Question

I read recently about memory barriers and the reordering issue and now I have some confusion about it.

Consider the following scenario:

private object _object1 = null;    
private object _object2 = null;
private bool _usingObject1 = false;

private object MyObject
{
    get 
    {
        if (_usingObject1)
        {
            return _object1;
        }
        else
        {
            return _object2;
        }
    }
    set 
    {
        if (_usingObject1)
        {
           _object1 = value;
        }
        else
        {
           _object2 = value;
        }
    }
}

private void Update()
{
    _usingMethod1 = true;
    SomeProperty = FooMethod();
    //..
    _usingMethod1 = false;
}

1. At Update method; is the _usingMethod1 = true statement always executed before getting or setting the property? or due to reordering issue we can not guarantee that?

2. Should we use volatile like

   private volatile bool _usingMethod1 = false;
   

3. If we use lock; can we guarantee then every statement within the lock will be executed in order like:

   private void FooMethod()
   {
       object locker = new object();
       lock (locker)
       {
           x = 1;
           y = a;
           i++;
       }
   }
   

Answer 1

The subject of memory barriers is quite complex. It even trips up the experts from time to time. When we talk about a memory barrier we are really combining two different ideas.

• Acquire fence: A memory barrier in which other reads & writes are not allowed to move before the fence.
• Release fence: A memory barrier in which other reads & writes are not allowed to move after the fence.

A memory barrier that creates only one of two is sometimes called a half-fence. A memory barrier that creates both is sometimes called a full-fence.

The volatile keyword creates half-fences. Reads of volatile fields have acquire semantics while writes have release semantics. That means no instruction can be moved before a read or after a write.

The lock keyword creates full-fences on both boundaries (entry and exit). That means no instruction can be moved either before or after each boundary.

However, all of this moot if we are only concerned with one thread. Ordering, as it is perceived by that thread, is always preserved. In fact, without that fundamental guarentee no program would ever work right. The real issue is with how other threads perceive reads and writes. That is where you need to be concerned.

So to answer your questions:

1. From a single thread’s perspective…yes. From another thread’s perspective…no.

2. It depends. That might work, but I need to have better understanding of what you are trying to acheive.

3. From another thread’s perspective…no. The reads and writes are free to move around within the boundaries of the lock. They just cannot move outside those boundaries. That is why it is important for other threads to also create memory barriers.