.NET Multithreading: Lock object with log when locked for to much time

问题

In system we have methods that lock an object by specific params. As implementation, we have LockManager with Enter method that receives a key for a lock, check if lock object exists in internal dictionary, if not, it creates it and then locks.

What I want to do, is to set "X expected time" for specific lock, and if an object was locked for more that X time, I want to write a message to our log.

Below is a source code for my lock manager:

public class MyLockManager<T>
{
    protected Dictionary<T, object> LockDictionary { get; private set; }

    public MyLockManager()
    {
        LockDictionary = new Dictionary<T, object>();
    }

    /// <summary>
    /// Enters a lock on the key.
    /// </summary>
    /// <param name="key">The key to lock.</param>
    public virtual void Enter(T key)
    {
        if (!LockDictionary.ContainsKey(key))
        {
            lock (LockDictionary)
            {
                if (!LockDictionary.ContainsKey(key))
                {
                    LockDictionary.Add(key, new object());
                }
            }
        }

        object lockObj = LockDictionary[key];
        Monitor.Enter(lockObj);
    }

    /// <summary>
    /// Releases the lock on the key.
    /// </summary>
    /// <param name="key">The key to release.</param>
    public virtual void Exit(T key)
    {
        if (LockDictionary.ContainsKey(key))
        {
            Monitor.Exit(LockDictionary[key]);
        }
    }
}

Now I want to add an additional method, lets say LockTimoutHandler(T key) that will be called, if object for specific key was locked for more than X time.

In order to do it, I want to add some logic to "Enter" and "Exit" methods. When Enter called, something will be somehow registered to run the LockTimoutHandler in X time, and when "Exit" called, that something will be somehow unregistered.

My question is what I can use instead of that something? How I can schedule the method to run in X time and if Exit occurred before, so remove the schedule. It must be very fast, since performance is very important in our case. I know about Timer object...it can execute method in delayed manner, but is it's performance good enough? What additional options I have to achieve that?

NOTE: Just to be clear, I am not talking about TryEnter. I am not trying to catch the case when the object can not be locked for specific amount of time, I want to catch objects that already locked for too much time.

Thanks!

回答1:

We had a similar rquirement and solved it this way:

When locking, set a timer to the timeout passing it a state object, that contains the key and a delegate whatever you desire: Logging, force-unlock, ... whatever is needed for your use case
When the timer fires, check for the key, if entry exists call delegate
Important: recycle the timer (e.q. in a threadsafe queue), do not let it go out of scope.
When you next need a timer, take one out of the recycle queue and manipulate the state object - only create a new one if you need to.

This will keep as many timers around as necessary, but no more and will incur the cost of allocation/deallocation only once. Since you can't change the timer's state object, you need to change its contents.

回答2:

I think you can do it simpler. Timer is very light weight object and I wouldn't bare with trying to limit their count. All timers are running in a special thread from threadpool and they are really cheap.

Just create a dictionary of timers (if it will be used from different threads you may want to change it to ConcurrentDictionary:

var timers = new Dictionary<T, Timer>();

When adding item to your list use this code to setup timeout:

var timer = new Timer(o => LogMessage("key {0} is being held too long", key));
timer.Change(timeout, Timeout.Infinite);
timers.Add(key, timer);

Note that timer will be executed only once -- after specified timeout. And when item is released just remove timer from the dictionary:

Timer timer;
if (timers.TryGetValue(key, out timer))
{
    timer.Dispose();
    timers.Remove(key);
}

来源：https://stackoverflow.com/questions/14030491/net-multithreading-lock-object-with-log-when-locked-for-to-much-time

标签

.net

multithreading

locking