Writing non-thread-safe code

[This was originally posted at http://timstall.dotnetdevelopersjournal.com/writing_nonthreadsafe_code.htm]

Multithreading is hard, and I'm certainly no expert at it. I like this post about Measuring how difficult a Threading bug is. Most application developers hover around phase 0 (totally reproducible) to 2 (solvable with locks). Application Developers constantly hear about how some piece of code isn't "thread safe". What exactly would that look like? How could you deterministically write non-thread safe code, like writing a unit test that fails your non-thread-safe object?

 

The trick is to run a unit test that opens up multiple threads, and then runs them in a loop 10,000 times to force the issue. The following code does something like that. Say we have a simple "Account" object with Credit and Debit instance methods, both sharing the same state (the _intBalance field). If you run the unit test "Threads_2_5", it opens up 2 threads, calling Credit in one and Debit in the other. Because Credit and Debit should cancel each other out, and they're "theoretically" called the same number of times, the final result should remain zero. But it's not - the test will fail (if it doesn't fail, increase the number of iterations to force more contention).

 

So, we have a reproducible multi-threading failure in our unit test. The Account object is not thread safe. However, we can apply the C# lock keyword to put a lock on the methods in question, which at least for this simple case, fixes the problem. I've shown how to apply the lock keyword in the commented-out lines of the Account object. I see two observations:

1. Without the C# lock keyword, this gets essentially random errors as contention (thread count or loops) increases.

2. Adding the lock keyword will prevent errors, but the code runs much slower.  This means it's also possible to abuse the lock keyword, perhaps adding it in places you don't need it, such that you get no benefit, but now your code runs slower. Tricky tricky.

Here's the code:

 

    public class Account
    {
      //private Object thisLock = new Object();

      private int _intBalance = 0;

      public void Credit()
      {
        //lock (thisLock)
          _intBalance++;
      }

      public void Debit()
      {
        //lock (thisLock)
          _intBalance--;
      }

      public int CurrentValue
      {
        get
        {
          return _intBalance;
        }
      }
    }

    private Account _account = null;
    private int _intMaxLoops = -1;

    private void RunThreads2(int intLoopMagnitude)
    {
      _intMaxLoops = Convert.ToInt32(Math.Pow(10, intLoopMagnitude));
      _account = new Account();

      Assert.AreEqual(0, _account.CurrentValue);

      Thread t1 = new Thread(new ThreadStart(Run_Credit));
      Thread t2 = new Thread(new ThreadStart(Run_Debit));

      t1.Start();
      t2.Start();

      t1.Join();
      t2.Join();

      //We did an equal number of credits and debits --> should still be zero.
      Assert.AreEqual(0, _account.CurrentValue);
    }

    private void Run_Credit()
    {
      for (int i = 0; i < _intMaxLoops; i++)
      {
        _account.Credit();
      }
    }

    private void Run_Debit()
    {
      for (int i = 0; i < _intMaxLoops; i++)
      {
        _account.Debit();
      }
    }

 
    [TestMethod]
    public void Threads_2_5()
    {
      RunThreads2(5);
    }
 

Someone who knows multi-threading much better than I do (and who hasn't yet started their own blog despite all my pleading), explained it well here:

With multithreading, it’s important to understand what’s happening at the assembly level.
For example, a statement like “count++” is actually “count = count + 1”. In assembly, this becomes something like:

1: mov eax, [esp + 10] ; Load ‘count’ into a register
2: add eax, eax, 1 ; Increment the register
3: mov [esp + 10], eax ; Store the register in ‘count’

With multithreading, both threads could run through this code at different rates. For example, start with ‘count = 7’. Thread ‘A’ could have executed (1), loading ‘7’ into ‘eax’. Thread ‘B’ then executes (1), (2), (3), also loading ‘7’ into ‘eax’, incrementing, and storing ‘count = 8’. It then executes 3 more times, setting ‘count = 11’. Thread ‘A’ finally starts running again, but it is out of sync! It then stores ‘count = 8’ because it didn’t get updated.

When using locks, that would prevent multiple threads from executing this code at the same time. So thread ‘A’ would make ‘count = 8’. Thread ‘B’ would make ‘count = 9’, etc.
 

The problem with locks is what happens if you ever need multiple locks. If thread ‘A’ grabs lock (1) and thread ‘b’ grabs lock (2), then neither of them would ever be able to grab both locks. In SQL, this throws an exception on one of the threads, forcing it to release its lock and try over. SQL can do this because the database uses transactions to modify the data. If anything goes wrong, the transaction rolls the database back to the previous values. Normal C# code can’t do this because there are no transactions. Modifying the data is immediate, and there is no undo. ;-)

Obviously there's a world more to multi-threading, but everyone has got to start somewhere.