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):

<         


        

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.

Answer 2

The example is unclear for two reasons:

1. It is too simple to fully show what's happening with the fences.
2. Albahari is including requirements for non-x86 architectures. See MSDN: "MemoryBarrier is required only on multiprocessor systems with weak memory ordering (for example, a system employing multiple Intel Itanium processors [which Microsoft no longer supports]).".

If you consider the following, it becomes clearer:

1. A memory barrier (full barriers here - .Net doesn't provide a half barrier) prevents read / write instructions from jumping the fence (due to various optimisations). This guarantees us the code after the fence will execute after the code before the fence.
2. "This serializing operation guarantees that every load and store instruction that precedes in program order the MFENCE instruction is globally visible before any load or store instruction that follows the MFENCE instruction is globally visible." See here.
3. x86 CPUs have a strong memory model and guarantee writes appear consistent to all threads / cores (therefore barriers #2 & #3 are unneeded on x86). But, we are not guaranteed that reads and writes will remain in coded sequence, hence the need for barriers #1 and #4.
4. Memory barriers are inefficient and needn't be used (see the same MSDN article). I personally use Interlocked and volatile (make sure you know how to use it correctly!!), which work efficiently and are easy to understand.

Ps. This article explains the inner workings of x86 nicely.