Strange deadlock in C# with single thread and no recursion

Question

I am running into a strange behavior of my C# application.

I am running the application in debug mode via VS2010 and after several hours the application just waits for a lock release but the only thread which is alive is the one that waits upon the lock to be released and there is not recursion in that function:

class ProductionNode        {  
   private readonly object _lock = new object();  
   public bool Activate( long jobId )  
   {  
       lock(_lock) // Doesn't go through here  
       {  
         return DAL.InsertJobIfNotExists(jobId)>0; //SQL PLAIN INSERT IF NOT EXIST COMMAND  
       }  
  }
  public void HasJobs()  
  {  
       lock(_lock)
       {
           return DAL.HasProductionJobs();
       }  
  }
 }

Again, when I am pausing the application using the VS2010 the only thread that uses the ProductionNode is the one that waits for the _lock object to be released.

To make things clear, the application might activate the production node in several threads but the in the given scenario where the deadlock occur the VS only displays a single thread which uses the ProductionNode object, the other threads uses other object types.

Any ideas?

Cheers, Doron

wild guess but maybe to lock an object it can't be `readonly`? — Dan D., Apr 27 '11 at 08:05
This is the only point in code where that lock gets ever taken? This part of the code looks perfectly fine to me. — CodesInChaos, Apr 27 '11 at 08:07
@Dan locking doesn't happen on the reference itself but on the instance. So `readonly` should have no effect. — CodesInChaos, Apr 27 '11 at 08:08
Why do you have a lock if you're not doing multithreading? Sounds fishy... — Lasse V. Karlsen, Apr 27 '11 at 08:09
The problem can obviously not be just in the code you provided. Please try to provide a minimal sample that reproduces the problem — jeroenh, Apr 27 '11 at 08:13
You have to be misunderstanding something from your debugging. It's not possible for a single thread to wait for a lock it holds itself - recursion or not; If it already holds the lock itself, it is permitted to enter subsequent protected blocks. — Andrew Barber, Apr 27 '11 at 08:34

Ben Schwehn · Answer 1 · 2011-04-27T09:21:51.260

I'd recommend using windbg (Dbugging Tools for Windows, included in the Windows SDK) and psscor2 to debug into your problem a little deeper.

Basically, in windbg try to:

1 . Load psscor2:

.load C:\tools\psscor2\amd64\psscor2 [replace with the path where you put the psscor2 dll]

2 . Try finding which threads are holding what locks using

!syncblk

3 . Look at what threads are running and compare to the threads that are holding locks

!threads

4 . Switch between threads

~Xs *[where X is the thread id you got from !threads and that you are interested in because it's holding a lock]*

look at the managed call stack of the current thread to find out what happened before the lock was acquired

!clrstack -a

EDIT:

Since you're presumably using .net 4, you might want to use SOSEX instead of psscor2. I haven't really used SOSEX so far, but I'd have a look at the deadlock command: !dlk

Xaqron · Answer 2 · 2011-04-27T08:38:20.743

Some hints that may help. I would go these steps:

Right click on _lock and find all references using Visual Studio context menu. Maybe you are using the lock somewhere else (you did in your update).
Make sure the lock is not static and ProductionNode class is not singleton
It is possible your thread is trapped inside DAL.InsertJobIfNotExists so logging it's entrance and exit would help tracing the problem root.

Strange deadlock in C# with single thread and no recursion

2 Answers2