Let's say there are two apartment-threaded COM objects, located in different apartments. Or maybe they're in different processes altogether. If one object calls a method on another, which in turn calls a method back on the first object, how does COM prevent the whole thing from deadlocking?
What you describe is called reentrancy.
The truth is that COM doesn’t do anything explicit to prevent reentrancy issues. It’s up to the implementer of each object to take precautions where needed, as applicable.
Funny enough, reentrancy in COM is far less common in real life than you would think. Object graphs in COM tend to be mostly trees, which do not exhibit reentrancy. When you have cycles it’s almost always because of objects exposing event-type functionality of some sort, typically Connection Points.
Event callbacks are very limited in scope and they trigger under the explicit control of each object’s code, so the programmer is able to easily time them so they occur at safe places (for example at/near the end of a method’s body after all the real work is done). This prevents serious reentrancy issues from developing.
But nothing stops you from coding something dangerous. For example, if an object triggers an event while its internal object state is inconsistent, all bets are off.
You mention deadlocks. Deadlocks require a locking mechanism of some sort (for example a Critical Section) and should be extremely rare to impossible in COM apartments for the reasons listed above. Any object that triggers an event while holding a lock is asking for serious trouble, and a deadlock is not the biggest of its worries: by virtue of being an STA object the reentrant call will run on the same thread, and it will be able to acquire the locks again and proceed right through, which means it’s very likely that the object will corrupt its internal state, cause a crash, or worse. Note that locks in an STA thread only make sense if the resources controlled by the lock are accessible to threads outside the object’s STA.
And finally, nothing in COM stops you from causing an infinite recursion loop and subsequent stack overflow either. For example, take two COM objects Obj1 and Obj2, with Obj2 implementing an event. We can have Obj1 call a pObj2->SomeMethod(…) which causes Obj2 to fire the event; then have obj1 listen (“sink”) to that event, and have that event handler call SomeMethod() again.
UPDATE:
Profound thanks to Remy Lebeau for pointing to in his comment something I had forgotten to discuss, via a link to CodeGuru article Understanding COM Apartments, Part I. And in the process I also learned something new myself I should have known about.
There is one aspect of reentrancy and locking to consider and that is what happens during inter-apartment calls (either STA<->STA, STA<->MTA, or even STA<->OutofProc). During an inter-apartment call the STA (caller's) thread needs to stall and wait for an answer to the call request; the response cannot (by definition) execute on the same thread. But it can't just fully block (e.g. WaitForSingleObject) waiting for the response because the thread needs to be able to respond and process not only potential callbacks to the original object, but also to callbacks to any other object inside of the same apartment. If it were to fully block, the COM infrastructure itself would be introducing the potential for a deadlock and you wouldn't even need a dependency cycle between objects. So the COM marshalling infrastructure uses a more complex form of Wait that can unblock for a few other situations (Hans Passat points to CoWaitForMultipleHandles which looks right to me but I don't know the infrastructure to that level). If an applicable callback occurs, the marshalling infrastructure will unblock and allow that call to enter the apartment and proceed.
This is a form of locking induced by the COM infrastructure itself, rather than one coded explicitly as part of the object's implementation, which is why I hadn't thought of bringing it up. So COM does in fact "do something to prevent deadlocks", but to prevent deadlock potentials induced by its own infrastructure.
The part that I hadn't consciously realized was that this mechanism is very selective. It only lets through COM calls that form part of the same causality chain, that is, a callback, a direct consequence of the call that the thread was waiting on. Other COM calls into the apartment have to queue up and wait for that call chain to conclude, and for the STA thread to return to the thread's message loop.1
1 It makes complete sense that it needs to be that way, but I don't think I ever realized it.
Related
I understand the reason behind STA, but don't really see the reason for MTA.
A COM object can be loaded without any apartments, right? That means it's already able to take calls asynchronously, since no one puts any constraints.
Where am I wrong?
First, your assumption is wrong: a COM object can not be created outside of any apartment. Generally a thread should only create COM objects if it has previously called CoInitialize or CoInitializeEx, which places it in an apartment. Otherwise, creation will usually fail. There is the edge case of the implicit multithreaded apartment (if another thread of the same process initialized it), but even then you would be in the MTA, just in an unreliable and hard to debug way. No COM object ever exists without being in an apartment.
The reason you want an MTA is that it is not necessarily the only apartment. A process can have one MTA and arbitrarily many STAs. Calls between the MTA and any of the STAs still need to be marshaled; if they weren't, one of the MTA threads could call an STA thread in an unsafe way.
In fact having at least one STA is the rule rather than the exception: The user interface wants to live in an STA, because it depends on messages (for example because of mouse clicks etc.) to be processed in sequence.
I've allocated some GPU global memory with cudaMalloc(), say, in the constructor of some class. Now it's time to destruct the instance I've constructed, and I have my instance's data pointer. The thing is, I'm worried maybe some mischievous code elsewhere has called cudaDeviceReset(), after which my cudaFree() will probably fail (I'll get an invalid device pointer error). So, how can can I tell whether my pointer is elligible for cudaFree()ing?
I don't believe you can do much about that.
About the best you can do is try and engineer the lifespan of objects which will call the CUDA APIs in their destructors to do so before context destruction. In practice, that means having them fall of of scope in a well defined fashion before the context is automatically or manually torn down.
For a call like cudaFree(), which is somewhat "fire and forget" anyway, the best thing to do might be to write your own wrapper for the call and explicitly catch and tastefully ignore any obvious error conditions which would arise if the call was made after context destruction-
Given what talonmies says, one might consider doing the converse:
wrap your cudaDeviceReset() calls to also regard a 'generation counter'.
Counter increases will be protected by a lock.
While you lock, you reset and increment the generation counter.
Wrap cudaMalloc() to also keep the generation index (you might need a class/struct for that) - obtained during allocation (which also locks).
Wrap cudaFree() to lock and only really cudaFree() if the reset generation has not changed.
... now, you might say "Is all that locking worth it? At worst, you'll get an error, it's not such a big deal." And, to be honest - I'm not sure it's worth it. You could make this somewhat less painful by using a Reader-Writer lock instead of a simple lock, where the allocate and free are just readers that can all access concurrently.
I know there are no destructors in Go since technically there are no classes. As such, I use initClass to perform the same functions as a constructor. However, is there any way to create something to mimic a destructor in the event of a termination, for the use of, say, closing files? Right now I just call defer deinitClass, but this is rather hackish and I think a poor design. What would be the proper way?
In the Go ecosystem, there exists a ubiquitous idiom for dealing with objects which wrap precious (and/or external) resources: a special method designated for freeing that resource, called explicitly — typically via the defer mechanism.
This special method is typically named Close(), and the user of the object has to call it explicitly when they're done with the resource the object represents. The io standard package does even have a special interface, io.Closer, declaring that single method. Objects implementing I/O on various resources such as TCP sockets, UDP endpoints and files all satisfy io.Closer, and are expected to be explicitly Closed after use.
Calling such a cleanup method is typically done via the defer mechanism which guarantees the method will run no matter if some code which executes after resource acquisition will panic() or not.
You might also notice that not having implicit "destructors" quite balances not having implicit "constructors" in Go. This actually has nothing to do with not having "classes" in Go: the language designers just avoid magic as much as practically possible.
Note that Go's approach to this problem might appear to be somewhat low-tech but in fact it's the only workable solution for the runtime featuring garbage-collection. In a language with objects but without GC, say C++, destructing an object is a well-defined operation because an object is destroyed either when it goes out of scope or when delete is called on its memory block. In a runtime with GC, the object will be destroyed at some mostly indeterminate point in the future by the GC scan, and may not be destroyed at all. So if the object wraps some precious resource, that resource might get reclaimed way past the moment in time the last live reference to the enclosing object was lost, and it might even not get reclaimed at all—as has been well explained by #twotwotwo in their respective answer.
Another interesting aspect to consider is that the Go's GC is fully concurrent (with the regular program execution). This means a GC thread which is about to collect a dead object might (and usually will) be not the thread(s) which executed that object's code when it was alive. In turn, this means that if the Go types could have destructors then the programmer would need to make sure whatever code the destructor executes is properly synchronized with the rest of the program—if the object's state affects some data structures external to it. This actually might force the programmer to add such synchronization even if the object does not need it for its normal operation (and most objects fall into such category). And think about what happens of those exernal data strucrures happened to be destroyed before the object's destructor was called (the GC collects dead objects in a non-deterministic way). In other words, it's much easier to control — and to reason about — object destruction when it is explicitly coded into the program's flow: both for specifying when the object has to be destroyed, and for guaranteeing proper ordering of its destruction with regard to destroying of the data structures external to it.
If you're familiar with .NET, it deals with resource cleanup in a way which resembles that of Go quite closely: your objects which wrap some precious resource have to implement the IDisposable interface, and a method, Dispose(), exported by that interface, must be called explicitly when you're done with such an object. C# provides some syntactic sugar for this use case via the using statement which makes the compiler arrange for calling Dispose() on the object when it goes out of the scope declared by the said statement. In Go, you'll typically defer calls to cleanup methods.
One more note of caution. Go wants you to treat errors very seriously (unlike most mainstream programming language with their "just throw an exception and don't give a fsck about what happens due to it elsewhere and what state the program will be in" attitude) and so you might consider checking error returns of at least some calls to cleanup methods.
A good example is instances of the os.File type representing files on a filesystem. The fun stuff is that calling Close() on an open file might fail due to legitimate reasons, and if you were writing to that file this might indicate that not all the data you wrote to that file had actually landed in it on the file system. For an explanation, please read the "Notes" section in the close(2) manual.
In other words, just doing something like
fd, err := os.Open("foo.txt")
defer fd.Close()
is okay for read-only files in the 99.9% of cases, but for files opening for writing, you might want to implement more involved error checking and some strategy for dealing with them (mere reporting, wait-then-retry, ask-then-maybe-retry or whatever).
runtime.SetFinalizer(ptr, finalizerFunc) sets a finalizer--not a destructor but another mechanism to maybe eventually free up resources. Read the documentation there for details, including downsides. They might not run until long after the object is actually unreachable, and they might not run at all if the program exits first. They also postpone freeing memory for another GC cycle.
If you're acquiring some limited resource that doesn't already have a finalizer, and the program would eventually be unable to continue if it kept leaking, you should consider setting a finalizer. It can mitigate leaks. Unreachable files and network connections are already cleaned up by finalizers in the stdlib, so it's only other sorts of resources where custom ones can be useful. The most obvious class is system resources you acquire through syscall or cgo, but I can imagine others.
Finalizers can help get a resource freed eventually even if the code using it omits a Close() or similar cleanup, but they're too unpredictable to be the main way to free resources. They don't run until GC does. Because the program could exit before next GC, you can't rely on them for things that must be done, like flushing buffered output to the filesystem. If GC does happen, it might not happen soon enough: if a finalizer is responsible for closing network connections, maybe a remote host hits its limit on open connections to you before GC, or your process hits its file-descriptor limit, or you run out of ephemeral ports, or something else. So it's much better to defer and do cleanup right when it's necessary than to use a finalizer and hope it's done soon enough.
You don't see many SetFinalizer calls in everyday Go programming, partly because the most important ones are in the standard library and mostly because of their limited range of applicability in general.
In short, finalizers can help by freeing forgotten resources in long-running programs, but because not much about their behavior is guaranteed, they aren't fit to be your main resource-management mechanism.
There are Finalizers in Go. I wrote a little blog post about it. They are even used for closing files in the standard library as you can see here.
However, I think using defer is more preferable because it's more readable and less magical.
I realise that I can't access Form controls from the DoWork event handler of a BackgroundWorker. (And if I try to, I get an Exception, as expected).
However, am I allowed to access other (custom) objects that exist on my Form?
For instance, I've created a "Settings" class and instantiated it in my Form and I seem to be able to read and write to its properties.
Is it just luck that this works?
What if I had a static class? Would I be able to access that safely?
#Engram:
You've got the gist of it - CrossThreadCalls are just a nice feature MS put into the .NET Framework to prevent the "bonehead" type of parallel programming mistakes. It can be overridden, as I'm guessing you've already found out, by setting the "AllowCrossThreadCalls" property on the class (and not on an instance of the class, e.g. set Label.AllowCrossThreadCalls and not lblMyLabel.AllowCrossThreadCalls).
But more importantly, you're right about the need to use some kind of locking mechanism. Whenever you have multiple threads of execution (be it threads, processes or whatever), you need to make sure that when you have one thread reading/writing to a variable, you probably don't want some other thread barging and changing that value under the feet of the first thread.
The .NET Framework actually provides several other mechanisms which might be more useful, depending on circumstances, than locking in code. The first is to use a Monitor class, which has the effect of locking a particular object. When you use this, other threads can continue to execute, as long as they don't try to lock that same object. Another very useful and common parallel-programming idea is the Mutex (or Semaphore). The Mutex is basically like a game of Capture the Flag between your threads. If one thread grabs the flag, no other threads can grab it until the first thread drops it. (A Semaphore is just like a Mutex, except that there can be more than one flag in a game.)
Obviously, none of these concepts will work in every particular problem - but having a few more tools to help you out might come in handy some day :)
You should communicate to the user interface through the ProgressChanged and RunWorkerCompleted events (and never the DoWork() method as you have noted).
In principle, you could call IsInvokeRequired, but the designers of the BackgroundWorker class created the ProgressChanged callback event for the purpose of updating UI elements.
[Note: BackgroundWorker events are not marshaled across AppDomain boundaries. Do not use a BackgroundWorker component to perform multithreaded operations in more than one AppDomain.]
MSDN Ref.
Ok, I've done some more research on this and I think have an answer. (Let the votes decide if I'm right!)
The answer is.. you can access any custom object that's in scope, however your access will not be thread-safe.
To ensure that it is thread-safe you should probably be using lock. The lock keyword prevents more than one thread executing a particular piece of code. (Subject to actually using it properly!)
The Cross Threading Exception that occurs when you try and access a Control is a safety mechanism designed especially for Controls. (It's easier and probably more efficient to get the user to make thread-safe calls then it is to design the controls themselves to be thread-safe).
You can't access controls that where created in one thread from another thread.
You can either use Settings class that you mentioned, or use InvokeRequired property and Invoke methods of control.
I suggest you look at the examples on those pages:
http://msdn.microsoft.com/en-us/library/ms171728.aspx
http://msdn.microsoft.com/en-us/library/system.windows.forms.control.invokerequired.aspx
I am working on a cocoa software and in order to keep the GUI responsive during a massive data import (Core Data) I need to run the import outside the main thread.
Is it safe to access those objects even if I created them in the main thread without using locks if I don't explicitly access those objects while the thread is running.
With Core Data, you should have a separate managed object context to use for your import thread, connected to the same coordinator and persistent store. You cannot simply throw objects created in a context used by the main thread into another thread and expect them to work. Furthermore, you cannot do your own locking for this; you must at minimum lock the managed object context the objects are in, as appropriate. But if those objects are bound to by your views a controls, there are no "hooks" that you can add that locking of the context to.
There's no free lunch.
Ben Trumbull explains some of the reasons why you need to use a separate context, and why "just reading" isn't as simple or as safe as you might think, in this great post from late 2004 on the webobjects-dev list. (The whole thread is great.) He's discussing the Enterprise Objects Framework and WebObjects, but his advice is fully applicable to Core Data as well. Just replace "EC" with "NSManagedObjectContext" and "EOF" with "Core Data" in the meat of his message.
The solution to the problem of sharing data between threads in Core Data, like the Enterprise Objects Framework before it, is "don't." If you've thought about it further and you really, honestly do have to share data between threads, then the solution is to keep independent object graphs in thread-isolated contexts, and use the information in the save notification from one context to tell the other context what to re-fetch. -[NSManagedObjectContext refreshObject:mergeChanges:] is specifically designed to support this use.
I believe that this is not safe to do with NSManagedObjects (or subclasses) that are managed by a CoreData NSManagedObjectContext. In general, CoreData may do many tricky things with the sate of managed objects, including firing faults related to those objects in separate threads. In particular, [NSManagedObject initWithEntity:insertIntoManagedObjectContext:] (the designated initializer for NSManagedObjects as of OS X 10.5), does not guarantee that the returned object is safe to pass to an other thread.
Using CoreData with multiple threads is well documented on Apple's dev site.
The whole point of using locks is to ensure that two threads don't try to access the same resource. If you can guarantee that through some other mechanism, go for it.
Even if it's safe, but it's not the best practice to use shared data between threads without synchronizing the access to those fields. It doesn't matter which thread created the object, but if more than one line of execution (thread/process) is accessing the object at the same time, since it can lead to data inconsistency.
If you're absolutely sure that only one thread will ever access this object, than it'd be safe to not synchronize the access. Even then, I'd rather put synchronization in my code now than wait till later when a change in the application puts a second thread sharing the same data without concern about synchronizing access.
Yes, it's safe. A pretty common pattern is to create an object, then add it to a queue or some other collection. A second "consumer" thread takes items from the queue and does something with them. Here, you'd need to synchronize the queue but not the objects that are added to the queue.
It's NOT a good idea to just synchronize everything and hope for the best. You will need to think very carefully about your design and exactly which threads can act upon your objects.
Two things to consider are:
You must be able to guarantee that the object is fully created and initialised before it is made available to other threads.
There must be some mechanism by which the main (GUI) thread detects that the data has been loaded and all is well. To be thread safe this will inevitably involve locking of some kind.
Yes you can do it, it will be safe
...
until the second programmer comes around and does not understand the same assumptions you have made. That second (or 3rd, 4th, 5th, ...) programmer is likely to start using the object in a non safe way (in the creator thread). The problems caused could be very subtle and difficult to track down. For that reason alone, and because its so tempting to use this object in multiple threads, I would make the object thread safe.
To clarify, (thanks to those who left comments):
By "thread safe" I mean programatically devising a scheme to avoid threading issues. I don't necessarily mean devise a locking scheme around your object. You could find a way in your language to make it illegal (or very hard) to use the object in the creator thread. For example, limiting the scope, in the creator thread, to the block of code that creates the object. Once created, pass the object over to the user thread, making sure that the creator thread no longer has a reference to it.
For example, in C++
void CreateObject()
{
Object* sharedObj = new Object();
PassObjectToUsingThread( sharedObj); // this function would be system dependent
}
Then in your creating thread, you no longer have access to the object after its creation, responsibility is passed to the using thread.