C# ConcurrentStack实现

我们通过C# Queue 和Stack的实现知道Stack是依靠数组实现的，那么ConcurrentStack的栈又是如何实现的了，然后它的线程安全又是怎么做到的了？ 来看看其code吧

 public class ConcurrentStack<T> : IProducerConsumerCollection<T>, IReadOnlyCollection<T>    {        private class Node        {            internal readonly T m_value; // Value of the node.            internal Node m_next; // Next pointer.            internal Node(T value)            {                m_value = value;                m_next = null;            }        }        private volatile Node m_head;         private const int BACKOFF_MAX_YIELDS = 8;                public ConcurrentStack(){}        public ConcurrentStack(IEnumerable<T> collection)        {            if (collection == null)            {                throw new ArgumentNullException("collection");            }            InitializeFromCollection(collection);        }                private void InitializeFromCollection(IEnumerable<T> collection)        {            // We just copy the contents of the collection to our stack.            Node lastNode = null;            foreach (T element in collection)            {                Node newNode = new Node(element);                newNode.m_next = lastNode;                lastNode = newNode;            }            m_head = lastNode;        }                public void Push(T item)        {            Node newNode = new Node(item);            newNode.m_next = m_head;            if (Interlocked.CompareExchange(ref m_head, newNode, newNode.m_next) == newNode.m_next)            {                return;            }            PushCore(newNode, newNode);        }                private void PushCore(Node head, Node tail)        {            SpinWait spin = new SpinWait();            do            {                spin.SpinOnce();                // Reread the head and link our new node.                tail.m_next = m_head;            }            while (Interlocked.CompareExchange(ref m_head, head, tail.m_next) != tail.m_next);        }                public bool TryPop(out T result)        {            Node head = m_head;            //stack is empty            if (head == null)            {                result = default(T);                return false;            }            if (Interlocked.CompareExchange(ref m_head, head.m_next, head) == head)            {                result = head.m_value;                return true;            }            return TryPopCore(out result);        }                private bool TryPopCore(out T result)        {            Node poppedNode;            if (TryPopCore(1, out poppedNode) == 1)            {                result = poppedNode.m_value;                return true;            }            result = default(T);            return false;        }         private int TryPopCore(int count, out Node poppedHead)        {            SpinWait spin = new SpinWait();            Node head;            Node next;            int backoff = 1;            Random r = new Random(Environment.TickCount & Int32.MaxValue); // avoid the case where TickCount could return Int32.MinValue            while (true)            {                head = m_head;                // Is the stack empty?                if (head == null)                {                    poppedHead = null;                    return 0;                }                next = head;                int nodesCount = 1;                for (; nodesCount < count && next.m_next != null; nodesCount++)                {                    next = next.m_next;                }                // Try to swap the new head.  If we succeed, break out of the loop.                if (Interlocked.CompareExchange(ref m_head, next.m_next, head) == head)                {                    poppedHead = head;                    return nodesCount;                }                // We failed to CAS the new head.  Spin briefly and retry.                for (int i = 0; i < backoff; i++)                {                    spin.SpinOnce();                }                backoff = spin.NextSpinWillYield ? r.Next(1, BACKOFF_MAX_YIELDS) : backoff * 2;            }        }    }

ConcurrentStack<T>里面有一个内部类Node，看到这里我们就知道ConcurrentStack<T>的栈是一开节点Node来做的一个链表，非常好理解。那么线程安全又是怎么做到的了？首先我们来看看Push放法，首先我们需要新实例一个Node，并且新Node的m_next指向现有m_head头节点【newNode.m_next = m_head】，然后在原子比较newNode.m_next 是否是m_head【Interlocked.CompareExchange(ref m_head, newNode, newNode.m_next) == newNode.m_next】，如果是那么把m_head改为newNode ，push操作完成。如果第一个线程newNode.m_next = m_head之后，有新的线程执行了Interlocked.CompareExchange(ref m_head, newNode, newNode.m_next) == newNode.m_next 那么push就需要执行PushCore方法；该方法先自旋一下，然后在Interlocked.CompareExchange(ref m_head, head, tail.m_next) != tail.m_next【这里head和tail是新节点，tail.m_next是指向m_head】，如果当前线程是最新最近的那个 ，那么这个Interlocked.CompareExchange(ref m_head, head, tail.m_next) == tail.m_next就为true，退出循环，否者自旋后再次比较赋值。那么TryPop的实现也是类似的，如果if (Interlocked.CompareExchange(ref m_head, head.m_next, head) == head)成立那么直接返回，否者调用TryPopCore方法。而TryPopCore方法的核心是   if (Interlocked.CompareExchange(ref m_head, next.m_next, head) == head)，如果成立则退出，否者自旋，至于自旋的次数来源于for (int i = 0; i < backoff; i++){ spin.SpinOnce(); }。是不是很简单了，但是也很巧妙啊。线程安全依靠SpinWait 的自旋和原子操作Interlocked.CompareExchange来实现的。