C#线程问题为什么这里无限循环？

Question

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

namespace ThreadExample
{
    public class Info
    {
        public int Counter;


        private static object _lock = new object();
        private List<Thread> ThreadList;

        public Info(int counter)
        {
            Counter = counter;

            ThreadList = new List<Thread>();

            ThreadList.Add(new Thread(ThreadBody));
            ThreadList.Add(new Thread(ThreadBody));

           ThreadList[0].Name = "t1";
           ThreadList[1].Name = "t2";

        }

        public void Start()
        {
            ThreadList.ForEach(t => t.Start(t.Name));
        }

        public void ThreadBody(object name)
        {
            while (Counter != 20)
            {
                lock (_lock)
                {
                    Counter++;

                    Console.WriteLine("Thread {0} : the value of the counter is {1}", name.ToString(), Counter);
                }
            }



        }

    }
}


using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ThreadExample
{
    class Program
    {
        static void Main(string[] args)
        {
            Info info = new Info(0);
            info.Start();
        }
    }
}

如果锁只是锁计数器++ 锁(_锁) { 计数器++； } 我没有无限循环，但如果锁像示例中那样，它会运行无限循环

Answer 1

可能是当 Counter 达到 19 时，两个线程都进入了循环，并且在它们再次测试该值之前它最终增加到 21。

您需要在读取 Counter 的值时持有锁。双重检查 Counter 可能就足够了(在 while 循环中再次读取它，同时持有锁)。但是，我对此不确定，因为我的头脑无法跟踪 native 、.NET、Java 等之间的各种线程内存模型的所有细节。即使在 .NET 上，ECMA 模型也明显不同于 MS 对其 CLR 的保证(参见 http://msdn.microsoft.com/en-us/magazine/cc163715.aspx 和 http://www.bluebytesoftware.com/blog/PermaLink,guid,543d89ad-8d57-4a51-b7c9-a821e3992bf6.aspx)。有关为什么双重检查可能有效或无效的更多详细信息，请搜索“双重检查锁定”- 显然应该简单的事情背后有很多复杂性。

例如，这是在我的机器上运行的片段:

Thread t1 : the value of the counter is 1
Thread t2 : the value of the counter is 2
Thread t2 : the value of the counter is 3
Thread t2 : the value of the counter is 4
Thread t2 : the value of the counter is 5
Thread t2 : the value of the counter is 6
Thread t2 : the value of the counter is 7
Thread t2 : the value of the counter is 8
Thread t2 : the value of the counter is 9
Thread t2 : the value of the counter is 10
Thread t2 : the value of the counter is 11
Thread t2 : the value of the counter is 12
Thread t2 : the value of the counter is 13
Thread t2 : the value of the counter is 14
Thread t2 : the value of the counter is 15
Thread t2 : the value of the counter is 16
Thread t2 : the value of the counter is 17
Thread t2 : the value of the counter is 18
Thread t2 : the value of the counter is 19
Thread t2 : the value of the counter is 20
Thread t1 : the value of the counter is 21
Thread t1 : the value of the counter is 22

... Thread t1 never stops ...

您会注意到 t2 在 Counter 达到 20 后停止，但 t1 没有注意到这一点。它已经进入循环(或决定进入循环)认为 Counter 是 1(或者可能是 2 或其他东西 - 只是不是 20)。