C#ReaderWriterLockSlim [操作系统入门]

Z时代
2024-01-10
分类：综合

ReaderWriterLockSlim-多线程读单线程写

注意事项

（1）线程A如果通过EnterReadLock获取到读锁（ReadLock）后，不能在持有读锁的情况下再申请获取写锁（WriteLock）

如果违反了这个规定就报如下错：
Write lock may not be acquired with read lock held. This pattern is prone to deadlocks. Please ensure that read locks are released before taking a write lock. If an upgrade is necessary, use an upgrade lock in place of the read lock.

（2）线程A如果通过EnterReadLock获取到读锁（ReadLock）后，不能在持有读锁的情况下再申请获取可升级读锁（UpgradeableReadLock）

如果违反了这个规定就报如下错：
Upgradeable lock may not be acquired with read lock held

（3）线程A如果通过EnterUpgradeableReadLock获取到可升级读锁（UpgradeableReadLockk）后，可以再持有可升级读锁的情况通过EnterWriteLock获取写锁

递归锁

递归锁：同一个线程获取到该锁后可以再获取该锁多次，不会产生死锁。

使用默认构造函数构造的ReaderWriterLockSlim是不允许锁递归的，如果一个线程已经获取到该锁后又获取该锁就会出错，如报错“Recursive upgradeable lock acquisitions not allowed in this mode”

示例如下：

解决方法：使用允许递归锁的构造方法（即new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion)）构造ReaderWriterLockSlim实例

如何判断代码逻辑里会触发递归锁呢，抓住两个要点“同一线程（即ManageThreadId一样）”和“同一个锁”，在线程池里运行的时候，由于可以重用工作线程，很容易触发递归锁的问题，比如这样：线程池线程1获取到UpgradeableReadLock锁后由于后面代码里有长时等待任务，调度器可能调度线程1运行队列里的另一个同样代码的任务，这时由于代码一样线程1再次申请获取UpgradeableReadLock锁（因为上一次他已经获取到该UpgradeableReadLock锁了），就会报错了

示例代码：

staticvoid Main()
        {
            ReaderWriterLockSlimTest();
            Console.Read();
        }
publicstaticvoid ReaderWriterLockSlimTest()
        {
            Console.WriteLine($"主线程Id : {Thread.CurrentThread.ManagedThreadId}");
// 定义共享资源
var list = new List<string>();
//  定义ReaderWriterLockSlim共享实例
//ReaderWriterLockSlim readerWriterLockSlim = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion); // 允许递归锁
            ReaderWriterLockSlim readerWriterLockSlim = new ReaderWriterLockSlim();
//TaskMethod0102("task1", 1, list, readerWriterLockSlim);
var task1 = Task.Run(() => TaskMethod0102("task1", 1, list, readerWriterLockSlim));
var task2 = Task.Run(() => TaskMethod0102("task2", 2, list, readerWriterLockSlim));
var task3 = Task.Run(() => TaskMethod0102("task3", 3, list, readerWriterLockSlim));
//var thread1 = new Thread(() => TaskMethod0102("task1", 1, list, readerWriterLockSlim));
//var thread2 = new Thread(() => TaskMethod0102("task2", 1, list, readerWriterLockSlim));
//var thread3 = new Thread(() => TaskMethod0102("task3", 1, list, readerWriterLockSlim));
        }
publicstaticstring TaskMethod0102(string taskName,double seconds,List<string>  list,ReaderWriterLockSlim readerWriterLockSlim)
        {
try
            {
                Console.WriteLine($"{taskName} want to Read  ,线程Id：{Thread.CurrentThread.ManagedThreadId}");
//readerWriterLockSlim.EnterReadLock(); // 尝试获取读锁
                readerWriterLockSlim.EnterUpgradeableReadLock(); // 尝试获取可升级读锁
                Console.WriteLine($"{taskName} success to Read ,集合总数：{(list == null ? 0 : list.Count)} ,线程Id：{Thread.CurrentThread.ManagedThreadId}");
                Console.WriteLine($"{taskName} want to Write ,线程Id：{Thread.CurrentThread.ManagedThreadId}");
                readerWriterLockSlim.EnterWriteLock();
                Console.WriteLine($"{taskName} success to Write ,线程Id：{Thread.CurrentThread.ManagedThreadId}");
                list.Add(taskName);
return"OK";
            }
catch(Exception ex)
            {
return"发生异常，异常信息：" + ex.Message;
            }
finally
            {
                Console.WriteLine($"{taskName } 释放所有的锁 ,线程Id：{Thread.CurrentThread.ManagedThreadId}");
                readerWriterLockSlim.ExitUpgradeableReadLock();
                readerWriterLockSlim.ExitWriteLock();
            }
        }