boost::atomic 实现 spinlock

最新推荐文章于 2024-01-15 19:05:59 发布

转载最新推荐文章于 2024-01-15 19:05:59 发布 · 1.5k 阅读

本文探讨了Boost库中SpinLock的实现方式及其使用atomic类型进行状态管理的方法。通过对比官方示例与自定义实现，深入解析compare_exchange_strong与compare_exchange_weak的区别，并讨论其在不同场景下的应用。

先看下boost给的例子，我觉得有问题：

#include <boost/atomic.hpp>

class spinlock {
private:
  typedef enum {Locked, Unlocked} LockState;
  boost::atomic<LockState> state_;

public:
  spinlock() : state_(Unlocked) {}

  void lock()
  {
    // 可能这里的boost::memory_order_acquire有原子操作的效果吧，偶不是很理解，不过我觉得这里应该用cae操作才对
    while (state_.exchange(Locked, boost::memory_order_acquire) == Locked) {
      /* busy-wait */
    }
  }
  void unlock()
  {
    // 这里都直接写不做检查了，更加难以理解
    state_.store(Unlocked, boost::memory_order_release);
  }
};

自己实现：

class CESpinLock : boost::noncopyable
{
private:
    typedef enum {emUnlocked = 0, emLocked} EM_LockState;

public:
    CESpinLock() : m_atomicState(emLocked)
    {
    }

public:
    void lock()
    {
        EM_LockState state = emUnlocked;
        while(false == m_atomicState.compare_exchange_strong(state, emLocked))
        {
            state = emUnlocked;
        }
    }

    void unlock()
    {
        EM_LockState state = emLocked;
        while(false == m_atomicState.compare_exchange_strong(state, emUnlocked))
        {
            state = emLocked;
        }
    }

private:
    boost::atomic<EM_LockState> m_atomicState;
};

可以适当的在false里边加一点sleep操作感觉。还有一点就是不太激烈这里的cae操作分两种 strong和weak

bool compare_exchange_weak(T & expected, T desired, memory_order success_order, memory_order failure_order)

bool compare_exchange_strong(T & expected, T desired, memory_order order)

Compare current value with expected, change it to desired if matches. Returns true if an exchange has been performed, and always writes the previous value back in expected. 实在不理解 May fail spuriously, so must generally be retried in a loop.的意义，不过看了代码，在win32的实现上，weak是调用了stong实现的。

有可能是我不理解后面的内存访问参数的意义，给下我自己的实现：

class CESpinLock : boost::noncopyable
{
private:
    typedef enum {emUnlocked = 0, emLocked} EM_LockState;

public:
    CESpinLock() : m_atomicState(emLocked)
    {
    }

public:
    void lock()
    {
        EM_LockState state = emUnlocked;
        while(false == m_atomicState.compare_exchange_strong(state, emLocked))
        {
            state = emUnlocked;
        }
    }

    void unlock()
    {
        EM_LockState state = emLocked;
        while(false == m_atomicState.compare_exchange_strong(state, emUnlocked))
        {
            state = emLocked;
        }
    }

private:
    boost::atomic<EM_LockState> m_atomicState;
};