Synchronization in .NET framework

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all Keep in mind that the garbage collector is free to move the object around at any time, and the location of the object may change For the sake of brevity, the output in the following listing only lists the sync block step:
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0:000> dd 0x01e358a0-0x4 l1 01e3589c 08000001
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This time, the sync block index looks more reasonable (0x08000001) From our earlier discussion on the object header, we know that the presence of bitmask 0x0800000 indicates that we are dealing with a sync block index If we use the syncblk command specifying 0x1 as the argument, we can now see
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0:000> !syncblk 0x1 Index SyncBlock MonitorHeld Recursion Owning Thread Info 1 00322fd4 1 1 003044f0 462c 0 AdvancedNETDebugging6Simple --------------------------------Total 1 CCW 0 RCW 0 ComClassFactory 0 Free 0 SyncBlock Owner 01e358a0
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The output of the syncblk command contains two primary sections The first section includes information relative to the lock being held The meaning of the columns is shown in Table 6-2 Table 6-2 Syncblk Column Description
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Column Description
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Index SyncBlock MonitorHeld Recursion Owning thread info
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The sync block index Address to the sync block data structure (undocumented) Number of monitors held How many times one and the same thread has acquired the lock First item is the pointer to the internal thread data structure, the second item is the OS thread id, and the third item is the debugger thread id First item is the pointer to the object whose lock is being held, and the item is the type of the object on which the lock is second held
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The other section of interest is not directly related to locks, but rather contains the total number of sync blocks in the sync block table, the number of Runtime Callable Wrappers (RCW) as well as the total number of COM Callable Wrappers (CCW) Finally, the Free line indicates how many sync blocks are available in the sync block table
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Understanding what happens internally when a lock is acquired on an object is incredibly useful when dealing with common synchronization problems such as deadlocks The syncblk command can be used to quickly identify which thread is holding which lock to determine the source of the deadlock Now, before we move onto discussing some of the most common synchronization problems and how they can be debugged, we have yet to explain that mysterious line in Listing 6-3:
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thisGetHashCode();
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The reason the above statement was added to the sample code was to ensure that a sync block was created As mentioned before, the object header can be used to manage locks that have been acquired on the object However, that is not the only use of the object header It can also be used to store other information about the object (such as hash codes, COM interoperability data, and application domain index) If the object is not synchronized (ie, a lock has never been taken) and if the application didn t perform an operation causing the object header to be used, the sync block will simply not exist for that object In our previous example, by calling GetHashCode, we essentially forced the creation of a sync block to illustrate how it is managed by the CLR Can a lock be managed without the existence of a sync block Yes, CLR 20 introduced what is known as a thin lock to allow for a far more efficient mechanism of managing locks In the case of a thin lock, the only piece of information that is stored in the object header is the thread ID of the acquiring thread (ie, no sync block) Based on this single piece of information, we can also infer that the thin lock is simply a spinning lock (because more information would be required to implement a lock that would efficiently wait) The thin lock does not, however, spin indefinitely; rather, only until a threshold has been reached If the lock cannot be acquired as the threshold is passed, an actual sync block is created that allows the storage of information required to do an efficient wait (such as an event)
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