Checking whether the current thread owns a lock

Question

Suppose I have the following code:

public class SomeClass()
{
    private readonly object _lock = new object();

    public void SomeMethodA()
    {
        lock (_lock)
        {
            SomeHelperMethod();

            //do something that requires lock on _lock
        }
    }

    public void SomeMethodB()
    {
        lock (_lock)
        {
            SomeHelperMethod();

            //do something that requires lock on _lock
        }
    }

    private void SomeHelperMethod()
    {
        lock (_lock)
        {
            //do something that requires lock on _lock
        }
    }
}

Locking inside SomeHelperMethod seems redundant and wasteful, since all callers have already taken a lock. However, simply removing the lock from SomeHelperMethod seems dangerous since we might later refactor the code and forget to lock the _lock object prior to calling SomeHelperMethod.

Ideally I could work around this by asserting that the current thread owns a lock on _lock inside SomeHelperMethod:

private void SomeHelperMethod()
{
    Debug.Assert(Monitor.HasLock(_lock));

    //do something that requires lock on _lock
}

But there does not appear to be such a method. Monitor.TryEnter does not help because locks are re-entrant. Therefore, if the current thread already owns the lock, TryEnter will still succeed and return true. The only time it will fail is if another thread owns the lock and the call times out.

So, does such a method exist? If not, why? It does not seem dangerous at all to me, since it merely tells you whether the current thread (not another thread) owns a lock or not.

score 12 · Accepted Answer · edited Aug 08 '19 at 05:29

12

This is supported in .NET 4.5 using Monitor.IsEntered(object).

edited Aug 08 '19 at 05:29

double-beep

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answered Aug 20 '13 at 07:19

Dave

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Awesome - thanks! And judging by the API docs, it was added for exactly the purpose I originally wanted this for: "*Use this method with diagnostic tools, such as the `Assert` method and the `Contract` class, to debug locking issues that involve the `Monitor` class.*" – Kent Boogaart Aug 20 '13 at 07:45

score 5 · Answer 2 · answered Feb 02 '09 at 11:18

5

Locking or re-locking are effectively free. The downside of this low cost is that you don't have as many features as the other synchronisation mechanisms. I would simply lock whenever a lock is vital, as you have above.

If you desparately want to omit 'unnecessary' locks without the risk involved with potential refactoring, add comments. If someone changes your code and it breaks, there is a comment there that explains why. If they still can't figure it out, that's their problem, not yours. A further alternative is to create a class for use as a locking object that contains a boolean you can flick on and off. However, the overheads introduced by doing this (including try/finally blocks, which are not free), are probably not worth it.

answered Feb 02 '09 at 11:18

Zooba

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Essentially you're saying what I suspected: there is no way to do this. Whilst not an ideal solution, this is the best answer. – Kent Boogaart Aug 23 '09 at 14:46
Update: but see Dave's answer which points out that this is now available in .NET 4.5 – Kent Boogaart Aug 20 '13 at 07:46
Re-locking should be almost free. Locking for sure isn't. It's cheap, but it isn't free. It requires at least one interlocked operation for the lock part and another for the unlock part -- assuming that there's no contention. Depending on the CPU this could mean about 100 cycles up to at least 10 times that on older CPUs. Compared to the typical call of a simple function call that's not a huge number, but I don't think one could call it "effectively free" ;) – Paul Groke Jul 03 '14 at 11:11

Hans Passant · Answer 3 · 2012-07-19T15:19:38.377

3

There's a fundamental problem with your request. Suppose there were a IsLockHeld() method and you'd use it like this:

if (not IsLockHeld(someLockObj)) then DoSomething()

This cannot work by design. Your thread could be pre-empted between the if() statement and the method call. Allowing another thread to run and lock the object. Now DoSomething() will run, even though the lock is held.

The only way to avoid this is to try to take the lock and see if it worked. Monitor.TryEnter().

You see the exact same pattern at work in Windows. There is no way to check if a file is locked by another process, other than trying to open the file. For the exact same reason.

edited Jul 19 '12 at 15:19

answered Jan 30 '09 at 19:08

Hans Passant

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6

But that's not how I'm using it. I'm using it as an assertion - as a way of knowing that my code is buggy. If the lock isn't held, there's a bug. I'm not using it to choose one code path over another. – Kent Boogaart Jan 30 '09 at 20:08
1

Yes, you're right. You'll need a wrapper for Monitor.Enter, that's ugly. – Hans Passant Feb 02 '09 at 18:55

score 2 · Answer 4 · answered Jun 07 '11 at 21:02

But there does not appear to be such a method. Monitor.TryEnter does not help because locks are re-entrant. Therefore, if the current thread already owns the lock, TryEnter will still succeed and return true. The only time it will fail is if another thread owns the lock and the call times out.

I found a workaround that works similar to Monitor.TryEnter() and yet doesn't have the re-entrant issues. Basically instead of using Monitor.TryEnter(), you can use Monitor.Pulse(). This function throws a SynchronizationLockException exception if the current thread doesn't hold the lock.

example:

try
{
    System.Threading.Monitor.Pulse(lockObj);
}
catch (System.Threading.SynchronizationLockException /*ex*/)
{
    // This lock is unlocked! run and hide ;)
}

Docs: http://msdn.microsoft.com/en-us/library/system.threading.monitor.pulse.aspx

Nice one, although this may have side effects if you (or someone) are actually using the `Wait()`/`Pulse()` mechanism for communication. — Sebastian Krysmanski, Mar 13 '13 at 12:29

score 1 · Answer 5 · answered Jan 30 '09 at 17:56

OK, I now understand your problem. Conceptually I am now thinking of a Semaphore object. You can easily check the semaphore value (a value greater or equal than 0 means the resource is available). Also, this does not actually lock the resource that being done by incrementing or decrementing the semaphore value. The Semaphore object is present in the .NET framework but i did not use it so I cannot provide a proper example. If necessary I can build a simple example and post it here. Let me know.

Alex.

L.E. I am unsure now if you actually have control to the synchronization mechanisms in the application. If you only have access to an object that could be locked or not, my solution could prove (once again) of no use.

The Semaphore APIs do not enable this scenario. Moreover, this is far less efficient than using a simple monitor. — Kent Boogaart, Jan 30 '09 at 20:19

csharpstudent · Answer 6 · 2012-07-01T01:16:42.287

I had similiar requirements of checking whether a lock is locked by the current thread, so I can do something like this.

public void Method1()
{
    if(!lockHeld()) lock();
    //DO something
}
public void Method2()
{
    if(!lockHeld()) lock();
    //DO something
}
public void Method3()
{
    if(!lockHeld()) lock();

    //Do something

    unlock()
}

So I wrote myself a class for that:

public class LockObject
{
    private Object internalLock;

    private bool isLocked;
    public bool IsLocked
    {
        get { lock (internalLock) return isLocked; }
        private set { lock (internalLock) isLocked = value; }
    }

    public LockObject()
    {
        internalLock = new object();
    }

    public void UsingLock(Action method)
    {
        try
        {
            Monitor.Enter(this);
            this.IsLocked = true;

            method();
        }
        finally
        {
            this.IsLocked = false;
            Monitor.Exit(this);
        }
    }
}

Then I can use it as:

    public void Example()
    {
        var obj = new LockObject();

        obj.UsingLock(() =>
        {
            //Locked, and obj.IsLocked = true
        });
    }

Note: I ommitted the Lock() Unlock() Methods on this class, but pretty much just wrappers to Monitor.Enter/Exit that set IsLocked to true. The example just illustrates that its very similar to lock(){ } in terms on style.

Might be unnecessary, but this is useful for me.

score 0 · Answer 7 · edited May 23 '17 at 10:30

If you need this under .NET 4.5 see the accepted answer from @Dave .

However, if you need this under .NET 3.5 or 4, you could replace the use of Monitor locks for a ReaderWriterLockSlim as follows.

According to Joeseph Albahari this will approximately double your (uncontested) locking overhead, but it is still really fast (est. 40nS). (Disclaimer, that page does not state if the tests were done using a recursive lock policy.)

public class SomeClass()
{
private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);

public void SomeMethodA()
{
    _lock.EnterWriteLock();
    try
    {
        SomeHelperMethod();

        //do something that requires lock on _lock
    }
    finally
    {
       _lock.ExitWriteLock();
    }
}

private void SomeHelperMethod()
{
    Debug.Assert(_lock.IsWriteLockHeld);

    //do something that requires lock on _lock
}
}

If you feel that is too non-performant you can of course write your own wrapper class around Monitor locks. However, other posts have mentioned a boolean flag which is misleading as the question is not "Is the lock held by any thread?" (which is a foolish and dangerous question to ask). The correct question is "Is the lock held by THIS thread?". No problem though, just make the 'flag' a ThreadLocal counter(for recursive lock support) and ensure updates are done after Monitor.Enter and before Monitor.Exit. As an improvement assert that the counter does not drop below 0 on "unlock" as that would indicate unbalanced Enter/Exit calls.

Another reason for the wrapper would be to avoid having someone slip in _lock.EnterReadLock calls. Depending on the usage of the class those might actually be helpful, but a surprising number of coders do not understand Reader-Writer Locks.

score 0 · Answer 8 · answered Jan 30 '09 at 18:29

0

I wish that was possible, that would make my code much, much cleaner.

answered Jan 30 '09 at 18:29

Jonathan Allen

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score 0 · Answer 9 · answered Feb 03 '09 at 08:42

I have no experience whatsoever with .NET programming, but in my own code (Delphi) I have once implemented such assertions by having custom Acquire() and Release() methods in a critical section class, with a private member variable for the id of the thread having acquired the cs. After EnterCriticalSection() the return value of GetCurrentThreadID was written, and before LeaveCriticalSection() the field value was reset.

But I don't know whether .NET allows a similar thing, or whether you could implement this in your program...

score -1 · Answer 10 · answered Jan 30 '09 at 19:32

-1

Not the best thing to do but ...

bool OwnsLock(object someLockObj)
{
  if (Monitor.TryEnter(someLockObj))
  {
    Monitor.Exit();
    return false;
  }
  return true;
}

Debug.Assert(OwnsLock(_someLockObject), "_someLockObject should be locked when this method is called")

answered Jan 30 '09 at 19:32

Jorge Córdoba

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Locking is re-entrant so this code does not work. The monitor will successfully take a lock even if the calling code already has the lock. – Kent Boogaart Jan 30 '09 at 20:11

Checking whether the current thread owns a lock

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