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the critical section as it is leaving it During this time, the critical section is still considered locked, but the releasing code clears the OwningThread field until the new thread enters the critical section at which point, it gets properly reset Because lock convoys can cause serious issues in your code, what are some of the mitigation techniques Unfortunately, mitigation techniques are not easy to come by for this problem The best answer is to simply redesign the application and remove the highly contentious lock Unfortunately, this is not always possible Another possible mitigation is to rely on spin counts when trying to acquire the critical section Using this technique can dramatically reduce the amount of time it takes to pass ownership (because a context switch is not required), and the convoy might be broken Unfortunately, this approach only works on multiprocessor machines The problem of lock convoys was deemed important enough that the Windows team decided to make a dramatic change to how critical sections worked in Windows 2003 Server SP1 Because the fundamental problem of lock convoys is that the owning thread has to pass ownership to the next thread in line (to honor the basic design goal of a fair lock), Microsoft decided to make a design change to the critical section and remove the fairness requirement If a thread does not have to pass ownership, a minimal amount of post-critical section release time would be spent, thereby eliminating the lock convoy problem A side effect stemming from removing fairness is that a thread that is woken up to enter the critical section would have to compete with brand new incoming threads also trying to compete for it, potentially resulting in thread starvation Nevertheless, the lock convoy problem was deemed important enough and trumped the potential of thread starvation To accommodate this change, a change had to be made for how the critical section is structured, and the approach agreed upon was to use the LockCount field to add the additional information required for an unfair lock More specifically, the LockCount has changed as shown in Figure 105
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High 30 bits indicate the count of waiting threads
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Waiter Woken bit
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Figure 105 All bits in the new LockCount field are now also stored in an inverted state, so when looking at the LockCount field in the debugger, it is important to flip the bits prior to making assumptions about its state As Figure 105 indicates, the high 30 bits of the LockCount field are pretty straightforward and contain the number of waiting threads
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Bit 1 is the Waiter Woken bit and indicates whether a thread that released the critical section has signaled the event to wake up another thread If the bit is set, it serves as an indication that another thread leaving the critical section should not set it as well The least significant bit is the Lock bit, and it indicates whether the critical section is in a locked state Let s take a look at a few examples (see Listings 1018, 1019, and 1020) of what the LockCount field resembles with critical sections in various states (Remember you must be running on Windows 2003 Server SP1 at a minimum) Listing 1018
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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 0 (null) (null) 0
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0x77ca5be0 -1 0 (null) (null) 0
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Listing 1019
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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 0 1 0x00000b04 (null) 0
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0x77ca5be0 -2 1 0x00000134 (null) 0
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