C#线程问题为什么无限循环在这里？

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();
        }
    }
}

如果锁只是锁定计数器++ lock(_lock){Counter ++; 我没有无限循环,但如果锁在示例中,则运行无限循环

Answer 1

可能是当Counter到达19时,两个线程都进入循环并且在它们再次测试值之前最终增加到21.

你需要在读取值时按住锁定Counter.双重检查Counter可能就足够了(while在握住锁的同时在循环内重新读取它).但是,我不确定这一点,因为我的头脑无法跟踪本机,.NET,Java等各种线程内存模型的所有细节.即使在.NET上,ECMA模型显然也不同于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).有关为什么重复检查可能会或可能不起作用的详细信息,请搜索"双重检查锁定" - 显然应该很简单的事情背后有很多复杂性.

例如,这是我机器上运行的片段:

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 ...

t2一旦Counter达到20,你会注意到停止,但是t1没有注意到.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).