How to find out on which thread a blocked thread is waiting

First get the stack of the blocked thread by

0:002> kb
ChildEBP RetAddr Args to Child
00edfdd8 7c90e9c0 7c8025db 0000026c 00000000 ntdll!KiFastSystemCallRet
00edfddc 7c8025db 0000026c 00000000 00000000 ntdll!ZwWaitForSingleObject+0xc
00edfe40 7c802542 0000026c ffffffff 00000000 kernel32!WaitForSingleObjectEx+0xa8
00edfe54 6640114a 0000026c ffffffff 00813190 kernel32!WaitForSingleObject+0x12
[...]

The first parameter passed to WaitForSingleObject is the handle to the thread this thread is waiting for (Precondition: we are waiting for a thread and not another synchronisation object).


We can get more information about this handle by

0:002> !handle 0000026c f
Handle 0000026c
Type Thread
Attributes 0
GrantedAccess 0x1f03ff:
Delete,ReadControl,WriteDac,WriteOwner,Synch
Terminate,Suspend,Alert,GetContext,SetContext,SetInfo,QueryInfo,SetToken,Impersonate,DirectImpersonate
HandleCount 7
PointerCount 10
Name
Object specific information
Thread Id b94.ff4
Priority 3
Base Priority -16

Now we identified the questionable thread with b94.ff4