.NET高级调试之sos命令输出看不懂怎么办

一:背景

1. 讲故事

很多.NET开发者在学习高级调试的时候,使用sos的命令输出会发现这里也看不懂那里也看不懂,比如截图中的这位朋友。


.NET高级调试属于一个偏冷门的领域,国内可观测的资料比较少,所以很多东西需要你自己去探究源代码,然后用各种调试工具去验证,相关源代码如下:

这一篇权当抛砖引玉,来引导一下如何去探索。

二:WinDbg 分析

1. 复原现象

为了方便讲述先上一段简单的测试代码,来观察一下线程列表中所谓的 lockcount 列。


 internal class Program
 {
 static void Main(string[] args)
 {
 Console.WriteLine("hello world...");
 Debugger.Break();
 }
 }

接下来用 windbg 附加一下,使用 !threads 命令观察输出。


0:000> !t
ThreadCount: 3
UnstartedThread: 0
BackgroundThread: 2
PendingThread: 0
DeadThread: 0
Hosted Runtime: no
 Lock 
 DBG ID OSID ThreadOBJ State GC Mode GC Alloc Context Domain Count Apt Exception
 0 1 14c8 000001A8B862DBB0 2a020 Preemptive 000001A8BA1DDEE0:000001A8BA1DF478 000001a8b8623c20 0 MTA 
 5 2 71e0 000001A8D25740B0 2b220 Preemptive 0000000000000000:0000000000000000 000001a8b8623c20 0 MTA (Finalizer) 
 7 3 31b0 000001A8B86E7730 102a220 Preemptive 0000000000000000:0000000000000000 000001a8b8623c20 0 MTA (Threadpool Worker) 

从上面的卦中可以看到 Lock Count=0,那这里的值取自源码的哪里呢?

2. 取自哪里

大家要知道每一行记录都是在coreclr Thread中摘取出来的字段,言外之意就是可以把 Thread 所有信息给展示出来,可以用 dt 命令,简化后如下:


0:000> dt coreclr!Thread 000001A8B862DBB0
 +0x000 __VFN_table : 0x00007ffd`d2f93c70 
 =00007ffd`d30ce8d8 m_DetachCount : 0n0
 =00007ffd`d30ce8d4 m_ActiveDetachCount : 0n0
 =00007ffd`d30cf4dc m_threadsAtUnsafePlaces : Volatile<long>
 +0x008 m_State : Volatile<enum Thread::ThreadState>
 +0x00c m_fPreemptiveGCDisabled : Volatile<unsigned long>
 +0x010 m_pFrame : 0x0000006f`df57e328 Frame
 +0x018 m_pDomain : 0x000001a8`b8623c20 AppDomain
 +0x020 m_dwLockCount : 0
 +0x024 m_ThreadId : 1
 ...

通过仔细分析卦中信息,可以发现这个 lockcount 其实就是 m_dwLockCount 字段,知道是这个字段之后接下来就是寻找coreclr源码啦,截图如下:

从源码注释中看的非常清楚,这个字段可用来跟踪5中锁。

  • critical section
  • spin lock
  • syncblock lock
  • EE Crst
  • GC lock

接下来搜索下源码看看 m_dwLockCount 是怎么更新的,可以发现如下两处,并且都是和同步块索引相关,截图如下:


inline void Thread::IncLockCount()
{
 LIMITED_METHOD_CONTRACT;
 _ASSERTE(GetThread() == this);
 m_dwLockCount++;
 _ASSERTE(m_dwLockCount != 0 || HasThreadStateNC(TSNC_UnbalancedLocks));
}
inline void Thread::DecLockCount()
{
 LIMITED_METHOD_CONTRACT;
 _ASSERTE(GetThread() == this);
 _ASSERTE(m_dwLockCount > 0 || HasThreadStateNC(TSNC_UnbalancedLocks));
 m_dwLockCount--;
}

接下来要做的一件事就是如何自增 m_dwLockCount 值 来模拟达到这位朋友的 1024。

3. 如何模拟复现

其实当你知道是跟踪这五种锁,要模拟就非常简单了,为了方便讲述上一段测试代码,利用 lock 让 m_dwLockCount 字段不断自增,参考如下:


 internal class Program
 {
 public static object[] locks;
 static void Main(string[] args)
 {
 locks = Enumerable.Range(0, 100).Select(i => new object()).ToArray();
 foreach (var item in locks)
 {
 Monitor.Enter(item);
 }
 Debugger.Break();
 Console.ReadLine();
 }
 }

接下来用 windbg 附加观察一下。


0:000> !t
ThreadCount: 3
UnstartedThread: 0
BackgroundThread: 2
PendingThread: 0
DeadThread: 0
Hosted Runtime: no
 Lock 
 DBG ID OSID ThreadOBJ State GC Mode GC Alloc Context Domain Count Apt Exception
 0 1 6c90 00000239840DDBB0 2a020 Preemptive 0000023985B2BB38:0000023985B2D478 00000239840d3c20 100 MTA 
 5 2 2178 00000239859640B0 2b220 Preemptive 0000000000000000:0000000000000000 00000239840d3c20 0 MTA (Finalizer) 
 7 3 6d00 0000023984197700 102a220 Preemptive 0000000000000000:0000000000000000 00000239840d3c20 0 MTA (Threadpool Worker) 
0:000> dt coreclr!Thread 00000239840DDBB0
 +0x000 __VFN_table : 0x00007ffd`d2f93c70 
 =00007ffd`d30ce8d8 m_DetachCount : 0n0
 =00007ffd`d30ce8d4 m_ActiveDetachCount : 0n0
 =00007ffd`d30cf4dc m_threadsAtUnsafePlaces : Volatile<long>
 +0x008 m_State : Volatile<enum Thread::ThreadState>
 +0x00c m_fPreemptiveGCDisabled : Volatile<unsigned long>
 +0x010 m_pFrame : 0x00000074`a937e888 Frame
 +0x018 m_pDomain : 0x00000239`840d3c20 AppDomain
 +0x020 m_dwLockCount : 0x64

这里顺便提一下,如果你想实时观察 m_dwLockCount 更改的线程栈信息,可以在 Thread+0x20处下一个ba硬件断点即可。


0:000> !t
ThreadCount: 3
UnstartedThread: 0
BackgroundThread: 2
PendingThread: 0
DeadThread: 0
Hosted Runtime: no
 Lock 
 DBG ID OSID ThreadOBJ State GC Mode GC Alloc Context Domain Count Apt Exception
 0 1 6188 00000290D550DBB0 2a020 Preemptive 00000290D6F4AD20:00000290D6F4B478 00000290d5503c20 0 MTA 
 5 2 686c 00000290EF3F40B0 2b220 Preemptive 0000000000000000:0000000000000000 00000290d5503c20 0 MTA (Finalizer) 
 7 3 312c 00000290D55C7740 102a220 Preemptive 0000000000000000:0000000000000000 00000290d5503c20 0 MTA (Threadpool Worker) 
0:000> ba w4 00000290D550DBB0+0x20
0:000> g
ModLoad: 00007ffd`cff80000 00007ffd`cffd1000 C:\Program Files\dotnet\shared\Microsoft.NETCore.App\3.1.32\System.Collections.dll
ModLoad: 00007ffd`d00d0000 00007ffd`d0103000 C:\Program Files\dotnet\shared\Microsoft.NETCore.App\3.1.32\System.Runtime.Extensions.dll
Breakpoint 1 hit
coreclr!JIT_MonEnter_Portable+0x65:
00007ffd`d298c8a5 4883c430 add rsp,30h
0:000> kL 5
 # Child-SP RetAddr Call Site
00 000000b5`f917e780 00007ffd`72e917f4 coreclr!JIT_MonEnter_Portable+0x65
01 000000b5`f917e7c0 00007ffd`d29b6ca3 0x00007ffd`72e917f4
02 000000b5`f917e860 00007ffd`d294cc62 coreclr!CallDescrWorkerInternal+0x83
03 (Inline Function) --------`-------- coreclr!CallDescrWorkerWithHandler+0x57
04 000000b5`f917e8a0 00007ffd`d2953c29 coreclr!MethodDescCallSite::CallTargetWorker+0x196
...

三:总结

还是那句话,国内.NET高级调试方面的资料甚少,要想理解sos命令输出是什么意思,切记多看源码,其实本篇重要的不是知识,而是告诉你如何去探究新知的方法。

作者:一线码农原文地址:https://www.cnblogs.com/huangxincheng/p/18034164

%s 个评论

要回复文章请先登录注册