使用愚蠢的currentHashMap进行并发更新

Question

我正在尝试folly::concurrentHashMap作为一个非常基本的示例。假设我们有n个线程，并且每个线程都试图在哈希映射中仅插入一个元素：key = 0和val=max(map[key], threadId)。

示例

运行3个线程：线程0，线程1，线程2（它们可以按任何顺序访问地图）

Before running threads: map.find(0) == map.end()

After thread 1 => map[0] will be 1

After thread 0 => map[0] will be 1 (i.e., max(1, 0))

After thread 2 => map[0] will be 2 (i.e., max(1, 2))

代码

folly::ConcurrentHashMap<KeyType, ValType> follyMap;
for (int i = 0; i < numThreads; i++) {
    insertionThreads.emplace_back(std::thread([&] {
    do {
      auto found = follyMap.find(key);
      if (found != follyMap.end()) {
        ValType oldVal = found->second;
        if (oldVal < val) {
          std::cout << cnt << std::endl;
          auto res = follyMap.assign_if_equal(key, oldVal, val);
          cont = !res;
        }
        break;
      } else {
        auto res = follyMap.insert(key, val);
        cont = !res.second;
      }
    } while (cont);
}

问题

问题是输出是不确定的且不正确。即，例如，对于前述示例，程序输出0、1或2。然而，正确值仅是2。