调试实战 | 使用 windbg 调试 DLL 卸载时的死锁

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前言

最近我们的程序在退出时会卡住，调查发现是在卸载dll时死锁了。大概流程是这样的：我们的dll在加载的时候会创建一个工作线程，在卸载的时候，会设置退出标志并等待之前开启的工作线程结束。为了研究这个经典的死锁问题，写了一个模拟程序，用到的dump文件及示例代码参考附件。

这也是几年前在项目中遇到的一个问题，我对之前的笔记进行了整理重新发布于此。

关键代码

主程序 WaitDllUnloadExe

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//WaitDllUnloadExe.cpp
#include "stdafx.h"
#include "windows.h"

int _tmain(int argc, _TCHAR* argv[])
{
  HMODULE module = LoadLibraryA(".\\DllUnload.dll");

  Sleep(5000);

  FreeLibrary(module);

  return 0;
}

DLL程序 DllUnload

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// dllmain.cpp
#include "stdafx.h"
#include "process.h"
HANDLE g_hThread;
bool g_quit = false;
unsigned __stdcall procThread(void *)
{
  while ( !g_quit )
  {
    OutputDebugStringA("procThread running.\n");
    Sleep(100);
  }

  OutputDebugStringA("==========================procThread quitting.\n");
  return 0;
}

unsigned __stdcall quitDemoProc(void *)
{
  int idx = 0;
  while ( idx++ < 5 )
  {
    OutputDebugStringA("quitDemoProc running!!!!!!!!.\n");
    Sleep(100);
  }

  OutputDebugStringA("--------------------------------------------------quitDemoProc quitting.\n");
  return 0;
}


BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
					 )
{
  switch (ul_reason_for_call)
  {
  case DLL_PROCESS_ATTACH:
    {
      g_hThread = (HANDLE)_beginthreadex(NULL, 0, &procThread, NULL, 0, NULL);
      CloseHandle((HANDLE)_beginthreadex(NULL, 0, &quitDemoProc, NULL, 0, NULL));
    }		
    break;
  case DLL_THREAD_ATTACH:
  case DLL_THREAD_DETACH:
    {
      OutputDebugStringA("------------DLL_THREAD_DETACH called.\n");
    }
    break;
  case DLL_PROCESS_DETACH:
    {
      OutputDebugStringA("------------DLL_PROCESS_DETACH begin wait...\n");
      g_quit = true;
      WaitForSingleObject(g_hThread, INFINITE);
      OutputDebugStringA("------------DLL_PROCESS_DETACH end wait...\n");
    }
    
    break;
  }
  return TRUE;
}

点我下载测试工程

分析

使用windbg打开dump文件。然后使用~kvn 列出所有线程的调用栈。

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0  Id: 1918.1924 Suspend: 1 Teb: 7efdd000 Unfrozen
 # ChildEBP RetAddr  Args to Child              
00 004af6f4 76150816 00000038 00000000 00000000 ntdll!NtWaitForSingleObject+0x15 (FPO: [3,0,0])
01 004af760 76781194 00000038 ffffffff 00000000 KERNELBASE!WaitForSingleObjectEx+0x98 (FPO: [Non-Fpo])
02 004af778 76781148 00000038 ffffffff 00000000 kernel32!WaitForSingleObjectExImplementation+0x75 (FPO: [Non-Fpo])
*** WARNING: Unable to verify checksum for DllUnload.dll
03 004af78c 6d0c15eb 00000038 ffffffff 00000000 kernel32!WaitForSingleObject+0x12 (FPO: [Non-Fpo])
04 004af86c 6d0c1e2b 6d0b0000 00000000 00000000 DllUnload!DllMain+0xdb (FPO: [Non-Fpo]) (CONV: stdcall) [c:\users\bianchengnan\documents\visual studio 2012\projects\waitdllunloadexe\dllunload\dllmain.cpp @ 55]
05 004af8b0 6d0c1d4f 6d0b0000 00000000 00000000 DllUnload!__DllMainCRTStartup+0xcb (FPO: [Non-Fpo]) (CONV: cdecl) [f:\dd\vctools\crt_bld\self_x86\crt\src\crtdll.c @ 508]
06 004af8c4 77139930 6d0b0000 00000000 00000000 DllUnload!_DllMainCRTStartup+0x1f (FPO: [Non-Fpo]) (CONV: stdcall) [f:\dd\vctools\crt_bld\self_x86\crt\src\crtdll.c @ 472]
07 004af8e4 77160000 6d0c10f0 6d0b0000 00000000 ntdll!LdrpCallInitRoutine+0x14
08 004af96c 77141221 6d0b0000 004af990 750227be ntdll!LdrpUnloadDll+0x375 (FPO: [Non-Fpo])
09 004af9b0 76151da7 6d0b0000 7efde000 004afaa4 ntdll!LdrUnloadDll+0x4a (FPO: [Non-Fpo])
*** WARNING: Unable to verify checksum for WaitDllUnloadExe.exe
0a 004af9c0 003a1425 6d0b0000 00000000 00000000 KERNELBASE!FreeLibrary+0x15 (FPO: [Non-Fpo])
0b 004afaa4 003a1989 00000001 0059a650 0059cf30 WaitDllUnloadExe!wmain+0x55 (FPO: [Non-Fpo]) (CONV: cdecl) [c:\users\bianchengnan\documents\visual studio 2012\projects\waitdllunloadexe\waitdllunloadexe\waitdllunloadexe.cpp @ 13]
0c 004afaf4 003a1b7d 004afb08 767833ca 7efde000 WaitDllUnloadExe!__tmainCRTStartup+0x199 (FPO: [Non-Fpo]) (CONV: cdecl) [f:\dd\vctools\crt_bld\self_x86\crt\src\crtexe.c @ 533]
0d 004afafc 767833ca 7efde000 004afb48 77139ed2 WaitDllUnloadExe!wmainCRTStartup+0xd (FPO: [Non-Fpo]) (CONV: cdecl) [f:\dd\vctools\crt_bld\self_x86\crt\src\crtexe.c @ 377]
0e 004afb08 77139ed2 7efde000 75022546 00000000 kernel32!BaseThreadInitThunk+0xe (FPO: [Non-Fpo])
0f 004afb48 77139ea5 003a107d 7efde000 00000000 ntdll!__RtlUserThreadStart+0x70 (FPO: [Non-Fpo])
10 004afb60 00000000 003a107d 7efde000 00000000 ntdll!_RtlUserThreadStart+0x1b (FPO: [Non-Fpo])

   1  Id: 1918.594 Suspend: 1 Teb: 7efda000 Unfrozen
 # ChildEBP RetAddr  Args to Child              
00 0090fc68 77138dd4 00000040 00000000 00000000 ntdll!NtWaitForSingleObject+0x15 (FPO: [3,0,0])
01 0090fccc 77138cb8 00000000 00000000 0059c5b8 ntdll!RtlpWaitOnCriticalSection+0x13e (FPO: [Non-Fpo])
02 0090fcf4 7715d349 772020c0 75d82382 00000000 ntdll!RtlEnterCriticalSection+0x150 (FPO: [Non-Fpo])
03 0090fd8c 7715d5c2 00000000 00000000 0090fdac ntdll!LdrShutdownThread+0x50 (FPO: [Non-Fpo])
04 0090fd9c 0f78e099 00000000 0059ec48 0090fde8 ntdll!RtlExitUserThread+0x2a (FPO: [Non-Fpo])
05 0090fdac 0f78e007 00000000 d910e7ee 00000000 MSVCR110D!_endthreadex+0x39 (FPO: [Non-Fpo])
06 0090fde8 0f78e1d1 0059ec48 0090fe00 767833ca MSVCR110D!_beginthreadex+0x1a7 (FPO: [Non-Fpo])
07 0090fdf4 767833ca 0059ec48 0090fe40 77139ed2 MSVCR110D!_endthreadex+0x171 (FPO: [Non-Fpo])
08 0090fe00 77139ed2 0059c5b8 75d8204e 00000000 kernel32!BaseThreadInitThunk+0xe (FPO: [Non-Fpo])
09 0090fe40 77139ea5 0f78e120 0059c5b8 00000000 ntdll!__RtlUserThreadStart+0x70 (FPO: [Non-Fpo])
0a 0090fe58 00000000 0f78e120 0059c5b8 00000000 ntdll!_RtlUserThreadStart+0x1b (FPO: [Non-Fpo])

#  2  Id: 1918.1960 Suspend: 1 Teb: 7efd7000 Unfrozen
 # ChildEBP RetAddr  Args to Child              
00 00a4f904 7719f826 75ec273a 00000000 00000000 ntdll!DbgBreakPoint (FPO: [0,0,0])
01 00a4f934 767833ca 00000000 00a4f980 77139ed2 ntdll!DbgUiRemoteBreakin+0x3c (FPO: [Non-Fpo])
02 00a4f940 77139ed2 00000000 75ec278e 00000000 kernel32!BaseThreadInitThunk+0xe (FPO: [Non-Fpo])
03 00a4f980 77139ea5 7719f7ea 00000000 00000000 ntdll!__RtlUserThreadStart+0x70 (FPO: [Non-Fpo])
04 00a4f998 00000000 7719f7ea 00000000 00000000 ntdll!_RtlUserThreadStart+0x1b (FPO: [Non-Fpo])

0号线程是主线程（线程id为1924），1号线程是子线程（线程id为594），2号线程（线程id为1960）是windbg插入的远程线程，用来中断到调试器。

0号线程在调用WaitForSingleObject时陷入了等待，我们来看它等什么。输入!handle 0x38 f

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!handle 0x38 f 
Handle 00000038
  Type         Thread
  Attributes   0
  GrantedAccess 0x1fffff:
         Delete,ReadControl,WriteDac,WriteOwner,Synch
         Terminate,Suspend,Alert,GetContext,SetContext,SetInfo,QueryInfo,SetToken,Impersonate,DirectImpersonate
  HandleCount   5
  PointerCount 8
  Name         <none>
  Object specific information
    Thread Id   1918.594
    Priority    10
    Base Priority 0

原来0号线程在等线程id为594的线程 。我们代码里确实有WaitForSingleObject(g_hThread, INFINITE); ，我们再来看看1号线程。从调用栈看来，1号线程已经在调用_endthreadex()准备关闭了，在关闭的过程中进入了一个关键段，并调用ntdll!NtWaitForSingleObject()进入等待。等待的句柄为0x40。输入!handle 0x40 f查看句柄的相关信息。

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0:002> !handle 0x40 f 
Handle 00000040
  Type          Event
  Attributes    0
  GrantedAccess 0x100003:
         Synch
         QueryState,ModifyState
  HandleCount   2
  PointerCount  4
  Name          <none>
  Object specific information
    Event Type Auto Reset
    Event is Waiting

我们发现句柄0x40对应的对象是Event，暂时先不管。使用万能死锁调试命令!cs -l看看（因为从调用堆栈来看1号线程是调用RtlEnterCriticalSection而死锁的。）

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0:002> !cs -l
-----------------------------------------
DebugInfo          = 0x77204360
Critical section   = 0x772020c0 (ntdll!LdrpLoaderLock+0x0)
LOCKED
LockCount          = 0x1
WaiterWoken        = No
OwningThread       = 0x00001924
RecursionCount     = 0x1
LockSemaphore      = 0x40
SpinCount          = 0x00000000

从输出结果可知，有一个锁住的关键段，被0号线程（线程id为0x00001924）拥有。而且这个死锁的关键段的成员LockSemaphore正是1号线程正在等待的句柄值。突然想起来《windows核心编程》上讲过关键段的结构，其中的LockSemaphore为Event类型的，具体参考第八章8.4节。

至此，终于真相大白了，0号线程在DllMain()内（ul_reason_for_call为DLL_PROCESS_DETACH）等待1号线程结束，而1号线程在结束的时候同样要调用DllMain()，并且ul_reason_for_call参数为DLL_THREAD_DETACH。由于对DllMain()的调用需要序列化，需要等待0号线程释放锁后，其它线程才能调用。而0号线程又在无限等待1号线程结束，故死锁。

注意：即使在DllMain()里调用DisableThreadLibraryCalls(hModule);也不管用，具体参考《windows核心编程》中的相关分析。

在winnt.h里找到了CriticalSection的定义，如下

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typedef struct _RTL_CRITICAL_SECTION {
    PRTL_CRITICAL_SECTION_DEBUG DebugInfo;

    //
    //  The following three fields control entering and exiting the critical
    //  section for the resource
    //

    LONG LockCount;
    LONG RecursionCount;
    HANDLE OwningThread;        // from the thread's ClientId->UniqueThread
    HANDLE LockSemaphore;
    ULONG_PTR SpinCount;        // force size on 64-bit systems when packed
} RTL_CRITICAL_SECTION, *PRTL_CRITICAL_SECTION;

总结

  • 不要在DllMain()里等待线程结束。

  • 使用!cs -l调试关键段死锁，真香。

参考资料

  • 《格蠹汇编》
  • 《windows核心编程》第八章（CriticalSection相关知识，尤其是8.4.1节） 第二十章（dll相关知识，尤其是20.2.5节）的相关内容。
  • Dynamic-Link Library Best Practices (Windows)
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