两个 unsigned 就是一把锁：Dragonfly 用 6 行 C++ 干掉了跨线程锁管理器

先看 Dragonfly 里最反直觉的六行代码，src/core/intent_lock.h:20-26：

// Returns true if lock was acquired. In any case, the intent is recorded.
bool Acquire(Mode m) {
   
  ++cnt_[m];
  if (cnt_[1