FIX: Deadlock in OLE DB Session Pooling in an STA COM Object (259753)

   Date         Time    Version       Size      File name     Platform
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   12/06/2000   19:41   2.60.7006.0   459,024   Oledb32.dll   x86 
				

MORE INFORMATION

Method calls to an STA COM object can be re-entered on the same thread, in much the same way as a recursive function call (where the same thread of execution can re-enter the same function). For example, if you peek at the Windows message loop by using the Win32 API call PeekMessage inside an STA COM object method, this can cause a second pending method call to re-enter the same STA COM object method on the same thread of execution. This is the design of STA COM objects and is not a bug. Under normal circumstances, this does not cause a deadlock, and both method calls execute normally.

In this specific case, the OLE DB code internally calls the Win32 API function CoCreateInstance to instantiate an internally used COM object. In certain circumstances, this can trigger STA COM object method call re-entrancy on Windows NT 4.0. In this particular case, a deadlock occurs in the OLE DB code because OLE DB obtains an internal lock while processing the first method call and then triggers STA re-entrancy while this lock is held. The original lock held by the first method call blocks the same thread inside of the second method call from obtaining another dependent lock in the OLE DB code. Because the first lock is held by the same thread, this lock will never be released, and this blocks other threads in the same process from using OLE DB.

The occurrence of this deadlock is rare because it requires a specific series of events to occur at precisely the same time. To determine if this specific problem has occurred, attach a debugger to the process and look for a call stack that enters some COM method X, then enters OLE DB code, then re-enters the same method X further up the stack, and then goes back into OLE DB code. The top of the stack of the deadlocked thread will contain the following function calls:

REFERENCES

For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base: