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。

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