Spin lock implementation in Windows and depends on it a double-checked singleton.

singleton.cpp

// we use double-lock check in singleton to solve the problem we met in spinlock1.cpp, but hey, that itself is a lock, it has to use some atomic intrusion/memory barrier to solve this synchronization problem

template<class T>
class SingletonHolder
{
private: 
    static volatile T* instance;
    static volatile int m_lockObj;
    SingletonHolder() {}
    
public:
    static T* get()
    {
        if(instance == NULL)
        {
            // TODO: use RAII to make it exception safe.
            Lock lock;
            lock.lock(m_lockObj);
            if(instance == NULL)
                instance = new T();
            lock.unlock(m_lockObj);
        }
        return instance;
    }
}

template<class T> SingletonHolder::instance = NULL;
template<class T> SingletonHolder::m_lockObj = 0;

spinlock0.cpp

// The right way
class Lock
{
    void lock(volatile int* lockObj)
    {
        while(true)
        {
            // CAS: compare and swap
            // synchronized: if(*lockObj == 0) *lockObj = 1; 
            if(interlockedCompareExchange(lockObj, 1, 0) == 0)
            {
                // lock acquired 
                break;
            }
        }
    }
    // release the lock
    void unlock(volatile int* lockObj)
    {
        // lock released
       *lockObj = 0; 
    }
};

spinlock1.cpp

// The wrong way - the check and assignment is not atomic, there is a race condition there.
class Lock
{
    void lock(volatile int* lockObj)
    {
        while(true)
        {
            if(*lockObj == 0)
            {
                *lockObj = 1;
                break;
            }
           
        }
    }
    // release the lock
    void unlock(volatile int* lockObj)
    {
        // lock released
       *lockObj= 0; 
    }
};

production-level的spinlock windows实现见此：http://www.codeproject.com/Articles/184046/Spin-Lock-in-C

volatile在C++标准中的定义保证一个线程的修改，在另外一个线程中是可见的。
但在windows下，微软做了更多：保证volatile的读写不会被乱序优化 （编译时）： http://msdn.microsoft.com/en-us/library/12a04hfd(v=vs.80).aspx