• 如何解决C#多线程编程中的资源竞争和线程死锁的问题?
  • 发布于 1个月前
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1. 资源竞争
当多个同时执行的线程需要同时对全局变量进行写读操作时,容易出现资源竞争的问题,导致运行结果出现多种情况。以下面的例子进行说明:
private static CancellationTokenSource cs = new CancellationTokenSource();
        private static int num = 5;
        private static object obj = new object();

        static void Main(string[] args)
        {
            Console.WriteLine("Main Start....");
            Task t1 = Task.Factory.StartNew(Test);
            Task t2 = Task.Factory.StartNew(Test);
            Task.WaitAll(new Task[] { t1, t2 });
            cs.Dispose();
            Console.WriteLine("Main end....");
            Console.ReadLine();
        }

        static void Test()
        {
            while (!cs.IsCancellationRequested)    //是否调用Cancel
            {
                Console.WriteLine("TaskId {0} Excute other code....num is {1}",Task.CurrentId,num);
                if(num == 5){
                    Thread.Sleep(50);        //为了方便观察,添加延时
                    num++;                      // 堆代码 duidaima.com
                    Console.WriteLine("TaskId {0} and Num is {1}", Task.CurrentId, num);
                    if (!cs.IsCancellationRequested)
                    {
                        cs.Cancel();       //取消操作
                    }
                }
            }
        }
 大多数情况,运行结果如下:
Main Start....
TaskId 1 Excute other code....num is 5
TaskId 2 Excute other code....num is 5
TaskId 2 and Num is 6
TaskId 1 and Num is 7
Main end....
任务1 首先运行至Thread.Sleep(50),等待50ms,cpu开始调度任务2运行至Thread.Sleep(50)。接着任务1 运行num++, 并往控制台输出结果num=6,然后任务2运行num++, 并往控制台输出num=7。但有时也会出现下面这种结果:
Main Start....
TaskId 1 Excute other code....num is 5
TaskId 2 Excute other code....num is 5
TaskId 2 and Num is 7
TaskId 1 and Num is 7
Main end....
任务1 首先运行至Thread.Sleep(50),等待50ms,cpu开始调度任务2运行至Thread.Sleep(50)。接着任务1 运行num++,cpu马上开始调度任务2运行num++,并往控制台输出num=7,最后调度任务1往控制台输出num=7。
 解决方式:只需加上线程锁lock, 便只会出现第一种运行结果,如下:
static void Test()
        {
            while (!cs.IsCancellationRequested)    //是否调用Cancel
            {
                Console.WriteLine("TaskId {0} Excute other code....num is {1}",Task.CurrentId,num);
                if(num == 5){
                    Thread.Sleep(50);        //为了方便观察,添加延时
                    lock (obj)               //只有一个线程可以操作
                    {
                        num++;                    
                        Console.WriteLine("TaskId {0} and Num is {1}", Task.CurrentId, num);
                    }
                    if (!cs.IsCancellationRequested)
                    {
                        cs.Cancel();       //取消操作
                    }
                }
            }
        }
 2.线程死锁
至少有2个线程被挂起,等待对方解锁,线程将无限等待下去。
private static int num = 5;
        private static object obj1 = new object();
        private static object obj2 = new object();

        static void Main(string[] args)
        {
            Console.WriteLine("Main Start....");
            Parallel.Invoke(LockTest1, LockTest2);
            Console.WriteLine("Main end....");
            Console.ReadLine();
        }

        static void LockTest1()
        {
            lock(obj1){
                lock(obj2){
                    Console.WriteLine("LockTest1 is running");
                }
            }
        }

        static void LockTest2()
        {
            lock (obj2)
            {
                lock (obj1)
                {
                    Console.WriteLine("LockTest2 is running");
                }
            }
        }
运行结果:
Main Start....
LockTest1 is running
LockTest2 is running
Main end....
看似正常,但这段程序在极少数的情况下,会出现死锁。例如CPU先运行LockTest1()中lock(obj1), 马上又运行LockTest2()中lock(obj2),这时LockTest1()会等待obj2 解锁,而LockTest2()会等待obj1解锁 ,形成死锁。

解决方式:在设计程序时,考虑好锁定的顺序,或者定义锁定超时。
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