1. 讲故事

前段时间有位朋友微信上找到我，说它的程序出现了卡死，让我帮忙看下是怎么回事？ 说来也奇怪，那段时间求助卡死类的dump特别多，被迫训练了一下对这类问题的洞察力 ，再次声明一下，我分析 dump 是免费的，没有某软高额的分析费用，你要问我图什么，图技术的精进。

回到正题，卡死类的问题分析入口点在于主线程此时在做什么，导致它不能处理自己的任务队列的原因是各种各样的，接下来上 windbg 分析一下。

1. 主线程此时在做什么

看主线程很简单，直接一个 k 命令搞定。

0:000> k
 # ChildEBP RetAddr
00 00f3ec2c 74ef6439     ntdll!NtWaitForSingleObject+0xc
01 00f3ec2c 70293370     KERNELBASE!WaitForSingleObjectEx+0x99
02 00f3ec5c 702933cc     clr!CLREventWaitHelper2+0x33
03 00f3ecac 70293319     clr!CLREventWaitHelper+0x2a
04 00f3ece4 7037bbcf     clr!CLREventBase::WaitEx+0x14b
05 00f3ecfc 70374f08     clr!WKS::GCHeap::WaitUntilGCComplete+0x35
06 00f3ed44 7032c2a2     clr!Thread::RareDisablePreemptiveGC+0x20b
07 00f3edc8 6d453712     clr!JIT_RareDisableHelper+0x22
08 00f3ee4c 6d4530d1     System_Windows_Forms_ni!System.Windows.Forms.Application+ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr, Int32, Int32)$##600550A+0x48e
09 00f3eea0 6d452f23     System_Windows_Forms_ni!System.Windows.Forms.Application+ThreadContext.RunMessageLoopInner(Int32, System.Windows.Forms.ApplicationContext)$##6005539+0x175
0a 00f3eecc 6d42b83d     System_Windows_Forms_ni!System.Windows.Forms.Application+ThreadContext.RunMessageLoop(Int32, System.Windows.Forms.ApplicationContext)$##6005538+0x4f
0b 00f3eee4 02d90c83     System_Windows_Forms_ni!System.Windows.Forms.Application.Run(System.Windows.Forms.Form)$##60005FA+0x35
...

从卦上信息看，主线程正在等待(WaitUntilGCComplete) GC 处理完成，接下来看下是哪一个线程触发了 GC 操作，并看下它正在 GC 三阶段的哪一步？

0:000> !t
ThreadCount:      33
UnstartedThread:  3
BackgroundThread: 29
PendingThread:    3
DeadThread:       0
Hosted Runtime:   no
                                                                         Lock
       ID OSID ThreadOBJ    State GC Mode     GC Alloc Context  Domain   Count Apt Exception
   0    1 17a0 0113ecd0     26020 Preemptive  00000000:00000000 01139490 0     STA
   2    2 2074 0114e0e0     2b220 Preemptive  00000000:00000000 01139490 0     MTA (Finalizer)
   5    3 27f4 011ddaa8   102a220 Preemptive  00000000:00000000 01139490 0     MTA (Threadpool Worker)
   8    4 2568 05bf0b90   1029220 Cooperative 00000000:00000000 01139490 1     MTA (GC) (Threadpool Worker) 

0:000> ~8s
eax=00000000 ebx=05bf0b90 ecx=00000000 edx=00000000 esi=00000000 edi=00000f6c
eip=77e5aa5c esp=098ce03c ebp=098ce0ac iopl=0         nv up ei pl nz na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000206
ntdll!NtWaitForSingleObject+0xc:
77e5aa5c c20c00          ret     0Ch
0:008> k
 # ChildEBP RetAddr
00 098ce0ac 74ef6439     ntdll!NtWaitForSingleObject+0xc
01 098ce0ac 70293370     KERNELBASE!WaitForSingleObjectEx+0x99
02 098ce0dc 702933cc     clr!CLREventWaitHelper2+0x33
03 098ce12c 70293319     clr!CLREventWaitHelper+0x2a
04 098ce164 7029333b     clr!CLREventBase::WaitEx+0x14b
05 098ce17c 7032a9b7     clr!CLREventBase::Wait+0x1a
06 098ce1fc 7032a9ef     clr!`anonymous namespace'::CreateSuspendableThread+0x165
07 098ce378 704322bd     clr!GCToEEInterface::CreateThread+0x15c
08 098ce3a8 704332e5     clr!WKS::gc_heap::prepare_bgc_thread+0x36
09 098ce3a8 70378585     clr!WKS::gc_heap::garbage_collect+0x215
0a 098ce3c8 7037868a     clr!WKS::GCHeap::GarbageCollectGeneration+0x1bd
0b 098ce3ec 70378703     clr!WKS::gc_heap::trigger_gc_for_alloc+0x1f
0c 098ce424 70396f6a     clr!WKS::gc_heap::try_allocate_more_space+0x152
0d 098ce46c 70397311     clr!WKS::gc_heap::allocate_large_object+0x51
0e 098ce478 702fde2d     clr!WKS::GCHeap::AllocLHeap+0x11
0f 098ce494 7063f1af     clr!AllocLHeap+0x4b
10 098ce4e8 7028f598     clr!FastAllocatePrimitiveArray+0xa7
11 098ce58c 0e2d3fb6     clr!JIT_NewArr1+0x126
12 098cf26c 6f1f1b5a     0xe2d3fb6
13 098cf274 6f188604     mscorlib_ni!System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.InvokeMoveNext(System.Object)$##6006FCB+0x1a
...

从卦中的 clr!GCToEEInterface::CreateThread 方法看，如果你没有这方面的分析经验，或许你已经无能为力了，以我的经验来说，既然卡死，那这里的 CreateThread 自然无法创建成功，创建不成功的原因在于 进程加载锁 LdrpLoaderLock 被别人持有了，导致它获取不到这把锁而一直茫然等待。

2. 谁持有了加载锁

要想找到谁持有了加载锁，可以在 ntdll 中搜 LdrpLoaderLock 字段，接下来使用 x 命令搜索。

0:008> x ntdll!LdrpLoaderLock
77f053b8          ntdll!LdrpLoaderLock = <no type information>
0:008> !cs 77f053b8
-----------------------------------------
Critical section   = 0x77f053b8 (ntdll!LdrpLoaderLock+0x0)
DebugInfo          = 0x77f0573c
LOCKED
LockCount          = 0x0
WaiterWoken        = No
OwningThread       = 0x00001314
RecursionCount     = 0x1
LockSemaphore      = 0x0
SpinCount          = 0x04000000

从卦中的 OwningThread = 0x00001314 迹象看，当前是线程 1314 持有了加载锁，看样子是要一生一世的持有。。。。 难怪程序不卡死。。。。 接下来切到这个线程看看到底咋这么痴情？

0:043> ~~[1314]s
eax=00000000 ebx=0589f784 ecx=00000000 edx=00000000 esi=00000000 edi=0589f770
eip=77e5c6bc esp=0589f72c ebp=0589f748 iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000202
ntdll!NtWaitForAlertByThreadId+0xc:
77e5c6bc c20800          ret     8
0:043> kb
 # ChildEBP RetAddr      Args to Child
00 0589f748 77e1336b     0ad114e8 00000000 00000000 ntdll!NtWaitForAlertByThreadId+0xc
01 0589f748 77e50630     00000000 00000000 ffffffff ntdll!RtlpWaitOnAddressWithTimeout+0x33
02 0589f78c 77e13299     00000004 00000000 00000000 ntdll!RtlpWaitOnAddress+0xa5
03 0589f7c8 77e2ed76     00000000 0ad114e0 0ad114e4 ntdll!RtlpWaitOnCriticalSection+0xaf
04 0589f7f0 77e2ec99     0589f814 0ac5d1b4 0ad114e4 ntdll!RtlpEnterCriticalSectionContended+0xd6
05 0589f7f8 0ac5d1b4     0ad114e4 00000003 0ac50000 ntdll!RtlEnterCriticalSection+0x49
06 0589f814 0ac5a890     00001314 0acddd2a 0ac50000 fwlibe1!xxx+0x13e3
07 0589f83c 77e5a9f6     0ac50000 00000003 00000000 fwlibe1!xxx+0x290
...

从卦中看，当前是 fwlibe1 这个非托管dll 准备获取 临界区 时卡死，接下来可以用 !cs 把 临界区变量 给提取出来看看到底谁在持有这个 临界区变量。

0:043> !cs 0ad114e4
-----------------------------------------
Critical section   = 0x0ad114e4 (fwlibe1!xxxx+0x3A1C2)
DebugInfo          = 0x0ff43360
LOCKED
LockCount          = 0x2
WaiterWoken        = No
OwningThread       = 0x00002378
RecursionCount     = 0x1
LockSemaphore      = 0xFFFFFFFF
SpinCount          = 0x020007d0

从卦中可以清晰的看到，当前临界区变量被 2378 号线程持有，接下来切到这个线程看看到底啥情况。

0:043> ~~[2378]s
eax=00000036 ebx=00002378 ecx=00000400 edx=00000157 esi=0ad114fc edi=0ad114e4
eip=0ac5cf88 esp=0caddf18 ebp=0cade7fc iopl=0         nv up ei pl nz na po nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00000202
fwlibe1!xxx+0x11b7:
0ac5cf88 8d45f0          lea     eax,[ebp-10h]

0:028> k
 # ChildEBP RetAddr
WARNING: Stack unwind information not available. Following frames may be wrong.
00 0cade7fc 0ac5a8df     fwlibe1!xxx+0x11b7
01 0cade938 0ac5abae     fwlibe1!xxx+0x22
02 0cadea20 09417ccd     fwlibe1!xxx+0x17
03 0cadebfc 09412c11     0x9417ccd
04 0caded2c 6f1f1b5a     0x9412c11
05 0caded34 6f188604     mscorlib_ni!System.Runtime.CompilerServices.AsyncMethodBuilderCore+MoveNextRunner.InvokeMoveNext(System.Object)$##6006FCB+0x1a
...

0:028> !clrstack
OS Thread Id: 0x2378 (28)
Child SP       IP Call Site
0cade9b4 0ac5cf88 [InlinedCallFrame: 0cade9b4]
0cade998 0941a0e9 *** WARNING: Unable to verify checksum for System.Data.ni.dll
DomainBoundILStubClass.IL_STUB_PInvoke(System.Object, UInt16, Int32, UInt16 ByRef)
0cade9b4 09417ccd [InlinedCallFrame: 0cade9b4] Focas.xxx(System.Object, UInt16, Int32, UInt16 ByRef)

从卦中看，托管层调用了 Focas.xxx 函数进入了 fwlibe1.dll ，最后有一句 WARNING: Stack unwind information not available. Following frames may be wrong. ，可能是展开有一定的问题，不过排查到这里，应该就是这个 Focas.xxx 的问题了。

3. 真相大白

接下来将我的排查结果跟朋友反馈了下，朋友排查下来说是最近改了这里面的连接方式所致，修改之后就搞定了。

这个案例告诉我们：只要代码还能跑，就不要动它，它要是不动了，你得要做好动的准备。