Software Diagnostics Library

We often say that a particular thread is blocked and/or it blocks other threads. At the same time we know that almost all threads are “blocked” to some degree except those currently running on processors. They are either preempted and in the ready lists, voluntary yielded their execution or they are waiting for some synchronization object. Therefore the notion of Blocked Thread is highly context and problem dependent and usually we notice them when comparing current thread stack traces with their expected normal stack traces. Here reference guides are indispensible especially those created for troubleshooting concrete products. Forthcoming Debugger Log Analyzer was partially incepted to address this problem.  

To show the diversity of “blocked” threads I propose the following thread classification:

Running threads

Their EIP (RIP) points to some function different from KiSwapContext:

3: kd> !running

System Processors f (affinity mask)
  Idle Processors 0

     Prcb      Current   Next   
  0  ffdff120  a30a9350            ................
  1  f7727120  a3186448            ................
  2  f772f120  a59a1b40            ................
  3  f7737120  a3085888            ................

3: kd> !thread a59a1b40
THREAD a59a1b40  Cid 0004.00b8  Teb: 00000000 Win32Thread: 00000000 RUNNING on processor 2
Not impersonating
DeviceMap                 e10028b0
Owning Process            a59aa648       Image:         System
Wait Start TickCount      1450446        Ticks: 1 (0:00:00:00.015)
Context Switch Count      308765            
UserTime                  00:00:00.000
KernelTime                00:00:01.250
Start Address nt!ExpWorkerThread (0×80880356)
Stack Init f7055000 Current f7054cec Base f7055000 Limit f7052000 Call 0
Priority 12 BasePriority 12 PriorityDecrement 0
ChildEBP RetAddr
f7054bc4 8093c55c nt!ObfReferenceObject+0×1c
f7054ca0 8093d2ae nt!ObpQueryNameString+0×2ba
f7054cbc 808f7d0f nt!ObQueryNameString+0×18
f7054d80 80880441 nt!IopErrorLogThread+0×197
f7054dac 80949b7c nt!ExpWorkerThread+0xeb
f7054ddc 8088e062 nt!PspSystemThreadStartup+0×2e
00000000 00000000 nt!KiThreadStartup+0×16

3: kd> .thread a59a1b40
Implicit thread is now a59a1b40

3: kd> r
Last set context:
eax=00000028 ebx=e1000228 ecx=e1002b30 edx=e1000234 esi=e1002b18 edi=0000001a
eip=8086c73e esp=f7054bc4 ebp=f7054ca0 iopl=0 nv up ei pl nz na po nc
cs=0008 ss=0010 ds=0023 es=0023 fs=0030 gs=0000 efl=00000202
nt!ObfReferenceObject+0×1c:
8086c73e 40              inc     eax
 
These threads can also be identified by RUNNING attribute in the output of !process 0 ff command applied for complete and kernel memory dumps.

Ready threads  

These threads can be seen in the output of !ready command or identified by READY attribute in the output of !process 0 ff command:

3: kd> !ready
Processor 0: No threads in READY state
Processor 1: Ready Threads at priority 11
    THREAD a3790790  Cid 0234.1108  Teb: 7ffab000 Win32Thread: 00000000 READY
    THREAD a32799a8  Cid 0234.061c  Teb: 7ff83000 Win32Thread: 00000000 READY
    THREAD a3961798  Cid 0c04.0c68  Teb: 7ffab000 Win32Thread: bc204ea8 READY
Processor 1: Ready Threads at priority 10
    THREAD a32bedb0  Cid 1fc8.1a30  Teb: 7ffad000 Win32Thread: bc804468 READY
Processor 1: Ready Threads at priority 9
    THREAD a52dcd48  Cid 0004.04d4  Teb: 00000000 Win32Thread: 00000000 READY
Processor 2: Ready Threads at priority 11
    THREAD a37fedb0  Cid 0c04.11f8  Teb: 7ff8e000 Win32Thread: 00000000 READY
Processor 3: Ready Threads at priority 11
    THREAD a5683db0  Cid 0234.0274  Teb: 7ffd6000 Win32Thread: 00000000 READY
    THREAD a3151b48  Cid 0234.2088  Teb: 7ff88000 Win32Thread: 00000000 READY
    THREAD a5099d80  Cid 0ecc.0d60  Teb: 7ffd4000 Win32Thread: 00000000 READY
    THREAD a3039498  Cid 0c04.275c  Teb: 7ff7d000 Win32Thread: 00000000 READY

If we look at these threads we would see that they were either scheduled to run because of a signaled object they were waiting for:

3: kd> !thread a3039498
THREAD a3039498  Cid 0c04.275c  Teb: 7ff7d000 Win32Thread: 00000000 READY
IRP List:
    a2feb008: (0006,0094) Flags: 00000870  Mdl: 00000000
Not impersonating
DeviceMap                 e10028b0
Owning Process            a399a770       Image:         svchost.exe
Wait Start TickCount      1450447        Ticks: 0
Context Switch Count      1069            
UserTime                  00:00:00.000
KernelTime                00:00:00.000
Win32 Start Address 0x001e4f22
LPC Server thread working on message Id 1e4f22
Start Address KERNEL32!BaseThreadStartThunk (0x77e617ec)
Stack Init f171b000 Current f171ac60 Base f171b000 Limit f1718000 Call 0
Priority 11 BasePriority 10 PriorityDecrement 0
ChildEBP RetAddr  Args to Child             
f171ac78 80833465 a3039498 a3039540 e7561930 nt!KiSwapContext+0x26
f171aca4 80829a62 00000000 00000000 00000000 nt!KiSwapThread+0x2e5
f171acec 80938d0c a301cad8 00000006 f171ad01 nt!KeWaitForSingleObject+0×346
f171ad50 8088978c 00000c99 00000000 00000000 nt!NtWaitForSingleObject+0×9a
f171ad50 7c8285ec 00000c99 00000000 00000000 nt!KiFastCallEntry+0xfc
03d9efa8 00000000 00000000 00000000 00000000 ntdll!KiFastSystemCallRet

3: kd> !object a301cad8
Object: a301cad8  Type: (a59a0720) Event
    ObjectHeader: a301cac0 (old version)
    HandleCount: 1  PointerCount: 3

Or they were boosted in priority:

3: kd> !thread a3790790
THREAD a3790790  Cid 0234.1108  Teb: 7ffab000 Win32Thread: 00000000 READY
IRP List:
    a2f8b7f8: (0006,0094) Flags: 00000900  Mdl: 00000000
Not impersonating
DeviceMap                 e10028b0
Owning Process            a554bcc8       Image:         lsass.exe
Wait Start TickCount      1450447        Ticks: 0
Context Switch Count      384            
UserTime                  00:00:00.000
KernelTime                00:00:00.000
Win32 Start Address RPCRT4!ThreadStartRoutine (0x77c7b0f5)
Start Address KERNEL32!BaseThreadStartThunk (0x77e617ec)
Stack Init f3ac1000 Current f3ac0ce8 Base f3ac1000 Limit f3abe000 Call 0
Priority 11 BasePriority 10 PriorityDecrement 0
ChildEBP RetAddr  Args to Child             
f3ac0d00 80831266 a3790790 a50f1870 a3186448 nt!KiSwapContext+0x26
f3ac0d20 8082833a 00000000 a50f1870 8098b56c nt!KiExitDispatcher+0xf8
f3ac0d3c 8098b5b9 a50f1870 00000000 00f5f8d0 nt!KeSetEventBoostPriority+0×156
f3ac0d58 8088978c a50f1870 00f5f8d4 7c8285ec nt!NtSetEventBoostPriority+0×4d
f3ac0d58 7c8285ec a50f1870 00f5f8d4 7c8285ec nt!KiFastCallEntry+0xfc
00f5f8d4 00000000 00000000 00000000 00000000 ntdll!KiFastSystemCallRet

3: kd> !object a50f1870
Object: a50f1870  Type: (a59a0720) Event
    ObjectHeader: a50f1858 (old version)
    HandleCount: 1  PointerCount: 15

Or were interrupted and queued to be run again:

3: kd> !thread a5683db0
THREAD a5683db0 Cid 0234.0274 Teb: 7ffd6000 Win32Thread: 00000000 READY
IRP List:
  a324d498: (0006,0094) Flags: 00000900 Mdl: 00000000
  a2f97a20: (0006,0094) Flags: 00000900 Mdl: 00000000
  a50c3e70: (0006,0190) Flags: 00000000 Mdl: a50a22d0
  a5167750: (0006,0094) Flags: 00000800 Mdl: 00000000
Not impersonating
DeviceMap e10028b0
Owning Process a554bcc8 Image: lsass.exe
Wait Start TickCount 1450447 Ticks: 0
Context Switch Count 9619
UserTime 00:00:00.156
KernelTime 00:00:00.234
Win32 Start Address RPCRT4!ThreadStartRoutine (0x77c7b0f5)
Start Address KERNEL32!BaseThreadStartThunk (0x77e617ec)
Stack Init f59f3000 Current f59f2d00 Base f59f3000 Limit f59f0000 Call 0
Priority 11 BasePriority 10 PriorityDecrement 0
ChildEBP RetAddr
f59f2d18 80a5c1ae nt!KiDispatchInterrupt+0xb1
f59f2d48 80a5c577 hal!HalpDispatchSoftwareInterrupt+0x5e
f59f2d54 80a59902 hal!HalEndSystemInterrupt+0x67
f59f2d54 77c6928d hal!HalpIpiHandler+0xd2 (TrapFrame @ f59f2d64)
00c5f908 00000000 RPCRT4!OSF_SCALL::GetBuffer+0×37

3: kd> .thread a5683db0
Implicit thread is now a5683db0

3: kd> r
Last set context:
eax=00000000 ebx=00000000 ecx=00000000 edx=00000000 esi=00000000 edi=00000000
eip=8088dba1 esp=f59f2d0c ebp=f59f2d2c iopl=0 nv up di pl nz na po nc
cs=0008 ss=0010 ds=0000 es=0000 fs=0000 gs=0000 efl=00000000
nt!KiDispatchInterrupt+0xb1:
8088dba1 b902000000 mov ecx,2

3: kd> ub
nt!KiDispatchInterrupt+0x8f:
8088db7f mov dword ptr [ebx+124h],esi
8088db85 mov byte ptr [esi+4Ch],2
8088db89 mov byte ptr [edi+5Ah],1Fh
8088db8d mov ecx,edi
8088db8f lea edx,[ebx+120h]
8088db95 call nt!KiQueueReadyThread (80833490)
8088db9a mov cl,1
8088db9c call nt!SwapContext (8088dbd0)

3: kd> u
nt!KiDispatchInterrupt+0xb1:
8088dba1 mov ecx,2
8088dba6 call dword ptr [nt!_imp_KfLowerIrql (80801108)]
8088dbac mov ebp,dword ptr [esp]
8088dbaf mov edi,dword ptr [esp+4]
8088dbb3 mov esi,dword ptr [esp+8]
8088dbb7 add esp,0Ch
8088dbba pop ebx
8088dbbb ret

We can get user space thread stack by using .trap command but we need to switch to its process context first:

3: kd> .process /r /p a554bcc8
Implicit process is now a554bcc8
Loading User Symbols

3: kd> kL
  *** Stack trace for last set context - .thread/.cxr resets it
ChildEBP RetAddr
00c5f908 77c7ed60 RPCRT4!OSF_SCALL::GetBuffer+0x37
00c5f924 77c7ed14 RPCRT4!I_RpcGetBufferWithObject+0x7f
00c5f934 77c7f464 RPCRT4!I_RpcGetBuffer+0xf
00c5f944 77ce3470 RPCRT4!NdrGetBuffer+0x2e
00c5fd44 77ce35c4 RPCRT4!NdrStubCall2+0x35c
00c5fd60 77c7ff7a RPCRT4!NdrServerCall2+0x19
00c5fd94 77c8042d RPCRT4!DispatchToStubInCNoAvrf+0x38
00c5fde8 77c80353 RPCRT4!RPC_INTERFACE::DispatchToStubWorker+0x11f
00c5fe0c 77c68e0d RPCRT4!RPC_INTERFACE::DispatchToStub+0xa3
00c5fe40 77c68cb3 RPCRT4!OSF_SCALL::DispatchHelper+0x149
00c5fe54 77c68c2b RPCRT4!OSF_SCALL::DispatchRPCCall+0x10d
00c5fe84 77c68b5e RPCRT4!OSF_SCALL::ProcessReceivedPDU+0x57f
00c5fea4 77c6e8db RPCRT4!OSF_SCALL::BeginRpcCall+0x194
00c5ff04 77c6e7b4 RPCRT4!OSF_SCONNECTION::ProcessReceiveComplete+0x435
00c5ff18 77c7b799 RPCRT4!ProcessConnectionServerReceivedEvent+0x21
00c5ff84 77c7b9b5 RPCRT4!LOADABLE_TRANSPORT::ProcessIOEvents+0x1b8
00c5ff8c 77c8872d RPCRT4!ProcessIOEventsWrapper+0xd
00c5ffac 77c7b110 RPCRT4!BaseCachedThreadRoutine+0x9d
00c5ffb8 77e64829 RPCRT4!ThreadStartRoutine+0x1b
00c5ffec 00000000 kernel32!BaseThreadStart+0x34

Waiting threads (wait originated from user space)

THREAD a34369d0  Cid 1fc8.1e88  Teb: 7ffae000 Win32Thread: bc6d5818 WAIT: (Unknown) UserMode Non-Alertable
    a34d9940  SynchronizationEvent
    a3436a48  NotificationTimer
Not impersonating
DeviceMap                 e12256a0
Owning Process            a3340a10       Image:         IEXPLORE.EXE
Wait Start TickCount      1450409        Ticks: 38 (0:00:00:00.593)
Context Switch Count      7091                 LargeStack
UserTime                  00:00:01.015
KernelTime                00:00:02.250
Win32 Start Address mshtml!CExecFT::StaticThreadProc (0x7fab1061)
Start Address kernel32!BaseThreadStartThunk (0x77e617ec)
Stack Init f252b000 Current f252ac60 Base f252b000 Limit f2528000 Call 0
Priority 11 BasePriority 10 PriorityDecrement 0
ChildEBP RetAddr 
f252ac78 80833465 nt!KiSwapContext+0x26
f252aca4 80829a62 nt!KiSwapThread+0x2e5
f252acec 80938d0c nt!KeWaitForSingleObject+0x346
f252ad50 8088978c nt!NtWaitForSingleObject+0x9a
f252ad50 7c8285ec nt!KiFastCallEntry+0xfc (TrapFrame @ f252ad64)
030dff08 7c827d0b ntdll!KiFastSystemCallRet
030dff0c 77e61d1e ntdll!NtWaitForSingleObject+0xc
030dff7c 77e61c8d kernel32!WaitForSingleObjectEx+0xac
030dff90 7fab08a3 kernel32!WaitForSingleObject+0×12
030dffa8 7fab109c mshtml!CDwnTaskExec::ThreadExec+0xae
030dffb0 7fab106e mshtml!CExecFT::ThreadProc+0×28
030dffb8 77e64829 mshtml!CExecFT::StaticThreadProc+0xd
030dffec 00000000 kernel32!BaseThreadStart+0×34

If we had taken user dump of iexplore.exe we would have seen the following stack trace there:

030dff08 7c827d0b ntdll!KiFastSystemCallRet
030dff0c 77e61d1e ntdll!NtWaitForSingleObject+0xc
030dff7c 77e61c8d kernel32!WaitForSingleObjectEx+0xac
030dff90 7fab08a3 kernel32!WaitForSingleObject+0×12
030dffa8 7fab109c mshtml!CDwnTaskExec::ThreadExec+0xae
030dffb0 7fab106e mshtml!CExecFT::ThreadProc+0×28
030dffb8 77e64829 mshtml!CExecFT::StaticThreadProc+0xd
030dffec 00000000 kernel32!BaseThreadStart+0×34

Another example:

THREAD a31f2438  Cid 1fc8.181c  Teb: 7ffaa000 Win32Thread: 00000000 WAIT: (Unknown) UserMode Non-Alertable
    a30f8c20  NotificationEvent
    a5146720  NotificationEvent
    a376fbb0  NotificationEvent
Not impersonating
DeviceMap                 e12256a0
Owning Process            a3340a10       Image:         IEXPLORE.EXE
Wait Start TickCount      1419690        Ticks: 30757 (0:00:08:00.578)
Context Switch Count      2            
UserTime                  00:00:00.000
KernelTime                00:00:00.000
Win32 Start Address USERENV!NotificationThread (0x76929dd9)
Start Address kernel32!BaseThreadStartThunk (0x77e617ec)
Stack Init f5538000 Current f5537900 Base f5538000 Limit f5535000 Call 0
Priority 10 BasePriority 10 PriorityDecrement 0
Kernel stack not resident.
ChildEBP RetAddr 
f5537918 80833465 nt!KiSwapContext+0x26
f5537944 80829499 nt!KiSwapThread+0x2e5
f5537978 80938f68 nt!KeWaitForMultipleObjects+0x3d7
f5537bf4 809390ca nt!ObpWaitForMultipleObjects+0x202
f5537d48 8088978c nt!NtWaitForMultipleObjects+0xc8
f5537d48 7c8285ec nt!KiFastCallEntry+0xfc (TrapFrame @ f5537d64)
0851fec0 7c827cfb ntdll!KiFastSystemCallRet
0851fec4 77e6202c ntdll!NtWaitForMultipleObjects+0xc
0851ff6c 77e62fbe kernel32!WaitForMultipleObjectsEx+0x11a
0851ff88 76929e35 kernel32!WaitForMultipleObjects+0×18
0851ffb8 77e64829 USERENV!NotificationThread+0×5f
0851ffec 00000000 kernel32!BaseThreadStart+0×34

Waiting threads (wait originated from kernel space)

Examples include explicit wait as a result from calling potentially blocking API:

THREAD a33a9740  Cid 1980.1960  Teb: 7ffde000 Win32Thread: bc283ea8 WAIT: (Unknown) UserMode Non-Alertable
    a35e3168  SynchronizationEvent
Not impersonating
DeviceMap                 e689f298
Owning Process            a342d3a0       Image:         explorer.exe
Wait Start TickCount      1369801        Ticks: 80646 (0:00:21:00.093)
Context Switch Count      1667                 LargeStack
UserTime                  00:00:00.015
KernelTime                00:00:00.093
Win32 Start Address Explorer!ModuleEntry (0x010148a4)
Start Address kernel32!BaseProcessStartThunk (0x77e617f8)
Stack Init f258b000 Current f258ac50 Base f258b000 Limit f2585000 Call 0
Priority 13 BasePriority 10 PriorityDecrement 1
Kernel stack not resident.
ChildEBP RetAddr 
f258ac68 80833465 nt!KiSwapContext+0x26
f258ac94 80829a62 nt!KiSwapThread+0x2e5
f258acdc bf89abd3 nt!KeWaitForSingleObject+0×346
f258ad38 bf89da43 win32k!xxxSleepThread+0×1be
f258ad4c bf89e401 win32k!xxxRealWaitMessageEx+0×12
f258ad5c 8088978c win32k!NtUserWaitMessage+0×14
f258ad5c 7c8285ec nt!KiFastCallEntry+0xfc (TrapFrame @ f258ad64)
0007feec 7739bf53 ntdll!KiFastSystemCallRet
0007ff08 7c8fadbd USER32!NtUserWaitMessage+0xc
0007ff14 0100fff1 SHELL32!SHDesktopMessageLoop+0×24
0007ff5c 0101490c Explorer!ExplorerWinMain+0×2c4
0007ffc0 77e6f23b Explorer!ModuleEntry+0×6d
0007fff0 00000000 kernel32!BaseProcessStart+0×23

and implicit wait when a thread yields execution to another thread voluntarily via explicit context swap:

THREAD a3072b68  Cid 1fc8.1d94  Teb: 7ffaf000 Win32Thread: bc1e3c20 WAIT: (Unknown) UserMode Non-Alertable
    a37004d8  QueueObject
    a3072be0  NotificationTimer
IRP List:
    a322be00: (0006,01fc) Flags: 00000000  Mdl: a30b8e30
    a30bcc38: (0006,01fc) Flags: 00000000  Mdl: a35bf530
Not impersonating
DeviceMap                 e12256a0
Owning Process            a3340a10       Image:         IEXPLORE.EXE
Wait Start TickCount      1447963        Ticks: 2484 (0:00:00:38.812)
Context Switch Count      3972                 LargeStack
UserTime                  00:00:00.140
KernelTime                00:00:00.250
Win32 Start Address ntdll!RtlpWorkerThread (0x7c839efb)
Start Address kernel32!BaseThreadStartThunk (0x77e617ec)
Stack Init f1cc3000 Current f1cc2c38 Base f1cc3000 Limit f1cbf000 Call 0
Priority 10 BasePriority 10 PriorityDecrement 0
ChildEBP RetAddr 
f1cc2c50 80833465 nt!KiSwapContext+0x26
f1cc2c7c 8082b60f nt!KiSwapThread+0x2e5
f1cc2cc4 808ed620 nt!KeRemoveQueue+0×417
f1cc2d48 8088978c nt!NtRemoveIoCompletion+0xdc
f1cc2d48 7c8285ec nt!KiFastCallEntry+0xfc (TrapFrame @ f1cc2d64)
06ceff70 7c8277db ntdll!KiFastSystemCallRet
06ceff74 7c839f38 ntdll!ZwRemoveIoCompletion+0xc
06ceffb8 77e64829 ntdll!RtlpWorkerThread+0×3d
06ceffec 00000000 kernel32!BaseThreadStart+0×34

THREAD a3612020  Cid 1980.1a48  Teb: 7ffd9000 Win32Thread: 00000000 WAIT: (Unknown) UserMode Alertable
    a3612098  NotificationTimer
Not impersonating
DeviceMap                 e689f298
Owning Process            a342d3a0       Image:         explorer.exe
Wait Start TickCount      1346718        Ticks: 103729 (0:00:27:00.765)
Context Switch Count      4            
UserTime                  00:00:00.000
KernelTime                00:00:00.000
Win32 Start Address ntdll!RtlpTimerThread (0x7c83d3dd)
Start Address kernel32!BaseThreadStartThunk (0x77e617ec)
Stack Init f2453000 Current f2452c80 Base f2453000 Limit f2450000 Call 0
Priority 10 BasePriority 10 PriorityDecrement 0
Kernel stack not resident.
ChildEBP RetAddr 
f2452c98 80833465 nt!KiSwapContext+0x26
f2452cc4 80828f0b nt!KiSwapThread+0x2e5
f2452d0c 80994812 nt!KeDelayExecutionThread+0×2ab
f2452d54 8088978c nt!NtDelayExecution+0×84
f2452d54 7c8285ec nt!KiFastCallEntry+0xfc (TrapFrame @ f2452d64)
0149ff9c 7c826f4b ntdll!KiFastSystemCallRet
0149ffa0 7c83d424 ntdll!NtDelayExecution+0xc
0149ffb8 77e64829 ntdll!RtlpTimerThread+0×47
0149ffec 00000000 kernel32!BaseThreadStart+0×34

Explicit waits in kernel can be originated from GUI threads and their message loops, for example, Main Thread. Blocked GUI Thread pattern can be seen as an example of a genuine Blocked Thread. Some “blocked” threads are just really Passive Threads.

- Dmitry Vostokov @ DumpAnalysis.org -

This entry was posted on Wednesday, February 27th, 2008 at 1:26 pm and is filed under Crash Dump Analysis, Crash Dump Patterns, Debugging, Stack Trace Collection. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.