I have a .NET 4 WCF service that maintains a thread-safe, in-memory, dictionary cache of objects (SynchronizedObject). I want to provide safe, concurrent access to read and modify both the collection and the objects in the collection. Safely modifying the objects and the cache can be accomplished with reader-writer locks.
I am running into trouble providing read access to an object in the cache. My Read method returns a SynchronizedObject, but I do not know how to elegantly ensure no other threads are modifying the object while WCF is serializing the SynchronizedObject.
I have tried placing the Read return clause inside the read-lock and setting a breakpoint in a custom XmlObjectSerializer. When the XmlObjectSerializer::WriteObject(Stream,object) method is called, a read-lock is not held on the SynchronizedObject.
I am specifically concerned with the following scenario:
Thread A calls Read(int). Execution continues until just after the return statement. By this point, the finally has also been executed, and the read lock on the SynchronizedObject has been released. Thread A's execution is interrupted.
Thread B calls Modify(int) for the same id. The write lock is available and obtained. Sometime between obtaining the write lock and releasing it, Thread B is interrupted.
Thread A restarts and serialization continues. Thread B has a write-lock on the same SynchronizedObject, and is in the middle of some critical section, but Thread A is reading the state of the SynchronizedObject and thus returns a potentially invalid object to the caller of Read(int).
I see two options:
Maintain a custom XmlObjectSerializer that grabs the read-lock before calling the base.WriteObject(Stream, object) method, and releases it after. I do not like this option because sub-classing and overriding a framework serialization function to perform a certain action if a the object to be serialized matches a certain type smells to me.
Create a deep-copy of a SynchronizedObject in the Read method while the read-lock is held, release the lock, and return the deep copy. I do not like this option because there will be many sub-classes of SynchronizedObject that I would have to implement and maintain correct deep-copiers for and deep-copies could be expensive.
What other options do I have? How should I implement the thread-safe Read method?
I have provided a dummy Service below for more explicit references:
public class Service : IService
{
IDictionary<int, SynchronizedObject> collection = new Dictionary<int, SynchronizedObject>();
ReaderWriterLockSlim rwLock = new ReaderWriterLockSlim();
public SynchronizedObject Read(int id)
{
rwLock.EnterReadLock();
try
{
SynchronizedObject result = collection[id];
result.rwLock.EnterReadLock();
try
{
return result;
}
finally
{
result.rwLock.ExitReadLock();
}
}
finally
{
rwLock.ExitReadLock();
}
}
public void ModifyObject(int id)
{
rwLock.EnterReadLock();
try
{
SynchronizedObject obj = collection[id];
obj.rwLock.EnterWriteLock();
try
{
// modify obj
}
finally
{
obj.rwLock.ExitWriteLock();
}
}
finally
{
rwLock.ExitReadLock();
}
}
public void ModifyCollection(int id)
{
rwLock.EnterWriteLock();
try
{
// modify collection
}
finally
{
rwLock.ExitWriteLock();
}
}
}
public class SynchronizedObject
{
public ReaderWriterLockSlim rwLock { get; private set; }
public SynchronizedObject()
{
rwLock = new ReaderWriterLockSlim();
}
}
New answer
Based on your new information and clearer scenario, I believe you want to use something similar to functional programming's immutability feature. Instead of serializing the object that could be changed, make a copy that no other thread could possibly access, then serialize that.
Previous (not valuable) answer
From http://msdn.microsoft.com/en-us/library/system.threading.readerwriterlockslim.enterwritelock.aspx:
If other threads have entered the lock
in read mode, a thread that calls the
EnterWriteLock method blocks until
those threads have exited read mode.
When there are threads waiting to
enter write mode, additional threads
that try to enter read mode or
upgradeable mode block until all the
threads waiting to enter write mode
have either timed out or entered write
mode and then exited from it.
So, all you need to do is call EnterWriteLock and ExitWriteLock inside ModifyObject(). Your attempt to make sure you have both a read and a write lock is actually stopping the code from working.
Related
I'm working with a class that manages a game's content and I'd like to know how I'm able to block calls to the dispose method without locking onto a shared dispose-lock? The worker methods themselves are thread-safe and can be executed in parallel, and that's why I don't want to lock those methods with a shared lock the dispose-method tries to acquire when it's fired.
public void DoSomething()
{
lock (method1Lock) { /* ... */ }
}
public void Dispose()
{
lock (method1Lock)
lock (method2Lock)
lock (methodXLock)
{
}
}
is not appropriate in this case as, as I previously mentioned, the methods (DoSomething in this case) can run in parallel.
Use a ReaderWriterLock. Let the Method... code take the read side and the Dispose the write side (which is exclusive).
I have a simple logger with producer consumer pattern based on BlockingCollection (code is below).
public class Logger
{
public Logger()
{
_messages = new BlockingCollection<LogMessage>(int.MaxValue);
_worker = new Thread(Work) {IsBackground = true};
_worker.Start();
}
~Logger()
{
_messages.CompleteAdding();
_worker.Join(); // Wait for the consumer's thread to finish.
//Some logic on closing log file
}
/// <summary>
/// This is message consumer thread
/// </summary>
private void Work()
{
while (!_messages.IsCompleted)
{
//Try to get data from queue
LogMessage message;
try
{
message = _messages.Take();
}
catch (ObjectDisposedException) { break; } //The BlockingCollection(Of T) has been disposed.
catch(InvalidOperationException){ continue; } //the BlockingCollection(Of T) is empty and the collection has been marked as complete for adding.
//... some simple logic to write 'message'
}
}
}
The problem is that application is not ending instantly with that. It takes 20-40 seconds to end an application and if I pause it with debugger in a middle, I see that:
1. GC.Finalize thread is set on _worker.Join();
2. _worker thread is on _messages.Take().
I would await that _messages.Take() is ended short after _messages.CompleteAdding(); But looks like it is not.
What's wrong with this finalization and how to better finalize worker thread in this situation?
P.S. I could simply drop _worker.Join() but then Work() can write something to closed file. I mean, this is concurrent non determined situation then.
Update
As a proof of concept I've renamed ~Logger() to Close() and call it at some point. It closes logger instantly. So _messages.Take() is ending right after _messages.CompleteAdding() as expected in this case.
The only explanation of the 20-40 seconds delay in ~Logger I see in high priority of the GC thread. Could there be another explanation?
In C#, Finalizers (aka destructors) are non-deterministic, which means you cannot predict when they will be called or in what order. For example in your code, it's entirely possible for the finalizer of _worker to be before after the finalizer for Logger. For this reason, you should never access managed objects (such as FileStreams etc) inside a finalizer, because the finalizers of other managed resources could have already completed, making their references invalid.
Also the finalizer will not be called until after the GC determines that a collection is necessary (due to the need for additional memory). In your case, the GC probably takes 20-40 seconds before it makes the required collection(s).
What you want to do is get rid of the finalizer and use the IDisposable interface instead (optionally with a Close() method that might provide better readability).
Then you would just call logger.Close() when it is no longer required.
void IDisposable.Dispose()
{
Close();
}
void Close()
{
_messages.CompleteAdding();
_worker.Join(); // Wait for the consumer's thread to finish.
//Some logic on closing log file
}
In general, only use a finalizer when you have unmanaged resources to clean up (for example, if you are using P/Invoke WinAPI function calls etc). If you are using only .Net classes, etc. you probably do not have any reason to use one. IDisposable is almost always the better choice, because it provides deterministic cleanup.
For more information on finalizers vs destructors, take a look here:
What is the difference between using IDisposable vs a destructor in C#?
Another change I would make in your code is using TryTake instead of Take. This gets rid of the need for the try/catch because it will not throw an exception when the collection is empty and CompleteAdding is called. It will simply return false.
private void Work()
{
//Try to get data from queue
LogMessage message;
while (_messages.TryTake(out message, Timeout.Infinite))
//... some simple logic to write 'message'
}
The two exceptions you catch in your code can still occur for other reasons such as accessing it after it is disposed or modifying the BlockingCollection's underlying collection (see MSDN for more info). But neither of those should occur in your code, because you don't hold a reference to the underlying collection, and you don't dispose of the BlockingCollection before the Work function is complete.
If you still wanted to catch those exceptions, just in case, you can place a try/catch block outside of the while loop (because you would NOT want to continue the loop after either exception occurs).
Finally, why do you specify int.MaxValue as the collection's capacity? You shouldn't do this unless you expect to routinely add close to that many messages to the collection.
So altogether, I would re-write your code as follows:
public class Logger : IDisposable
{
private BlockingCollection<LogMessage> _messages = null;
private Thread _worker = null;
private bool _started = false;
public void Start()
{
if (_started) return;
//Some logic to open log file
OpenLogFile();
_messages = new BlockingCollection<LogMessage>(); //int.MaxValue is the default upper-bound
_worker = new Thread(Work) { IsBackground = true };
_worker.Start();
_started = true;
}
public void Stop()
{
if (!_started) return;
// prohibit adding new messages to the queue,
// and cause TryTake to return false when the queue becomes empty.
_messages.CompleteAdding();
// Wait for the consumer's thread to finish.
_worker.Join();
//Dispose managed resources
_worker.Dispose();
_messages.Dispose();
//Some logic to close log file
CloseLogFile();
_started = false;
}
/// <summary>
/// Implements IDiposable
/// In this case, it is simply an alias for Stop()
/// </summary>
void IDisposable.Dispose()
{
Stop();
}
/// <summary>
/// This is message consumer thread
/// </summary>
private void Work()
{
LogMessage message;
//Try to get data from queue
while(_messages.TryTake(out message, Timeout.Infinite))
WriteLogMessage(message); //... some simple logic to write 'message'
}
}
As you can see, I added Start() and Stop() methods to enable/disable queue processing. If you want, you can call Start() from your constructor, but in general, you probably don't want expensive operations (such as thread creation) in a constructor. I used Start/Stop instead of Open/Close, because it seemed to make more sense for a logger, but that's just a personal preference, and either pair would work fine. As I mentioned before, you don't even have to use a Stop or Close method. Simply adding Dispose() is enough, but some classes (like Streams etc) use Close or Stop as an alias for Dispose just to make the code more readable.
I have the following code that I want to achieve the following with.
Check if a value is in cache
If in cache, get the value from it and proceed
If not in cache, perform the logic to enter it in cache but do this async as the operation to do such may take a long period of time and I dont want to hold up the user
As you will see in my code I place a lock on the cache in the async thread. Is my setup below thread safe? And by placing the lock will this mean that the cache will not be accessible for other threads to read from cache while the async operation takes place. I do not want a circumstance where the cache is locked in an async thread preventing other requests from accessing it.
There is also a chance that the same request may be called by several threads hence the lock.
Any recommendations as how I could improve the code would be great.
// Check if the value is in cache
if (!this.Cache.Contains(key))
{
// Perform processing of files async in another thread so rendering is not slowed down
ThreadPool.QueueUserWorkItem(delegate
{
lock (this.Cache)
{
if (!this.Cache.Contains(key))
{
// Perform the operation to get value for cache here
var cacheValue = operation();
this.Cache.Add(key, cacheValue);
}
}
});
return "local value";
}
else
{
// Return the string from cache as they are present there
return this.Cache.GetFilename(key);
}
Note: this.Cache represents a cache object.
The application is a web application on .net 3.5.
How about changing the delegate to look like this:
var cacheValue = operation();
lock (this.Cache)
{
if (!this.Cache.Contains(key))
{
// Perform the operation to get value for cache here
this.Cache.Add(key, cacheValue);
}
}
This kind of coding locks the dictionary for a very short time. You can also try using ConcurrentDictionary that mostly doesn't to any locking at all.
Alex.
There are several problems with your code. Problems include: calling Cache.Contains outside a lock while other threads may be modifying the collection; invoking operation within a lock which may cause deadlocks; etc.
Here's a thread-safe implementation of a cache that satisfies all your requirements:
class Cache<TKey, TValue>
{
private readonly ConcurrentDictionary<TKey, Task<TValue>> items;
public Cache()
{
this.items = new ConcurrentDictionary<TKey, Task<TValue>>();
}
public Task<TValue> GetAsync(TKey key, Func<TKey, TValue> valueFactory)
{
return this.items.GetOrAdd(key,
k => Task.Factory.StartNew<TValue>(() => valueFactory(k)));
}
}
The GetAsync method works as follows: First it checks if there is a Task in the items dictionary for the given key. If there is no such Task, it runs valueFactory asynchronously on the ThreadPool and stores the Task object that represents the pending asynchronous operation in the dictionary. Code calling GetAsync can wait for the Task to finish, which will return the value calculated by valueFactory. This all happens in an asynchronous, non-blocking, thread-safe manner.
Example usage:
var cache = new Cache<string, int>();
Task<int> task = cache.GetAsync("Hello World", s => s.Length);
// ... do something else ...
task.Wait();
Console.WriteLine(task.Result);
Looks like a standard solution, except for the retrieval in the background thread. It will be thread safe as long as all other bits of the code that use the cache also take out a lock on the same cache reference before modifying it.
From your code, other threads will still be able to read from the cache (or write to it if they don't take out a lock(). The code will only block at the point a lock() statement is encountered.
Does the return "local value" make sense? Would you not need to retrieve the item in that function anyway in the case of a cache miss?
I have two working threads.I have locked both with a same lock, but threadB is getting executed before threadA, so exception came.I locked both using the same lock object.Thread B is using delegate function.How can I solve the issue.
Detailed Information:
I have a class called StateSimulation.
Inside that there are two functions called
a) OnSimulationCollisionReset
b) OnSimulationProgressEvent
Implementation is like this:
private void OnSimulationCollisionReset()
{
Thread XmlReset = new Thread(XmlResetFn);
XmlReset.Start();
}
private void OnSimulationProgressEvent()
{
DataStoreSingleTon.Instance.IsResetCompleted = true;
Thread ThrdSimulnProgress = new Thread(SimulnProgress);
ThrdSimulnProgress.Start();
}
where SimulnProgress() and XmlResetFn() are as follows:
private void SimulnProgress()
{
//uses a delegate
UIControlHandler.Instance.ShowSimulationProgress();
}
private void XmlResetFn()
{
DataStoreSingleTon.Instance.GetFPBConfigurationInstance().ResetXmlAfterCollision();
}
In which OnSimulationProgressEvent() is using a delegate function.
Both showSimulationProgress and ResetXML...() uses a same resource FPBArrayList.
My requirement is SimulationProgressEvent() should work only after Reset..(). In resetXML..() I clear the FPBList.
In SimulationProgress() I access FPBList[i] where i:0--->size;
I have locked both functions using a same lock object.I expected, reset() will complete first. But after entering to reset, before complete reset, showProgress() started and exception occured..
How to solve my issue?
This is how I locked the functions
public System.Object lockThis = new System.Object();
private void SimulnProgress()
{
lock (lockThis)
{
UIControlHandler.Instance.ShowSimulationProgress();
}
}
private void XmlResetFn()
{
lock (lockThis)
{
DataStoreSingleTon.Instance.GetFPBConfigurationInstance().ResetXmlAfterCollision();
}
}
Please give a solution.
Regards
Nidhin KR
It's not a good idea to write multithreaded code that assumes or requires that execution on different threads occurs in a particular order. The whole point of multithreading is to allow things to be executed independently of each other. Independently means no particular order is expressed or implied. CPU time might not be distributed evenly between the two threads, for example, particularly is one thread is waiting for an external signaling event and the other thread is in a compute loop.
For your particular code, it seems very odd that IsResetCompleted = true; is set in the OnSimulationProgressEvent handler. The completion state of the Reset activity should be set by the Reset activity, not by some other event executing in another thread assuming "If we're here, the work in the other thread must be finished."
You should review your design and identify your assumptions and dependencies between threads. If thread B must not proceed until after thread A has completed something, you should first reexamine why you're putting this work in different threads, and then perhaps use a synchronization object (such as an AutoResetEvent) to coordinate between the threads.
The key point here is if you take a sequential task and split it into multiple threads, but the threads use locks or synch objects to serialize their execution, then there is no benefit to using multiple threads. The operation is still sequential.
Locks are intended to prevent several threads from entering a given section of code simultaneously. They are not intended to synchronize the threads in any other way, like, making them execute code in some specific order.
To enforce the execution order you need to implement some signalling between your threads.
Have a look at Synchronization Primitives, specifically, Auto/ManualResetEvent is probably what you want.
I am not sure if I understand the question entirely, but if your requirement is simply that you want to prevent the body of SimulnProgress from executing before XmlResetfn has executed at least once, you can do:
public readonly object lockThis = new object();
private readonly ManualResetEvent resetHandle = new ManualResetEvent(false);
private void SimulnProgress()
{
resetHandle.WaitOne();
lock (lockThis)
{
UIControlHandler.Instance.ShowSimulationProgress();
}
}
private void XmlResetFn()
{
lock (lockThis)
{
DataStoreSingleTon.Instance.GetFPBConfigurationInstance().ResetXmlAfterCollision();
}
resetHandle.Set();
}
In my application I have a form that starts synchronization process and for number of reasons I want to allow only one synchronization to run at a time. So I've added a static bool field to my form indicating whether sync is in progress and added a lock to set this field to true if it wasn't already set so that first thread could start synchronization but when it's running every other thread that will try to start it will terminate.
My code is something like this:
internal partial class SynchronizationForm : Form
{
private static volatile bool workInProgress;
private void SynchronizationForm_Shown(object sender, EventArgs e)
{
lock (typeof(SynchronizationForm))
{
if (!workInProgress)
{
workInProgress = true;
}
else
{
this.Close();
}
}
}
}
This is working well but when I run Code Analysis on my project I'm getting the following warning message:
CA2002 : Microsoft.Reliability : 'SynchronizationForm.SynchronizationForm_Shown(object, EventArgs)' locks on a reference of type 'Type'. Replace this with a lock against an object with strong-identity.
Can anyone explain to me what's wrong with my code and how can I improve it to make the warning gone. What does it mean that object has a strong-identity?
What is wrong is that you are locking on something public (typeof(SynchronizationForm)) which is accessible everywhere from your code and if some other thread locks on this same thing you get a deadlock. In general it is a good idea to lock only on private static objects:
private static object _syncRoot = new object();
...
lock (_syncRoot)
{
}
This guarantees you that it's only SynchronizationForm that could possess the lock.
From the MSDN explanation of the rule
An object is said to have a weak identity when it can be directly accessed across application domain boundaries. A thread that tries to acquire a lock on an object that has a weak identity can be blocked by a second thread in a different application domain that has a lock on the same object.
Since you can't necessarily predict what locks another AppDomain might take, and since such locks might need to be marshalled and would then be expensive, this rule makes sense to me.
The problem is that typeof(SynchronizationForm) is not a private lock object, which means that any other piece of code could use it to lock on, which could result in deadlock. For example if some other code did this:
var form = new SynchronizationForm();
lock(typeof(SynchronizationForm))
{
form.SomeMethodThatCausesSynchronizationForm_ShownToBeCalled();
}
Then deadlock will occur. Instead you should delcare a private lock object in the SynchronizationForm class and lock on that instead.
The System.Type object of a class can conveniently be used as the mutual-exclusion lock for static methods of the class.
Source: http://msdn.microsoft.com/en-us/library/aa664735(VS.71).aspx
To add to Doug's answer, what you have here is a locking mechanism which should only be used in static methods, being used in an instance method.