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Listing 1020
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Old Critical Section +0x000 DebugInfo +0x004 LockCount +0x008 RecursionCount +0x00c OwningThread +0x010 LockSemaphore +0x014 SpinCount New Critical Section +0x000 DebugInfo +0x004 LockCount +0x008 RecursionCount +0x00c OwningThread +0x010 LockSemaphore +0x014 SpinCount : : : : : : 0x7c97c8c0 1 1 0x00000b04 0x000007f4 0
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0x77ca5be0 -6 1 0x00000134 0x00000010 0
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To make it easier to understand the LockCount field of the new critical sections, let s look at a binary breakdown of the LockCount field of the critical section in Listing 1020 To get a binary representation of the LockCount field, we use the dyd command:
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0:003> dyd 01002008+0x4 3 2 1 10987654 32109876 54321098 0100200c 11111111 11111111 11111111 01002010 00000000 00000000 00000000 01002014 00000000 00000000 00000001 01002018 00000000 00000000 00000000 0100201c 00000000 00000000 00000000 01002020 00000000 00000000 00000000 01002024 00000000 00000000 00000000 01002028 00000000 00000000 00000000 0 76543210
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11111010 00000001 11111000 00010000 00000000 00000000 00000000 00000000
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The first thing to remember is that the bits are all inverted, so prior to trying to figure out the lock state, we need to flip the bits again:
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00000000 00000000 00000000 00000101
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The Lock Bit (bit 0) is 1, which means that the critical section is locked The Waiter Woken bit (bit 1) is 0, which means that no waiters have yet been woken The remaining 30 bits is the number of waiters, which in our case is 1 Fortunately, the !cs
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extension command has been updated to recognize this change on the affected platforms and can display the information in a friendlier way For example, if we used the same critical section as preceding as a parameter to the !cs extension command, we would see the following:
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0:003> !cs 01002008 --------------------Critical section = 0x01002008 (10LockConv!g_cs+0x0) DebugInfo = 0x77965be0 LOCKED LockCount = 0x1 WaiterWoken = No OwningThread = 0x000001f8 RecursionCount = 0x1 LockSemaphore = 0x10 SpinCount = 0x00000000
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The critical section is in a LOCKED state, has one waiter, and no waiters have been woken
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Direct Usage of Critical Section Fields
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Because the critical section structure is directly available to the developers, there have been cases in which developers make assumptions about the meaning of the fields One such example is the LockCount field, where application code uses the LockCount field directly to see if the critical section is free or being held It should go without saying that looking at the critical section fields directly rather than going through the critical section API(s) is a big no-no Although the structure itself is documented, Microsoft reserves the right to change that structure in any shape it deems necessary If you have code that directly manipulates the critical section fields, you run the risk of your application not working between Windows versions
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Managing Critical Sections
Properly managing critical sections is just as important as managing any other form of resource Mismanagement can lead to devastating situations that are extremely difficult and time-consuming to debug In this part of the chapter, we take a look at some of the most common problems when managing critical sections
10
Synchronization
The four most common types of problems seen when managing critical sections are
Use of a critical section before initialization Use of a critical section after deletion Overreleasing the critical section Underreleasing the critical section
To understand the first problem, usage of a critical section before initialization, we must first be able to identify what a properly initialized critical section resembles Listing 1021 shows the contents of an initialized critical section Listing 1021
+0x000 DebugInfo : 0x7c97c8c0 +0x000 Type : 0 +0x002 CreatorBackTraceIndex : 0 +0x004 CriticalSection : 0x0007ff2c +0x008 ProcessLocksList : _LIST_ENTRY [ 0x7c97c0c8 - 0x7c97c8a8 ] +0x010 EntryCount : 0 +0x014 ContentionCount : 0 +0x018 Spare : [2] 0 +0x004 LockCount : -1 +0x008 RecursionCount : 0 +0x00c OwningThread : (null) +0x010 LockSemaphore : (null) +0x014 SpinCount : 0
All fields of the critical section in Listing 1021 have proper default values, and the CriticalSection field of the DebugInfo structure points back to the actual critical section structure Now, imagine that a thread had managed to call EnterCriticalSection before it had been initialized The net result is most likely an access violation with a stack trace, as follows:
ChildEBP 0007fe98 0007ff18 0007ff20 0007ff44 0007ffc0 0007fff0 RetAddr 7c94243c 7c90104b 01001152 010012a0 7c816fd7 00000000 Args to Child c0000008 00090000 0007ff2c 01001152 0007ff2c 0007ff50 00000001 00034e50 00090000 0007fa0c 01001171 00000000 0007fa0c 0007ff2c 77c3ac60 00033338 7ffde000 78746341 ntdll!RtlRaiseStatus+0x26 ntdll!RtlpWaitForCriticalSection+0x204 ntdll!RtlEnterCriticalSection+0x46 simple!main+0x12 simple!mainCRTStartup+0x12f kernel32!BaseProcessStart+0x23