我应该*始终*同步访问从多个线程使用的所有双字段/属性/变量？

Question

Note I tend to write lock-free code so I try to avoid any types of lock where possible. Instead I just use while(true) loop because I have a lot of CPU power.

According to that http://msdn.microsoft.com/en-us/library/aa691278%28VS.71%29.aspx double variable update is not atomic.

I'm concerned about two issues:

• if one thread modify variable of field or property and another thread read it at the same time i want to have either previous or new value, but I don't want to receive something strange. Ie if one thread changes value from 5.5 to 15.15 I want to have in another thread one of these two numbers, but not 5.15 or 15.5 or anything else.
• if one thread already updated value and another thread read it after that I want to receive latest, up to date, value. I was thinking that volatile keyword can help with that but in seems it can't, because "Volatile does not guarantee freshness of a value. It prevents some optimizations, but does not guarantee thread synchronization." as said here are c# primitive arrays volatile?

Questions:

• Am I correct that without synchronization both of these issues may appear?
• If you can give me short example which proves that without synchronization it will not work - that would be nice
• How should I access double field or variable or property to have always real up to date value? Will "synchronization" guarantee "freshness"? What would be the fastest way for that? spinlock or something?

Currently I use a lot of double and decimal variables/field/properties in my program and almost everythig works fine, so I really confused because I ofthen access them from different threads without any synchronization and that just works... But now I'm thinking that probably it would be better to use float to have "built-in syncrhonization"

Answer 1

Yes, you need to do something. double and decimal are not guaranteed to be atomic , so if you don't protect it you could get a torn value - ie your first bullet is entirely correct.

Re volatile ; it is moot; you are not allowed to have a volatile field that is double or decimal , so the simplest answer is: lock .

Getting double to fail is a royal PITA; but here's a torn-value example featuring decimal (note the numbers of success/fail will change each iteration, even though the data is the same; this is the randomness of thread scheduling):

using System;
using System.Threading;
static class Program
{
    private static decimal shared ;
    static void Main()
    {
        Random random = new Random(12345);
        decimal[] values = new decimal[20];
        Console.WriteLine("Values:");
        for (int i = 0; i < values.Length; i++)
        {
            values[i] = (decimal)random.NextDouble();
            Console.WriteLine(values[i]);
        }
        Console.WriteLine();
        object allAtOnce = new object();
        int waiting = 10;
        shared = values[0];
        int correct = 0, fail = 0;
        for(int i = 0 ; i < 10 ; i++)
        {
            Thread thread = new Thread(() =>
            {
                lock(allAtOnce)
                {
                    if (Interlocked.Decrement(ref waiting) == 0)
                    {
                        Monitor.PulseAll(allAtOnce);
                    } else
                    {
                        Monitor.Wait(allAtOnce);
                    }
                }
                for(int j = 0 ; j < 1000 ; j++)
                {
                    for(int k = 0 ; k < values.Length ; k++)
                    {
                        Thread.MemoryBarrier();
                        var tmp = shared;
                        if(Array.IndexOf(values, tmp) < 0)
                        {
                            Console.WriteLine("Invalid value detected: " + tmp);
                            Interlocked.Increment(ref fail);
                        } else
                        {
                            Interlocked.Increment(ref correct);
                        }
                        shared = values[k];
                    }
                }
                if (Interlocked.Increment(ref waiting) == 10)
                {
                    Console.WriteLine("{0} correct, {1} fails",
                        Interlocked.CompareExchange(ref correct, 0, 0),
                        Interlocked.CompareExchange(ref fail, 0, 0));
                    Console.WriteLine("All done; press any key");
                    Console.ReadKey();
                }
            });
            thread.IsBackground = false;
            thread.Start();
        }
    }
}

The key point; the language makes no guarantees for the atomicity of double . In reality, I expect you'll be fine, but most subtle problems caused by threading are due to using "I expect" instead of "I can guarantee".

Answer 2

If you want to guarantee that a block of code will be executed and finished before another thread manipulates it, surround that block of code with a lock .

You may be lucky, and threads may never battle over using a variable, but to make sure that it never happens, making sure precautions are taken wouldn't hurt.

Take a look here - this might help: http://msdn.microsoft.com/en-us/library/ms173179%28v=vs.80%29.aspx

Answer 3

a general answer would be - updates should be synchronized for all "shared" variables. For exact answer need to see the code snippet.

Answer 4

Yes, you need to lock to be sure you get the correct result if multiple threads read/write a double at the same time.

Here's a failing example

[TestFixture]
public class DoubleTest
{
    private double theDouble;

    [Test]
    public void ShouldFailCalledParallell()
    {
        theDouble = 0;
        const int noOfTasks = 100;
        const int noOfLoopInTask = 100;
        var tasks = new Task[noOfTasks];
        for (var i = 0; i < noOfTasks; i++)
        {
            tasks[i] = new Task(() =>
                                    {
                                        for (var j = 0; j < noOfLoopInTask; j++)
                                        {
                                            theDouble++;
                                        }
                                    });
        }
        foreach (var task in tasks)
        {
            task.Start();
        }
        Task.WaitAll(tasks);
        theDouble.Should().Be.EqualTo(noOfTasks * noOfLoopInTask);
    }
}