurrent::map(vec.begin(), vec.end(),task);

        f.waitForFinished(); // BUG: may hang there

        count++;

        /*
        // waiting most of thread in thread pool expire
        while (QThreadPool::globalInstance()->activeThreadCount() > 0)
        {
            QTest::qSleep(50);
        }
        */

        // launch a task only calculation
        Task2 task2;
        QFuture<void> f2 = QtConcurrent::map(vec.begin(), vec.end(), task2);

        f2.waitForFinished(); // BUG: may hang there

        qDebug() << count ;
    }
    return 0;
}