CAS

Compare And Swap 比较并交换  硬件同步原语 

.NET通过 System.Threading.Interlocked.CompareExchang重载实现CAS

 

自旋锁CAS实现

public class SpinLock
    {
        private volatile int locked;

        public void Aquire()
        {
            while (System.Threading.Interlocked.CompareExchange(ref locked, 1, 0) != 0) 
           {
               //‘浪费’CPU周期 重试
           }
        }

        public void Release()
        {
            locked = 0;
        }
    

 因为自旋锁浪费CPU周期所以它不适合保护长时间的操作，如数据库访问，磁盘大文件写操作，网络发包等。

 如果保护的代码很快执行完毕，自旋锁很有用了，如修改对象字段，增加变量值，向简单集合插入数据等。

 

 public static void DoWithCAS<T>(ref T location, Func<T, T> generator) where T : class
        {
            T temp, replace;
            do
            {
                temp = location;
                replace = generator(temp);
            } while (Interlocked.CompareExchange(ref location, replace, temp) != temp);
        }

 单向链表无锁栈实现：

public class LockFreeStack<T>
        {
            private class Node
            {
                public T Data;
                public Node Next;
            }
            private Node head;
            public void Push(T element)
            {
                Node node = new Node { Data = element };
                DoWithCAS(ref head, h =>
                {
                    node.Next = h;
                    return node;
                });
            }
            public bool TryPop(out T element)
            {
                //DoWithCAS does not work here because we need early termination semantics
                Node node;
                do
                {
                    node = head;
                    if (node == null)
                    {
                        element = default(T);
                        return false; //bail out – nothing to return
                    }
                } while (Interlocked.CompareExchange(ref head, node.Next, node) != node);
                element = node.Data;
                return true;
            }
        

 codes refer