.net 锁定手动重置事件时出现死锁
锁定manualResetEvent实例时遇到死锁。我想不出怎么解决它。我将感谢任何帮助 我在一个类中有两个由不同线程执行的方法:.net 锁定手动重置事件时出现死锁,.net,deadlock,manualresetevent,.net,Deadlock,Manualresetevent,锁定manualResetEvent实例时遇到死锁。我想不出怎么解决它。我将感谢任何帮助 我在一个类中有两个由不同线程执行的方法: private ManualResetEvent _event = new ManualResetEvent (true); private void process(){ ... lock(_event){ _event.WaitOne(); ... } } internal void Stop(){ _event.Reset(
private ManualResetEvent _event = new ManualResetEvent (true);
private void process(){
...
lock(_event){
_event.WaitOne();
...
}
}
internal void Stop(){
_event.Reset();
lock(_event){
...
}
}
第一个线程触发了锁,并在_event.WaitOne()中被阻止
socond线程执行了行_event.Reset();并在尝试执行锁定(\u事件)时被阻止
我认为当线程1在WaitOne上被阻塞时,锁应该被释放。我想我错了。我不知道该怎么修。
b、 t.w-我添加了锁,因为锁块中的代码应该在两个线程中同步
再次感谢并为这篇冗长的帖子感到抱歉。我猜您可能对Monitor.Wait(对象)和ManualResetEvent.WaitOne()感到困惑 监视。等待(对象)释放锁并等待,直到它获得锁。ManualResetEvent.WaitOne()阻塞当前线程,直到事件句柄收到信号 我还建议不要同时将ManualResetEvent对象用作锁。即使编译器不会生成错误,这也可能会像您现在可能遇到的那样造成混乱
1.你为什么会陷入僵局 首先,简短的回答是:您错过了Set的重置 我已经复制了您的代码(将大括号更改为我喜欢的样式),我将在注释中解释此问题:
private ManualResetEvent _event = new ManualResetEvent (true);
private void process()
{
//...
lock(_event)
{
_event.WaitOne(); //Thread A is here waiting _event to be set
//...
}
}
internal void Stop()
{
_event.Reset(); //But thread B just did reset _event
lock(_event) //And know thread B is here waiting... nobody is going to set _event
{
//...
}
}
清楚了这一部分,让我们继续讨论解决方案
2.解决僵局 由于我们将使用
.Set()
交换.Reset()
,因此我们还必须将手动重置事件的默认状态从true
更改为false
因此,要解决死锁,请按如下[tested]编辑代码:
private ManualResetEvent _event = new ManualResetEvent (false);
private void process()
{
//...
lock(_event)
{
_event.WaitOne(); //Thread A will be here waiting _event to be set
//...
}
}
internal void Stop()
{
_event.Set(); //And thread B will set it, so thread a can continue
lock(_event) //And when thread a releases the lock on _event thread b can enter
{
//...
}
}
private ManualResetEvent _event = new ManualResetEvent (false);
private ReaderWriterLockSlim _readWrite = new ReaderWriterLockSlim();
private void process()
{
//...
_readWrite.EnterReadLock();
_event.WaitOne();
try
{
//...
}
finally
{
_readWrite.ExitReadLock();
}
}
internal void Stop()
{
//there are three relevant thread positions at the process method:
//a) before _readWrite.EnterReadLock();
//b) before _event.WaitOne();
//c) after _readWrite.EnterReadLock();
_event.Set(); //Threads at position b start to advance
Thread.Sleep(1); //We want this thread to preempt now!
_event.Reset(); //And here we stop them
//Threads at positions a and b wait where they are
//We wait for any threads at position c
_readWrite.EnterWriteLock();
try
{
//...
}
finally
{
_readWrite.ExitWriteLock();
//Now the threads in position a continues...
// but are halted at position b
//Any thread in position b will wait until Stop is called again
}
}
上面的代码不仅强制只有一个线程可以同时进入锁,而且强制进入进程
的线程将等待,直到有一个线程调用停止
3.但是你有一个比赛条件。。。修好它。
工作没有完成,因为上面的代码患有种族疾病。要理解为什么,想象一下在多个线程调用进程
的情况下会发生什么。只有一个线程将进入锁,并等待调用Stop
并设置_事件,然后可以继续。现在,考虑如果调用停止的线程在调用<代码>事件> SETE()/代码>之后被抢先发生了什么,则在<代码>事件中的等待线程继续。现在,您无法判断等待进入进程
锁定的另一个线程是否将进入,或者在停止
中抢占的线程是否将继续并以该方法进入锁定。这是一个比赛条件,我不认为你想要那个特别的
也就是说,我为您提供了一个更好的解决方案(经过测试):
private ManualResetEvent _event = new ManualResetEvent (false);
private void process()
{
//...
lock(_event)
{
_event.WaitOne(); //Thread A will be here waiting _event to be set
//...
}
}
internal void Stop()
{
_event.Set(); //And thread B will set it, so thread a can continue
lock(_event) //And when thread a releases the lock on _event thread b can enter
{
//...
}
}
private ManualResetEvent _event = new ManualResetEvent (false);
private ReaderWriterLockSlim _readWrite = new ReaderWriterLockSlim();
private void process()
{
//...
_readWrite.EnterReadLock();
_event.WaitOne();
try
{
//...
}
finally
{
_readWrite.ExitReadLock();
}
}
internal void Stop()
{
//there are three relevant thread positions at the process method:
//a) before _readWrite.EnterReadLock();
//b) before _event.WaitOne();
//c) after _readWrite.EnterReadLock();
_event.Set(); //Threads at position b start to advance
Thread.Sleep(1); //We want this thread to preempt now!
_event.Reset(); //And here we stop them
//Threads at positions a and b wait where they are
//We wait for any threads at position c
_readWrite.EnterWriteLock();
try
{
//...
}
finally
{
_readWrite.ExitWriteLock();
//Now the threads in position a continues...
// but are halted at position b
//Any thread in position b will wait until Stop is called again
}
}
阅读代码中的注释以了解其工作原理。简单地说,它需要一个读写锁来允许多个线程进入方法进程
,但只允许一个线程进入停止
。尽管已经做了额外的工作,以确保调用方法进程
的线程将等待,直到有线程调用方法停止
4.现在你遇到了再入问题。。。修好它。
上面的解决方案更好。。。这并不意味着完美。怎么了?嗯,如果您递归地调用Stop,或者同时从两个不同的线程调用Stop,它将无法正常工作,因为第二个调用可能会在第一个调用执行时使线程进程提前。。。我想你不想这样。显然,使用读写锁足以防止多个线程调用方法Stop
,但事实并非如此
为了解决这个问题,我们需要确保Stop一次只执行一次。您可以使用lock执行此操作:
private ManualResetEvent _event = new ManualResetEvent (false);
private ReaderWriterLockSlim _readWrite = new ReaderWriterLockSlim();
//I'm going to use _syncroot, you can use any object...
// as long as you don't lock on it somewhere else
private object _syncroot = new object();
private void process()
{
//...
_readWrite.EnterReadLock();
_event.WaitOne();
try
{
//...
}
finally
{
_readWrite.ExitReadLock();
}
}
internal void Stop()
{
lock(_syncroot)
{
//there are three relevant thread positions at the process method:
//a) before _readWrite.EnterReadLock();
//b) before _event.WaitOne();
//c) after _readWrite.EnterReadLock();
_event.Set(); //Threads at position b start to advance
Thread.Sleep(1); //We want this thread to preempt now!
_event.Reset(); //And here we stop them
//Threads at positions a and b wait where they are
//We wait for any threads at position c
_readWrite.EnterWriteLock();
try
{
//...
}
finally
{
_readWrite.ExitWriteLock();
//Now the threads in position a continues...
// but are halted at position b
//Any thread in position b will wait until Stop is called again
}
}
}
为什么我们需要读写锁您可能会问-如果我们使用lock来确保只有一个线程进入方法Stop
因为读写锁还允许方法Stop
处的线程停止调用方法process
的新线程,同时允许已经存在的线程执行并等待它们完成
为什么我们需要ManualResetEvent
?-您可能会问-如果我们已经有读写锁来控制方法进程中线程的执行
因为在调用方法Stop
之前,读写锁无法阻止方法进程
中的代码执行
所以,你知道我们需要这些。。。还是我们
好吧,这取决于你有什么样的行为,所以如果我解决了一个和你不一样的问题,我会在下面提供一些替代解决方案
5.具有替代行为的替代解决方案
锁是很容易理解,但它是有点太多了我的口味。。。特别是,如果不需要确保每个对Stop的并发调用都有机会允许线程在方法进程中执行
如果是这种情况,则可以按如下方式重写代码:
private ManualResetEvent _event = new ManualResetEvent (false);
private ReaderWriterLockSlim _readWrite = new ReaderWriterLockSlim();
private int _stopGuard;
private void process()
{
//...
_readWrite.EnterReadLock();
_event.WaitOne();
try
{
//...
}
finally
{
_readWrite.ExitReadLock();
}
}
internal void Stop()
{
if(Interlocked.CompareExchange(_stopGuard, 1, 0) == 0)
{
//there are three relevant thread positions at the process method:
//a) before _readWrite.EnterReadLock();
//b) before _event.WaitOne();
//c) after _readWrite.EnterReadLock();
_event.Set(); //Threads at position b start to advance
Thread.Sleep(1); //We want this thread to preempt now!
_event.Reset(); //And here we stop them
//Threads at positions a and b wait where they are
//We wait for any threads at position c
_readWrite.EnterWriteLock();
try
{
//...
}
finally
{
_readWrite.ExitWriteLock();
//Now the threads in position a continues...
// but are halted at position b
//Any thread in position b will wait until Stop is called again
}
}
}
还没有正确的行为?好的,我们再看一个
6.具有替代行为的替代解决方案。。。再一次
这次我们将看到如何允许多线程进入方法进程
,甚至在调用方法停止
之前
private ReaderWriterLockSlim _readWrite = new ReaderWriterLockSlim();
private int _stopGuard;
private void process()
{
//...
_readWrite.EnterReadLock();
try
{
//...
}
finally
{
_readWrite.ExitReadLock();
}
}
internal void Stop()
{
if(Interlocked.CompareExchange(_stopGuard, 1, 0) == 0)
{
//there are two relevant thread positions at the process method:
//a) before _readWrite.EnterReadLock();
//b) after _readWrite.EnterReadLock();
//We wait for any threads at position b
_readWrite.EnterWriteLock();
try
{
//...
}
finally
{
_readWrite.ExitWriteLock();
//Now the threads in position a continues...
// and they will continue until halted when Stop is called again
}
}
}
不是你想要的吗
好吧,我放弃。。。让我们回到基本点
7.还有你已经知道的
…为了完整性,如果您只需要确保两个方法的访问是同步的,并且您可以允许进程中的方法在任何时候运行,那么您只需使用锁就可以做到这一点。。。你已经知道了