The collection object that I have is a singleton type, which contains a list of particular object, each index in the list is read by multiple threads, so that they can query an integer property value to be used by a thread local variable. Does this case need any safety using synchronization, in my view No, but posting the question to be doubly sure.
There's no update happening to the object mentioned above on the multiple threads, they are just reading. In my view even ReaderWriterLockSlim needn't be used here, since there's no write. Please confirm my understanding.
Code is something like:
Here NumOfLocs, threadProp are specific to a thread and collection count and objects doesn't change, while threads are reading, they are just fixed in the beginning during initialization
int NumOfLocs = collectionObject.LocCollection.Count;
int threadProp = collectionObject.LocCollection[index].Prop
You wouldn't need synchronization if you're only reading the collection. If you wanted to update the collection however, there's a list of Thread-safe collection classes available in System.Collections.Concurrent which you could use. See here for the MSDN documentation.
Usually functions that are meant to read the state do not change the state. But sometimes some function of some object will change some internal object's state, contrary to common sense. This for instance could happen if object is caching something, or rearranging internal structure. It is impossible to tell up front what some object does in any of its functions, without knowing the internal workings of the object.
If it's standard .NET object then there's probably a documentation for it that will tell you if the object is thread safe for reading. If it's some third party object then you have to ask that third party. If you coded the object then only you know.
Related
You can sometimes reuse the object itself for the lock, but quite often it is advised to use a different object anyway.
Don't we have a lot more typesafety and a lot better intention if there would just be a keyword for lock?
private object _MyLock = new object();
// someone would now be able to reassign breaking everything
_MyLock = new object();
lock ( _MyLock )
...
VS
private lock _MyLock;
// compiler error
_MyLock = new object();
lock ( _MyLock )
...
Before I get downvotes that you can't guess someones intention: I'm pretty sure the language designers had a good reason and there are more knowledgable coders here that now why. I ask this to understand better programming principles.
Note that by using an object as a monitor, you can already do all the things you can do with any other object: Pass references so that multiple classes can share the same monitor, keep them in arrays, keep them inside other data structures.
If you had a special type of declaration for a lockable, you would need special syntax for passing it to a function, storing a reference to it inside another instance, associating it with a type instead of an instance, creating arrays (e.g. for LockMany operation), and so on.
Using the language rules that already exist for objects to handle all these common and not-so-common usages makes the language a whole lot simpler.
Don't we have a lot more typesafety and a lot better intention if there would just be a keyword for lock?
It's not about type safety at all. It's about thread safety.
Sometimes that means running the same code in a single lock over and over. Perhaps you have a single large array, where some of your operations might need to swap two elements, and you want to make sure things are synchronized during the swap. In this kind of context, even a simple lock keyword by itself, where the object is created for you behind the scenes, might be good enough.
Sometimes you're sharing an object among very different sets of code. Now you need multiple lock sections that coordinate using a common object. In this case, the code you're talking about seems to make sense. Letting the compiler create a lock object for you isn't good enough because the different lock sections would not coordinate, but you also want to make sure the common lock object is fixed, and doesn't change somehow. For example, maybe you're working through an array with multiple threads, and you have different operations that might modify a shared index value that indicates which element is considered current or active. Each of these operations should lock on the same object.
But sometimes you share multiple object instances (often of the same type) among several sets of code. Think producer/consumer pattern, where multiple consumers from different threads need to coordinate access to a shared queue, and the consumers themselves are multi-threaded. In this kind of case, a single common lock object would be okay to retrieve an element from the queue, but a single shared object in different sections of the consumer could become a bottleneck for the application. Instead, you would only want to lock once per active object/consumer. You need the lock section to accept a variable that indicates which object needs protection, without locking your entire data set.
One solution may be defining lock object as readonly.
static readonly object lockObject = new object();
In this case compiler prevents renewing and assigning new object to lockobject.
If I have a ConcurrentDictionary and use the TryGetValue within an if statement, does this make the if statement's contents thread safe? Or must you lock still within the if statement?
Example:
ConcurrentDictionary<Guid, Client> m_Clients;
Client client;
//Does this if make the contents within it thread-safe?
if (m_Clients.TryGetValue(clientGUID, out client))
{
//Users is a list.
client.Users.Add(item);
}
or do I have to do:
ConcurrentDictionary<Guid, Client> m_Clients;
Client client;
//Does this if make the contents within it thread-safe?
if (m_Clients.TryGetValue(clientGUID, out client))
{
lock (client)
{
//Users is a list.
client.Users.Add(item);
}
}
Yes you have to lock inside the if statement the only guarantee you get from concurrent dictionary is that its methods are thread save.
The accepted answer could be misleading, depending on your point of view and the scope of thread safety you are trying to achieve. This answer is aimed at people who stumble on this question while learning about threading and concurrency:
It's true that locking on the output of the dictionary retrieval (the Client object) makes some of the code thread safe, but only the code that is accessing that retrieved object within the lock. In the example, it's possible that another thread removes that object from the dictionary after the current thread retrieves it. (Even though there are no statements between the retrieval and the lock, other threads can still execute in between.) Then, this code would add the Client object to the Users list even though it is no longer in the concurrent dictionary. That could cause an exception, synchronization, or race condition.
It depends on what the rest of the program is doing. But in the scenario I'm describing, it would be safer to put the lock around the entire dictionary retrieval. And then a regular dictionary might be faster and simpler than a concurrent dictionary, as long as you always lock on it while using it!
While both of the current answers are technically true I think that the potential exists for them to be a little misleading and they don't express ConcurrentDictionary's big strengths. Maybe the OP's original way of solving the problem with locks worked in that specific circumstance but this answer is aimed more generally towards people learning about ConcurrentDictionary for the first time.
Concurrent Dictionary is designed so that you don't have to use locks. It has several specialty methods designed around the idea that some other thread could modify the object in the dictionary while you're currently working on it. For a simple example, the TryUpdate method lets you check to see if a key's value has changed between when you got it and the moment that you're trying to update it. If the value that you've got matches the value currently in the ConcurrentDictionary you can update it and TryUpdate returns true. If not, TryUpdate returns false. The documentation for the TryUpdate method can make this a little confusing because it doesn't make it explicitly clear why there is a comparison value but that's the idea behind the comparison value. If you wanted to have a little more control around adding or updating, you could use one of the overloads of the AddOrUpdate method to either add a value for a key if it doesn't exist at the moment that you're trying to add it or update the value if some other thread has already added a value for the key that is specified. The context of whatever you're trying to do will dictate the appropriate method to use. The point is that, rather than locking, try taking a look at the specialty methods that ConcurrentDictionary provides and prefer those over trying to come up with your own locking solution.
In the case of OP's original question, I would suggest that instead of this:
ConcurrentDictionary<Guid, Client> m_Clients;
Client client;
//Does this if make the contents within it thread-safe?
if (m_Clients.TryGetValue(clientGUID, out client))
{
//Users is a list.
client.Users.Add(item);
}
One might try the following instead*:
ConcurrentDictionary<Guid, Client> m_Clients;
Client originalClient;
if(m_Clients.TryGetValue(clientGUID, out originalClient)
{
//The Client object will need to implement IEquatable if more
//than an object instance comparison needs to be done. This
//sample code assumes that Client implements IEquatable.
//If copying a Client is not trivial, you'll probably want to
//also implement a simple type of copy in a method of the Client
//object. This sample code assumes that the Client object has
//a ShallowCopy method to do this copy for simplicity's sake.
Client modifiedClient = originalClient.ShallowCopy();
//Make whatever modifications to modifiedClient that need to get
//made...
modifiedClient.Users.Add(item);
//Now update the value in the ConcurrentDictionary
if(!m_Clients.TryUpdate(clientGuid, modifiedClient, originalClient))
{
//Do something if the Client object was updated in between
//when it was retrieved and when the code here tries to
//modify it.
}
}
*Note in the example above, I'm using TryUpate for ease of demonstrating the concept. In practice, if you need to make sure that an object gets added if it doesn't exist or updated if it does, the AddOrUpdate method would be the ideal option because the method handles all of the looping required to check for add vs update and take the appropriate action.
It might seem like it's a little harder at first because it may be necessary to implement IEquatable and, depending on how instances of Client need to be copied, some sort of copying functionality but it pays off in the long run if you're working with ConcurrentDictionary and objects within it in any serious way.
While going through some database code looking for a bug unrelated to this question, I noticed that in some places List<T> was being used inappropriately. Specifically:
There were many threads concurrently accessing the List as readers, but using indexes into the list instead of enumerators.
There was a single writer to the list.
There was zero synchronization, readers and writers were accessing the list at the same time, but because of code structure the last element would never be accessed until the method that executed the Add() returned.
No elements were ever removed from the list.
By the C# documentation, this should not be thread safe.
Yet it has never failed. I am wondering, because of the specific implementation of the List (I am assuming internally it's an array that re-allocs when it runs out of space), it the 1-writer 0-enumerator n-reader add-only scenario accidentally thread safe, or is there some unlikely scenario where this could blow up in the current .NET4 implementation?
edit: Important detail I left out reading some of the replies. The readers treat the List and its contents as read-only.
This can and will blow. It just hasn't yet. Stale indices is usually the first thing that goes. It will blow just when you don't want it to. You are probably lucky at the moment.
As you are using .Net 4.0, I'd suggest changing the list to a suitable collection from System.Collections.Concurrent which is guaranteed to be thread safe. I'd also avoid using array indices and switch to ConcurrentDictionary if you need to look up something:
http://msdn.microsoft.com/en-us/library/dd287108.aspx
Because of it has never failed or your application doesn't crash that doesn't mean that this scenario is thread safe. for instance suppose the writer thread does update a field within the list, lets say that is was a long field, at the same time the reader thread reading that field. the value returned maybe a bitwise combination of the two fields the old one and the new one! that could happen because the reader thread start reading the value from memory but before it finishes reading it the writer thread just updated it.
Edit: That of course if we suppose that the reader threads will just read all the data without updating anything, I am sure that they doesn't change the values of the arrays them self but, but they could change a property or field within the value they read. for instance:
for (int index =0 ; index < list.Count; index++)
{
MyClass myClass = list[index];//ok we are just reading the value from list
myClass.SomeInteger++;//boom the same variable will be updated from another threads...
}
This example not talking about thread safe of the list itself rather than the shared variables that the list exposed.
The conclusion is that you have to use a synchronization mechanism such as lock before interaction with the list, even if it has only one writer and no item removed, that will help you prevent tinny bugs and failure scenarios you are dispensable for in the first place.
Thread safety only matters when data is modified more than once at a time. The number of readers does not matter. Even when someone is writing while someone reads, the reader either gets the old data or the new, it still works. The fact that elements can only be accessed after the Add() returns, prevents parts of the element being read seperately. If you would start using the Insert() method readers could get the wrong data.
It follows then, that if the architecture is 32 bits, writing a field bigger than 32 bits, such as long and double, is not a thread safe operation; see the documentation for System.Double:
Assigning an instance of this type is not thread safe on all hardware platforms because the
binary representation of that instance might be too large to assign in a single atomic
operation.
If the list is fixed in size, however, this situation matters only if the List is storing value types greater than 32 bits. If the list is only holding reference types, then any thread safety issues stem from the reference types themselves, not from their storage and retrieval from the List. For instance, immutable reference types are less likely to cause thread safety issues than mutable reference types.
Moreover, you can't control the implementation details of List: that class was mainly designed for performance, and it's likely to change in the future with that aspect, rather than thread safety, in mind.
In particular, adding elements to a list or otherwise changing its size is not thread safe even if the list's elements are 32 bits long, since there is more involved in inserting, adding, or removing than just placing the element in the list. If such operations are needed after other threads have access to the list, then locking access to the list or using a concurrent list implementation is a better choice.
First off, to some of the posts and comments, since when was documentation reliable?
Second, this answer is more to the general question than the specifics of the OP.
I agree with MrFox in theory because this all boils down to two questions:
Is the List class is implemented as a flat array?
If yes, then:
Can a write instruction be preempted in the middle of a write>
I believe this is not the case -- the full write will happen before anything can read that DWORD or whatever. In other words, it will never happen that I write two of the four bytes of a DWORD and then you read 1/2 of the new value and 1/2 of the old one.
So, if you're indexing an array by providing an offset to some pointer, you can read safely without thread-locking. If the List is doing more than just simple pointer math, then it is not thread safe.
If the List was not using a flat array, I think you would have seen it crash by now.
My own experience is that it is safe to read a single item from a List via index without thread-locking. This is all just IMHO though, so take it for what it's worth.
Worst case, such as if you need to iterate through the list, the best thing to do is:
lock the List
create an array the same size
use CopyTo() to copy the List to the array
unlock the List
then iterate through the array instead of the list.
in (whatever you call the .net) C++:
List<Object^>^ objects = gcnew List<Object^>^();
// in some reader thread:
Monitor::Enter(objects);
array<Object^>^ objs = gcnew array<Object^>(objects->Count);
objects->CopyTo(objs);
Monitor::Exit(objects);
// use objs array
Even with the memory allocation, this will be faster than locking the List and iterating through the entire thing before unlocking it.
Just a heads up though: if you want a fast system, thread-locking is your worst enemy. Use ZeroMQ instead. I can speak from experience, message-based synch is the right way to go.
I have a list of objects which will be accessible by many objects across many threads. To ensure thread safety I have made the list and its object read-only. My only concern is the iterators of the List<> object because I remember reading something about iterator thread safety issues. Do I have a problem?
For clarification: in the BarObservable class, the List < Bar > bar is read-only. The individual bars of the list are also read-only. The MarketDataAdaptor class uses BarService to add new bars to BarsObservable class. The diagram doesn't show this but the IBarObservers are passed a reference to the List < Bar > . They can't write to it but they do use the iterator of the List. Meanwhile the final bar is updated and once finalized a new bar is added to the end of the list.
As I understand it, you currently provide two immutability guarantees:
There is an unchanging reference(s) to a List<Bar> object.
The Bar type itself is immutable or is, by convention, instances of it are not mutated after they are added to the list.
Neither of these is sufficient to deal with any concurrent reader / writer scenarios since the List<T> type itself is not thread-safe.
If you have multiple unsynchronized writers, you could corrupt the list.
If you have a single writer, with reader(s) on other threads, you probably won't corrupt the list. On the other hand, the readers won't work correctly. If you're lucky, iterating the list in the middle of a write will throw a "Collection changed during enumeration" exception. If you're not, your program will silently lose its functional correctness.
Now you could try synchronizing access to the list with locks, ReaderWriterLockSlims, etc. How you do this will be specific to the Producer / Consumer relationships of your particular situation. For example, you could lock mutation during enumeration by either of these:
Hold up an attempt to mutate as long as there is an active undisposed enumerator.
Every time an enumerator is requested, block writers. Copy the list to another list; return its enumerator, and then unblock writers.
But I would suggest, if you are on .NET 4.0, to take a look at the thread-safe collection classes in the System.Collections.Concurrent namespace. In particular, you may find the BlockingCollection<T> class to be exactly what you need.
Finally, I would look at the overall design to see if you can solve this problem in a lock-free manner.
I have two threads, a producer thread that places objects into a generic List collection and a consumer thread that pulls those objects out of the same generic List. I've got the reads and writes to the collection properly synchronized using the lock keyword, and everything is working fine.
What I want to know is if it is ok to access the Count property without first locking the collection.
JaredPar refers to the Count property in his blog as a decision procedure that can lead to race conditions, like this:
if (list.Count > 0)
{
return list[0];
}
If the list has one item and that item is removed after the Count property is accessed but before the indexer, an exception will occur. I get that.
But would it be ok to use the Count property to, say, determine the initial size a completely different collection? The MSDN documentation says that instance members are not guaranteed to be thread safe, so should I just lock the collection before accessing the Count property?
I suspect it's "safe" in terms of "it's not going to cause anything to go catastrophically wrong" - but that you may get stale data. That's because I suspect it's just held in a simple variable, and that that's likely to be the case in the future. That's not the same as a guarantee though.
Personally I'd keep it simple: if you're accessing shared mutable data, only do so in a lock (using the same lock for the same data). Lock-free programming is all very well if you've got appropriate isolation in place (so you know you've got appropriate memory barriers, and you know that you'll never be modifying it in one thread while you're reading from it in another) but it sounds like that isn't the case here.
The good news is that acquiring an uncontested lock is incredibly cheap - so I'd go for the safe route if I were you. Threading is hard enough without introducing race conditions which are likely to give no significant performance benefit but at the cost of rare and unreproducible bugs.