Code example proven to fail w/o volatile

Question

Below is a C# code example which is a verbatim translation of a broken Java code (which has proven to break (i. e. the 2nd thread may fail to observe the change of sha

Answer 1

This should get stuck in an infinite loop (I can't test it right now)

public class Test
{
  private bool loop = true;

  public static void Main()
  {
    Test test = new Test();
    Thread thread = new Thread(DoStuff);
    thread.Start(test);

    Thread.Sleep(1000);

    test.loop = false;
    Console.WriteLine("loop is now false");
  }

  private static void DoStuff(object o) {
    Test test = (Test)o;
    Console.WriteLine("Entering loop");
    while (test.loop) {

    }
    Console.WriteLine("Exited loop");
  }

}

Answer 2

Try running the RELEASE build of the following program (do NOT run it from the debugger otherwise the demonstration won't work - so run the release build via "Debug | Start without debugging"):

using System;
using System.Threading;
using System.Threading.Tasks;

namespace Demo
{
    internal class Program
    {
        private void run()
        {
            Task.Factory.StartNew(resetFlagAfter1s);
            int x = 0;

            while (flag)
                ++x;

            Console.WriteLine("Done");
        }

        private void resetFlagAfter1s()
        {
            Thread.Sleep(1000);
            flag = false;
        }

        private volatile bool flag = true;

        private static void Main()
        {
            new Program().run();
        }
    }
}

The program will terminate after a second.

Now remove the volatile from the line

private volatile bool flag = true; // <--- Remove volatile from this

After doing so, the program will now never terminate. (Tested on Windows 8 x64, .Net 4.5)

Please note, however, that in some cases it is more appropriate to use Thread.MemoryBarrier() rather than declaring the variable volatile, i.e.:

while (flag)
{
    Thread.MemoryBarrier();
    ++x;
}

For more information, see http://blogs.msdn.com/b/brada/archive/2004/05/12/130935.aspx