c# - 这个无锁 .NET 队列线程安全吗？

Question

我的问题是，下面包含的用于单读取器单写入器队列类的类是否是线程安全的？这种队列称为无锁，即使队列满了也会阻塞。数据结构的灵感来自 Marc Gravell's implementation of a blocking queue在 StackOverflow。

该结构的要点是允许单个线程将数据写入缓冲区，而另一个线程读取数据。所有这一切都需要尽快发生。

article at DDJ by Herb Sutter 中描述了类似的数据结构。 , 除了实现是在 C++ 中。另一个区别是我使用普通链表，我使用数组链表。

我不是只包含一段代码，而是包含带有注释的整个代码，并带有一个宽松的开源许可证(麻省理工学院许可证 1.0)，以防有人发现它有用，并想使用它(按原样或修改)。

这与 Stack Overflow 上关于如何创建阻塞并发队列的其他问题有关(参见 Creating a blockinq Queue in .NET 和 Thread-safe blocking queue implementation in .NET)。

代码如下:

using System;
using System.Collections.Generic;
using System.Threading;
using System.Diagnostics;

namespace CollectionSandbox
{
    /// This is a single reader / singler writer buffered queue implemented
    /// with (almost) no locks. This implementation will block only if filled 
    /// up. The implementation is a linked-list of arrays.
    /// It was inspired by the desire to create a non-blocking version 
    /// of the blocking queue implementation in C# by Marc Gravell
    /// https://stackoverflow.com/questions/530211/creating-a-blocking-queuet-in-net/530228#530228
    class SimpleSharedQueue<T> : IStreamBuffer<T>
    {
        /// Used to signal things are no longer full
        ManualResetEvent canWrite = new ManualResetEvent(true);

        /// This is the size of a buffer 
        const int BUFFER_SIZE = 512;

        /// This is the maximum number of nodes. 
        const int MAX_NODE_COUNT = 100;

        /// This marks the location to write new data to.
        Cursor adder;

        /// This marks the location to read new data from.
        Cursor remover;

        /// Indicates that no more data is going to be written to the node.
        public bool completed = false;

        /// A node is an array of data items, a pointer to the next item,
        /// and in index of the number of occupied items 
        class Node
        {
            /// Where the data is stored.
            public T[] data = new T[BUFFER_SIZE];

            /// The number of data items currently stored in the node.
            public Node next;

            /// The number of data items currently stored in the node.
            public int count;

            /// Default constructor, only used for first node.
            public Node()
            {
                count = 0;
            }

            /// Only ever called by the writer to add new Nodes to the scene
            public Node(T x, Node prev)
            {
                data[0] = x;
                count = 1;

                // The previous node has to be safely updated to point to this node.
                // A reader could looking at the point, while we set it, so this should be 
                // atomic.
                Interlocked.Exchange(ref prev.next, this);
            }
        }

        /// This is used to point to a location within a single node, and can perform 
        /// reads or writers. One cursor will only ever read, and another cursor will only
        /// ever write.
        class Cursor
        {
            /// Points to the parent Queue
            public SimpleSharedQueue<T> q;

            /// The current node
            public Node node;

            /// For a writer, this points to the position that the next item will be written to.
            /// For a reader, this points to the position that the next item will be read from.
            public int current = 0;

            /// Creates a new cursor, pointing to the node
            public Cursor(SimpleSharedQueue<T> q, Node node)
            {
                this.q = q;
                this.node = node;
            }

            /// Used to push more data onto the queue
            public void Write(T x)
            {
                Trace.Assert(current == node.count);

                // Check whether we are at the node limit, and are going to need to allocate a new buffer.
                if (current == BUFFER_SIZE)
                {
                    // Check if the queue is full
                    if (q.IsFull())
                    {
                        // Signal the canWrite event to false
                        q.canWrite.Reset();

                        // Wait until the canWrite event is signaled 
                        q.canWrite.WaitOne();
                    }

                    // create a new node
                    node = new Node(x, node);
                    current = 1;
                }
                else
                {
                    // If the implementation is correct then the reader will never try to access this 
                    // array location while we set it. This is because of the invariant that 
                    // if reader and writer are at the same node: 
                    //    reader.current < node.count 
                    // and 
                    //    writer.current = node.count 
                    node.data[current++] = x;

                    // We have to use interlocked, to assure that we incremeent the count 
                    // atomicalluy, because the reader could be reading it.
                    Interlocked.Increment(ref node.count);
                }
            }

            /// Pulls data from the queue, returns false only if 
            /// there 
            public bool Read(ref T x)
            {
                while (true)
                {
                    if (current < node.count)
                    {
                        x = node.data[current++];
                        return true;
                    }
                    else if ((current == BUFFER_SIZE) && (node.next != null))
                    {
                        // Move the current node to the next one.
                        // We know it is safe to do so.
                        // The old node will have no more references to it it 
                        // and will be deleted by the garbage collector.
                        node = node.next;

                        // If there is a writer thread waiting on the Queue,
                        // then release it.
                        // Conceptually there is a "if (q.IsFull)", but we can't place it 
                        // because that would lead to a Race condition.
                        q.canWrite.Set();

                        // point to the first spot                
                        current = 0;

                        // One of the invariants is that every node created after the first,
                        // will have at least one item. So the following call is safe
                        x = node.data[current++];
                        return true;
                    }

                    // If we get here, we have read the most recently added data.
                    // We then check to see if the writer has finished producing data.
                    if (q.completed)
                        return false;

                    // If we get here there is no data waiting, and no flagging of the completed thread.
                    // Wait a millisecond. The system will also context switch. 
                    // This will allow the writing thread some additional resources to pump out 
                    // more data (especially if it iself is multithreaded)
                    Thread.Sleep(1);
                }
            }
        }

        /// Returns the number of nodes currently used.
        private int NodeCount
        {
            get
            {
                int result = 0;
                Node cur = null;
                Interlocked.Exchange<Node>(ref cur, remover.node);

                // Counts all nodes from the remover to the adder
                // Not efficient, but this is not called often. 
                while (cur != null)
                {
                    ++result;
                    Interlocked.Exchange<Node>(ref cur, cur.next);
                }
                return result;
            }
        }

        /// Construct the queue.
        public SimpleSharedQueue()
        {
            Node root = new Node();
            adder = new Cursor(this, root);
            remover = new Cursor(this, root);
        }

        /// Indicate to the reader that no more data is going to be written.
        public void MarkCompleted()
        {
            completed = true;
        }

        /// Read the next piece of data. Returns false if there is no more data. 
        public bool Read(ref T x)
        {
            return remover.Read(ref x);
        }

        /// Writes more data.
        public void Write(T x)
        {
            adder.Write(x);
        }

        /// Tells us if there are too many nodes, and can't add anymore.
        private bool IsFull()
        {
            return NodeCount == MAX_NODE_COUNT;  
        }
    }
}

Answer 1

Microsoft Research CHESS 应该被证明是测试您的实现的好工具。