find a thread safety bug

gistfile1.cs

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

# Test

namespace ConsoleApplication3
{
    class ThreadUnsafe
    {
        public static int _val1 = 1;
        public static int _val2 = 1;

        private static readonly object syncObj = new object();

        public void Go()
        {
            lock (syncObj)
            {
                if (_val2 != 0)
                {
                    Thread.Sleep(0);
                    var i = _val1/_val2;
                }
            }
            _val2 = 0;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
           ThreadUnsafe threadUnsafe1 = new ThreadUnsafe();
           ThreadUnsafe threadUnsafe2 = new ThreadUnsafe();

           for (int i = 0; i < 10000; i++)
           {
               Task task = new Task(threadUnsafe1.Go);
               Task task1 = new Task(threadUnsafe2.Go);

               task.Start();
               task1.Start();

               try
               {
                   Task.WaitAll(task, task1);
               }
               catch (AggregateException ex)
               {
                   Console.WriteLine(i);
                   Console.WriteLine(ex);
               }

               ThreadUnsafe._val2 = 1;
           }
            
        }
    }
}