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++;
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。