How can I find the reason for a hung finalizer queue?

Question

I have an application that experiences a slow memory leak from the word go.

Using ANTS Memory Profiler I can see that all of the leaked memory is being held by the finalizer queue's GC root.

I suspect what may have happened is that the finalizer is deadlocked waiting on a lock to become available.

None of our classes implement explicit finalizers, we avoid them as a rule, this makes me think the lock might related to a system or library class.

I've used SOS.dll to take a look at the contents of the finalizer queue and if I am interpreting it correctly then it reports the first item to be an instance System.Threading.Thread However I am unsure if the head of the queue actually represents the currently being disposed object or the next object to be disposed.

• Are there any tricks I can use to find out what is being finalized?
• Is there a way I can find out what lock the finalizer thread is waiting on?
• Can any extra debugging be turned on to trace the finalizer thread's actions?
• What else can I look at?

Update

The finalizer thread's stack appears as follows:

ntdll.dll!_ZwWaitForSingleObject@12()  + 0x15 bytes  
ntdll.dll!_ZwWaitForSingleObject@12()  + 0x15 bytes  
user32.dll!_NtUserPostMessage@16()  + 0x15 bytes     

kernel32.dll!_WaitForSingleObjectExImplementation@12()  + 0x43 bytes     
kernel32.dll!_WaitForSingleObject@8()  + 0x12 bytes  
ole32.dll!GetToSTA()  + 0x72 bytes   

ole32.dll!CRpcChannelBuffer::SwitchAptAndDispatchCall()  - 0x1939 bytes  
ole32.dll!CRpcChannelBuffer::SendReceive2()  + 0xa6 bytes    
ole32.dll!CAptRpcChnl::SendReceive()  + 0x5b7 bytes  
ole32.dll!CCtxComChnl::SendReceive()  - 0x14b97 bytes    
ole32.dll!NdrExtpProxySendReceive()  + 0x43 bytes    
rpcrt4.dll!@NdrpProxySendReceive@4()  + 0xe bytes    
rpcrt4.dll!_NdrClientCall2()  + 0x144 bytes  
ole32.dll!_ObjectStublessClient@8()  + 0x7a bytes    
ole32.dll!_ObjectStubless@0()  + 0xf bytes   

ole32.dll!CObjectContext::InternalContextCallback()  - 0x511f bytes  
ole32.dll!CObjectContext::ContextCallback()  + 0x8f bytes    
clr.dll!CtxEntry::EnterContext()  + 0x119 bytes  

clr.dll!RCWCleanupList::ReleaseRCWListInCorrectCtx()  + 0x2bb bytes  

clr.dll!RCWCleanupList::CleanupAllWrappers()  - 0x20fb0 bytes    
clr.dll!SyncBlockCache::CleanupSyncBlocks()  + 0x1ec6 bytes  
clr.dll!Thread::DoExtraWorkForFinalizer()  + 0x411b5 bytes   

clr.dll!WKS::GCHeap::FinalizerThreadWorker()  + 0x8b bytes   
clr.dll!Thread::DoExtraWorkForFinalizer()  + 0xb6e76 bytes   
clr.dll!Thread::ShouldChangeAbortToUnload()  - 0x5f8 bytes   
clr.dll!Thread::ShouldChangeAbortToUnload()  - 0x53d bytes   
clr.dll!ManagedThreadBase_NoADTransition()  + 0x35 bytes     
clr.dll!ManagedThreadBase::FinalizerBase()  + 0xf bytes  
clr.dll!WKS::GCHeap::FinalizerThreadStart()  + 0xfb bytes    
clr.dll!Thread::intermediateThreadProc()  + 0x48 bytes   
kernel32.dll!@BaseThreadInitThunk@12()  + 0x12 bytes     
ntdll.dll!___RtlUserThreadStart@8()  + 0x27 bytes    
ntdll.dll!__RtlUserThreadStart@8()  + 0x1b bytes     

Answer 1

Looks to me you are having a problem with a COM server. The call stack shows it is trying to make the IUnknown::Release() call on a single-threaded COM object. The ReleaseRCWListInCorrectCtx() call sets it off, the _NtUserPostMessage@16() is the call that marshals the request to the STA that owns the COM object.

The typical cause is creating COM objects and not pumping a message loop. A hard requirement for STA threads. You avoid it by creating them on the main UI thread and never blocking it.