Question

I trying to implement synchronization algorithm in C#, without success.

Why isn’t the following code thread safe?

using System;
using System.Threading;

namespace SoftwareLockTest
{
    class Program
    {
        private static volatile bool _isLocked1 = false;
        private static volatile bool _isLocked2 = false;
        private static volatile int _count = 0;

        static void Main(string[] args)
        {
            Thread thread2 = new Thread(Thread2Work);
            thread2.Start();

            Thread1Work();
        }

        public static void Thread1Work()
        {
            while (true)
            {
                _isLocked1 = true;

                while (_isLocked2)
                {
                    _isLocked1 = false;
                    while (_isLocked2) ;
                    _isLocked1 = true;
                }

                CriticalSection();
                _isLocked1 = false;
            }
        }

        public static void Thread2Work()
        {
            while (true)
            {
                _isLocked2 = true;

                while (_isLocked1)
                {
                    _isLocked2 = false;
                    while (_isLocked1) ;
                    _isLocked2 = true;
                }

                CriticalSection();
                _isLocked2 = false;
            }
        }

        private static void CriticalSection()
        {
            if (_count != 0)
            {
                Console.WriteLine("NOT THREAD SAFE 1");
                _count = 0;
            }

            _count++;

            if (_count != 1)
            {
                Console.WriteLine("NOT THREAD SAFE 2");
            }

            _count--;

            if (_count != 0)
            {
                Console.WriteLine("NOT THREAD SAFE 3");
            }
        }
    }
}

Answer 1

The problem is that a write followed by a read may be reordered (even with “volatile”). You need to call Thread.MemoryBarrier(); in front of the four “while (_isLockedX)” loops.

Please read http://www.albahari.com/threading/part4.aspx for an explanation of memory barriers and volatile.

For any real project, please prefer existing lock implementations instead of trying to make your own.