我ReaderWriterLockSlim
用来保护一些操作。我想偏爱读者而不是作者,这样当读者长时间持有锁并且作者试图获取写锁时,进一步的读者不会被作者的尝试阻塞(如果作者被屏蔽了lock.EnterWriteLock()
)。
为此,我认为作者可以TryEnterWriteLock
在循环中使用短暂的超时,以便后续读者仍然能够获得读锁,而作者不能。然而,令我惊讶的是,我发现一个不成功的调用TryEnterWriteLock
改变了锁的状态,无论如何都会阻塞未来的读者。概念验证代码:
System.Threading.ReaderWriterLockSlim myLock = new System.Threading.ReaderWriterLockSlim(System.Threading.LockRecursionPolicy.NoRecursion);
System.Threading.Thread t1 = new System.Threading.Thread(() =>
{
Console.WriteLine("T1:{0}: entering read lock...", DateTime.Now);
myLock.EnterReadLock();
Console.WriteLine("T1:{0}: ...entered read lock.", DateTime.Now);
System.Threading.Thread.Sleep(10000);
});
System.Threading.Thread t2 = new System.Threading.Thread(() =>
{
System.Threading.Thread.Sleep(1000);
while (true)
{
Console.WriteLine("T2:{0}: try-entering write lock...", DateTime.Now);
bool result = myLock.TryEnterWriteLock(TimeSpan.FromMilliseconds(1500));
Console.WriteLine("T2:{0}: ...try-entered write lock, result={1}.", DateTime.Now, result);
if (result)
{
// Got it!
break;
}
System.Threading.Thread.Yield();
}
System.Threading.Thread.Sleep(9000);
});
System.Threading.Thread t3 = new System.Threading.Thread(() =>
{
System.Threading.Thread.Sleep(2000);
Console.WriteLine("T3:{0}: entering read lock...", DateTime.Now);
myLock.EnterReadLock();
Console.WriteLine("T3:{0}: ...entered read lock!!!!!!!!!!!!!!!!!!!", DateTime.Now);
System.Threading.Thread.Sleep(8000);
});
这段代码的输出是:
T1:18-09-2015 16:29:49: entering read lock...
T1:18-09-2015 16:29:49: ...entered read lock.
T2:18-09-2015 16:29:50: try-entering write lock...
T3:18-09-2015 16:29:51: entering read lock...
T2:18-09-2015 16:29:51: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:51: try-entering write lock...
T2:18-09-2015 16:29:53: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:53: try-entering write lock...
T2:18-09-2015 16:29:54: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:54: try-entering write lock...
T2:18-09-2015 16:29:56: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:56: try-entering write lock...
T2:18-09-2015 16:29:57: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:57: try-entering write lock...
T2:18-09-2015 16:29:59: ...try-entered write lock, result=False.
T2:18-09-2015 16:29:59: try-entering write lock...
如您所见,即使线程 2(“写入器”)尚未获得写入器锁并且它不在EnterWriteLock
调用中,线程 3 也会被永久阻塞。我可以看到类似的行为ReaderWriterLock
。
我做错什么了吗?如果不是,当作家排队时,我必须有哪些选择来支持读者?