Correctness Hazards in .NET

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Let' s begin by examining various kinds of correctness hazards This cate gory is full of data race problems of different sorts, but also includes sub tleties around lock recursion and reentrancy We'll also see some unique problems that arise due to locks and application shutdown This includes the possibility of orphaning locks indefinitely
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Data Races All imperative programs contain fundamental assumptions about state, control flow, and the intertwined relationship between the two This rela tionship is not always explicitly called out, but, should you violate one of the assumptions, your code is apt to do strange things For example, if we have just written the value 5 to some memory location x, can subsequent
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lines of code safely assume it will continue containing the value 5 as x is reread over and over again
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Some Type myObj myObj x int a int b
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myObj x ; I I St i l l 5 myObj x ; I I What a bout now
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If multiple threads can access myObj at once, this code is apt to break if it assumes that both a and b will contain the value 5 Another thread could write to x in between the execution of the two separate reads Preventing this situation requires some concurrency safety: isolation (private state), data synchronization, or immutability But what if you forget to add the necessary concurrency safety Or what if you do it incorrectly We won't dwell too long on this particular problem We already discussed data races at great length in 2, Synchronization and Time, so you should know that doing these things causes your program to crash, hang, or cor rupt important application and system state Many assumptions commonly made by sequentially oriented software are quickly invalidated by concurrency due to unexpected interactions between many threads running different parts of your program simul taneously Another way of explaining this is in terms of invariants All algorithms and data structures have invariants, even if they aren't explic itly called out Invariants are important to be conscientious of when pro gramming because, when broken, the surrounding program logic behaves unexpectedly Understanding and documenting invariants is tremendously helpful in building correct and robust concurrent systems The term "invariant" sounds overly abstract Here are a few concrete examples
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Methods have preconditions that represent conditions that the method assumes to be true in order to function correctly Sometimes preconditions pertain to arguments to a method, in which case they are typically checked by argument validation logic Other times, pre conditions pertain to surrounding state and the implementation may assert (or just assume) that they are true
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Similarly, methods have postconditions that specify the state of the returned and surrounding state after the method has finished executing
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Obj ect invariants apply to a single object and describe expected legal states in which the object may be For example, we might assume that the current index for a list backed by an array is always within legal range, that is, points to a valid index in the array Were this ever to be untrue, the object's methods would probably not work correctly, that is, method preconditions often include the object's invariants Control flow invariants are like object invariants, but are more ad hoc and local For example, once we've exited a loop, we might expect some set of conditions to hold Or, as in our x 5 example above, we might assume some earlier assignments still hold true
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Some systems even allow checking of invariants in a structured way For example, the language Eiffel (see Further Reading, Meyer) is well known for its first class support, and research systems such as Spec# from Microsoft Research (see Further Reading, Barnett, Leino, Schulte) extend existing imperative languages (in this case, C#) with similar support for checking invariants Use of such systems is not widespread on Windows, so most invariants take the form of asserts sprinkled throughout your code base The relationship between invariants and race conditions is fundamental If your program can reach a state in which an invariant doesn't hold for state that is visible among multiple threads, your program has a race condition Broken invariants cannot be sidestepped because many logical operations entail multiple physical steps to complete In between steps, state may be left inconsistent If you can write your data structures so invariants hold at each atomic state update, you've built one capable of lock freedom and might use this to your advantage when it comes to building scalable code But for most cases, the practical implication is that state must be protected by synchro nization or be kept isolated for the duration of said broken invariants When locking is involved, we often say that invariants must hold at lock entry and exit boundaries
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