Why do I need a memory barrier?

Question

C# 4 in a Nutshell (highly recommended btw) uses the following code to demonstrate the concept of MemoryBarrier (assuming A and B were run on different threads):

class Foo{
  int _answer;
  bool complete;
  void A(){
    _answer = 123;
    Thread.MemoryBarrier(); // Barrier 1
    _complete = true;
    Thread.MemoryBarrier(); // Barrier 2
  }
  void B(){
    Thread.MemoryBarrier(); // Barrier 3;
    if(_complete){
      Thread.MemoryBarrier(); // Barrier 4;
      Console.WriteLine(_answer);
    }
  }
}

they mention that Barriers 1 & 4 prevent this example from writing 0 and Barriers 2 & 3 provide a freshness guarantee: they ensure that if B ran after A, reading _complete would evaluate to true.

I'm not really getting it. I think I understand why Barriers 1 & 4 are necessary: we don't want the write to _answer to be optimized and placed after the write to _complete (Barrier 1) and we need to make sure that _answer is not cached (Barrier 4). I also think I understand why Barrier 3 is necessary: if A ran until just after writing _complete = true, B would still need to refresh _complete to read the right value.

I don't understand though why we need Barrier 2! Part of me says that it's because perhaps Thread 2 (running B) already ran until (but not including) if(_complete) and so we need to insure that _complete is refreshed.

However, I don't see how this helps. Isn't it still possible that _complete will be set to true in A but yet the B method will see a cached (false) version of _complete? Ie, if Thread 2 ran method B until after the first MemoryBarrier and then Thread 1 ran method A until _complete = true but no further, and then Thread 1 resumed and tested if(_complete) -- could that if not result in false?

Answer 1

Barrier #2 guarentees that the write to _complete gets committed immediately. Otherwise it could remain in a queued state meaning that the read of _complete in B would not see the change caused by A even though B effectively used a volatile read.

Of course, this example does not quite do justice to the problem because A does nothing more after writing to _complete which means that the write will be comitted immediately anyway since the thread terminates early.

The answer to your question of whether the if could still evaluate to false is yes for exactly the reasons you stated. But, notice what the author says regarding this point.

Barriers 1 and 4 prevent this example from writing “0”. Barriers 2 and 3 provide a freshness guarantee: they ensure that if B ran after A, reading _complete would evaluate to true.

The emphasis on "if B ran after A" is mine. It certainly could be the case that the two threads interleave. But, the author was ignoring this scenario presumably to make his point regarding how Thread.MemoryBarrier works simpler.

By the way, I had a hard time contriving an example on my machine where barriers #1 and #2 would have altered the behavior of the program. This is because the memory model regarding writes was strong in my environment. Perhaps, if I had a multiprocessor machine, was using Mono, or had some other different setup I could have demonstrated it. Of course, it was easy to demonstrate that removing barriers #3 and #4 had an impact.