Hi everyoneI was considering starting to put some code in this blog, but was lacking the proper motivation. But I found one: trivia game!

So welcome to the first instalment of

Can You Spot the Deadlock?

Rules are simple: I publish some code which exhibits some concurrency related problem and ask some questions about it such as:

• What concurrency issue(s) is(are) present in the code? (deadlock, race condition, livelock…)
• Where does it occurs or what are the impacts?
• How can it be fixed

Please post your answer(s) in the comment section, I will provide the expected answers one week after first submission. Of course, debugging is not allowed here.
So without further ado, here is the first one, quite easy for a starter.

Where is the deadlock?

Which single line needs to be changed to fix this?

Problem is now closed, please see the solution.

class Program
{
  /// Main function
  static void Main(string[] args)
  {
    DeadLock1 sample1 = new DeadLock1();
    sample1.Start();
  }
}
/// First instance of 'Can you Spot the DeadLock?
/// Let's start easy to put things in motion
internal class DeadLock1
{
  private Thread _firstRunner;
  private Thread _secondRunner;
  private bool _stopSignal = false;
  private readonly object _synchro = new object();

  public DeadLock1()
  {
    _firstRunner = new Thread(FirstRunner);
    _secondRunner = new Thread(SecondRunner);
  }

  // start your engines
  public void Start()
  {
    _firstRunner.Start();
    _secondRunner.Start();
    Thread.Sleep(100);
    lock (_synchro)
    {
      _stopSignal = true;
      Monitor.Pulse(_synchro);
    }
    _firstRunner.Join();
    _secondRunner.Join();
  }
  // first thread logic
  private void FirstRunner()
  {
     lock (_synchro)
     {
       if (!_stopSignal)
       {
         Monitor.Wait(_synchro);
       }
     }
   }
  // second thread logic
  private void SecondRunner()
  {
    lock (_synchro)
    {
      if (!_stopSignal)
      {
        Monitor.Wait(_synchro);
      }
    }
  }
}

Several quality comments can be made about this code, but please focus on the multithreading issues. Bonus points if the second design weakness is identified!

PS: I’ll wait a few days before approving comments not to disclose the solution. I already have at least one good answer (but not perfect) and one wrong one. Keep commenting!

edited to explicitly initialize _stopSignal for clarity.