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Listing 102 shows the type of the handle, its attributes, its granted access, its handle counts, and so on It also gives information on the type of event (auto reset), as well as the state of the event, which in this particular case happens to be set Another interesting piece of information is the name of the event (set to <none>) As part of the event creation, it is possible to name an event, thereby enabling the event to be used across processes rather than just within a single process Two or more processes agree on an event name, and when trying to open an event with that particular name, the event will either be created, if it s the first call, or the reference count on the existing event will simply be incremented
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Critical sections are most commonly used to protect shared resources among threads by guaranteeing exclusive access (that is, only one thread is capable of gaining access to the resource) To illustrate the usage of a critical section, imagine the following piece of pseudo-code:
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1 Enter Critical Section 2 Access Shared Resource 3 Leave Critical Section
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Furthermore, imagine two threads (T1 and T2) both executing the preceding code, trying to get access to the shared resource Let s assume that T1 gets to step 1 first The first thing that happens when T1 tries to enter the critical section is that it checks to see if the critical section is available (that is, that no other thread is currently inside the critical section) Because that is the case, T1 enters the critical section and starts accessing the shared resource in step 2 Now, a context switch occurs, and T2 is
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allowed to run and gets to step 1 and tries to enter the critical section Because T1 already owns the critical section, T2 is instructed to wait at the critical section entry point until T1 leaves the critical section Another context switch occurs, and T1 finishes by executing step 3 and leaves the critical section At the next context switch, T2 enters the critical section and execution continues The way that a thread waits for a critical section to become available is different between single-processor and multiprocessor machines On single-processor machines, the thread really does go into an efficient wait state (kernel transition), whereas on multiprocessor machines, the thread might try to spin X number of times in hopes that the critical section will become available while spinning This is to avoid the expense of going into a wait state, which requires a kernel transition and context switch Let s take a closer look at the memory layout of a critical section The underlying critical section data structure is RTL_CRITICAL_SECTION and can be viewed by using the dt command:
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0:001> dt +0x000 +0x004 +0x008 +0x00c +0x010 +0x014 RTL_CRITICAL_SECTION DebugInfo : Ptr32 _RTL_CRITICAL_SECTION_DEBUG LockCount : Int4B RecursionCount : Int4B OwningThread : Ptr32 Void LockSemaphore : Ptr32 Void SpinCount : Uint4B
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The individual fields in the RTL_CRITICAL_SECTION structure are discussed in more detail here:
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DebugInfo The DebugInfo field is a system-allocated companion structure that contains
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an assortment of augmented information about the critical section (discussed later)
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LockCount
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This field indicates how many threads are waiting to acquire the critical section It is by default initialized to 1, which indicates that the critical section has not been acquired A value of 0 or more indicates that it has been acquired To find out how many other threads are waiting for the critical section, the following formula can be used: Number of waiting threads=LockCount-RecursionCount+1
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In Windows 2003 Server SP1 and later, this field has changed into a bit field to eliminate a very common problem with critical sections known as the lock convoy problem Later in the chapter, we take a closer look at what a lock convoy is and how to detect it
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