6.1 Parallelism: race conditions, C#

6.1 Parallelism: race conditions

//6.1 Parallelism: race conditions

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Threading;
using System.Threading.Tasks;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Threads introduce a lot of new interesting bug types. One of them is called race condition. 
            //These are hard to catch and even hard to produce intentionally!
            int total_threads = 0;

for(int i=0; i<100; i++)
            {
                ThreadPool.QueueUserWorkItem(f =>
                                             {
                                                 if(total_threads == 50)
                                                    Console.WriteLine("50!");
                                                 
                                                 total_threads++;
                                             });
            }
   
            Thread.Sleep(5000);
            
            //Anyway this code has race condition and sometimes (maybe very rarely) it will print more than just one line "50!" or won't print anything at all.
            //It is because variable total_threads is accesed from many threads "simultaneously". And this could happen:
            //Thread A checks for value: it's 49 and goes to next command (total_threads++;)
            //But then Thread B kicks in and checks for value and it is still 49, so it goes to next command (total_threads++;)
            //Thread A kicks in back and continues where it was interrupted and executes total_threads++; making value equal to 50
            //Thread B kicks in back and continues where it was interrupted and executes total_threads++; making value equal to 51
            //And in this case no thread will have total_threads == 50 so nothing will be printed.
            
            //The main point is that threads can be interrupted at any point and resumed later and anything in other threads can happen in between
            //Even this instruction - total_threads++; can be interrupted in the midle, because it actually consists of several assembly instructions:
            //mov $ptr, eax - move memory value to eax register
            //inc eax       - increment by one
            //mov eax, $ptr - write back to memory
            //and interruption can happen after any of these instructions
            
            //How to avoid problems like this one? Locks!
        }
    }
}


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