C h a pter 6: D a t a a n d Co n t ro l Syn c h ro n i z a t i o n in .NET

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C h a pter 6: D a t a a n d Co n t ro l Syn c h ro n i z a t i o n
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elR locks The CLR provides "monitors" as the managed code equivalent to critical regions and Win32' s critical sections Any CLR object can be used as a mon itor, which can be accessed through the System T h r e a d i n g Mo n it o r class's static methods There's no need to initialize or delete a monitor explicitly You allocate the object on the GC heap and the CLR will take care of any ini tialization and management of internal data structures needed to support synchronization Each monitor is logically comprised of two things: a critical section and a condition variable Physically, the monitor does not include a Windows C R I T I CA L_S E CTION, but it behaves much as though it does We will defer discussion of the condition variable aspect of monitors until later in this chapter and focus for now on how to make use of its mutually exclusive locking capabilities Note also that managing a monitor object is just like managing any other kind of object in an object-oriented system Encapsulation is important so as not to accidentally leak the target of synchronization, enabling users of your type to interfere with internal synchronization This is why it's gen erally seen as a bad practice to lock on t h i s inside of an instance method And, as with Win32 critical sections, you can decide to associate monitors with static variables or as fields of individual objects At first it might seem convenient that you can lock on any CLR object, but it's almost always a better idea to explicitly manage locks as you would native critical sections Synchronization is difficult to begin with, and being thoughtful and disci plined about how locks are managed, what they protect, and so forth, is very important Explicitly walling off your objects meant for synchroniza tion from the rest is a good first step in this direction
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The Mo n i t o r E nt e r static method acquires the monitor associated with the object passed as an argument and the Mo n i t o r E x i t method leaves it
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p u b l i c stat i c void E n t e r ( o b j e c t obj ) j p u b l i c s t a t i c void E x i t ( o b j e c t obj ) j
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If the target monitor, o b j , is already held by another thread when you call E nt e r, the calling thread will block until the owning thread releases it
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The CLR uses Win32 events to implement waiting, which get allocated on demand and pooled among monitors Because monitors use kernel objects internally, they exhibit the same roughly-FIFO behavior that the OS syn chronization mechanisms also exhibit (described in the previous chapter) Monitors are unfair, so if another thread sneaks in and acquires the lock before an awakened waiting thread tries to acquire the lock, the sneaky thread is permitted to acquire the lock Trying to call E x i t on a monitor, o b j , that i s not held b y the current CLR thread causes a System T h r e a d i n g Syn c h ro n i z a t i o n Loc k E xc e pt ion exception to be thrown The monitor itself still remains in a completely valid state CLR monitors support recursive acquires by maintaining an internal recursion counter, so if a thread owns the monitor when a call to E n t e r is made, the acquisition succeeds and the counter is incremented When E x i t is called, this counter is decremented Once it hits 0, the monitor is released, waiting threads are awakened, and other threads may freely acquire it Each call to E nt e r must, therefore, have only one matching call to E x i t As mentioned earlier, recursion can cause some subtle problems, because it is dangerous to rely on invariants that would normally hold at critical region boundaries
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Ensuring a Thread Always Leaves the Monitor As discussed earlier with Win32 critical sections, you'll typically want to use a try / finally block to guarantee your lock is released, even in the face of an exception And, as also already noted, this sometimes is dangerous to do An excep tion from within a critical region often implies that data protected by that region has (possibly) become corrupt, so releasing the lock is usually the wrong thing to do It's often too cumbersome and time con suming to take the extra effort to validate state invariants for the extremely rare case that an exception occurs, so most programs simply don' t do it Using a try/finally might look something like this:
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