A lock-free user space "mutex" implemented by std

std_futex.h

#include <condition_variable>
#include <mutex>
#include <atomic>
/**
 * A lock-free user space "mutex" implemented by std
 *
 * The core idea is very simple:
 * 1. when there is no contention, we use atomic release/acquire synchronization to do a lock-free lock/unlock.
 * 2. when there is contention, we use the syscall lock/unlock.
 *
 * Note:
 * cpp20 introduced `wait` `notify_one` `notify_all` to atomic types, it might be better than condition variable.
 */
class Futex {
private:
  std::atomic<int> m_counter{0};
  std::mutex mutex;
  std::condition_variable cv;

public:
  void lock() {
    if (m_counter.fetch_add(1, std::memory_order_acquire) > 0) {
      std::unique_lock<std::mutex> lock(mutex);
      // std::cout << "Non-lock-free wait" << std::endl;
      cv.wait(lock);
      // std::cout << "Non-lock-free locked" << std::endl;
    } else {
      // std::cout << "lock-free locked" << std::endl;
    }
  }

  void unlock() {
    if (m_counter.fetch_add(-1, std::memory_order_release) == 1) {
      // std::cout << "lock-free unlocked" << std::endl;
    } else {
      std::unique_lock<std::mutex> lock(mutex);
      cv.notify_one();
      // std::cout << "Non-lock-free unlocked" << std::endl;
    }
  }
};