How to correctly read an Interlocked.Increment'ed int field?

Solution 1:

If you want to guarantee that the other thread will read the latest value, you must use Thread.VolatileRead(). (*)

The read operation itself is atomic so that will not cause any problems but without volatile read you may get old value from the cache or compiler may optimize your code and eliminate the read operation altogether. From the compiler's point of view it is enough that the code works in single threaded environment. Volatile operations and memory barriers are used to limit the compiler's ability to optimize and reorder the code.

There are several participants that can alter the code: compiler, JIT-compiler and CPU. It does not really matter which one of them shows that your code is broken. The only important thing is the .NET memory model as it specifies the rules that must be obeyed by all participants.

(*) Thread.VolatileRead() does not really get the latest value. It will read the value and add a memory barrier after the read. The first volatile read may get cached value but the second would get an updated value because the memory barrier of the first volatile read has forced a cache update if it was necessary. In practice this detail has little importance when writing the code.

Solution 2:

A bit of a meta issue, but a good aspect about using Interlocked.CompareExchange(ref value, 0, 0) (ignoring the obvious disadvantage that it's harder to understand when used for reading) is that it works regardless of int or long. It's true that int reads are always atomic, but long reads are not or may be not, depending on the architecture. Unfortunately, Interlocked.Read(ref value) only works if value is of type long.

Consider the case that you're starting with an int field, which makes it impossible to use Interlocked.Read(), so you'll read the value directly instead since that's atomic anyway. However, later in development you or somebody else decides that a long is required - the compiler won't warn you, but now you may have a subtle bug: The read access is not guaranteed to be atomic anymore. I found using Interlocked.CompareExchange() the best alternative here; It may be slower depending on the underlying processor instructions, but it is safer in the long run. I don't know enough about the internals of Thread.VolatileRead() though; It might be "better" regarding this use case since it provides even more signatures.

I would not try to read the value directly (i.e. without any of the above mechanisms) within a loop or any tight method call though, since even if the writes are volatile and/or memory barrier'd, nothing is telling the compiler that the value of the field can actually change between two reads. So, the field should be either volatile or any of the given constructs should be used.

My two cents.

Solution 3:

You're correct that you do not need a special instruction to atomically read a 32bit integer, however, what that means is you will get the "whole" value (i.e. you won't get part of one write and part of another). You have no guarantees that the value won't have changed once you have read it.

It is at this point where you need to decide if you need to use some other synchronization method to control access, say if you're using this value to read a member from an array, etc.

In a nutshell, atomicity ensures an operation happens completely and indivisibly. Given some operation A that contained N steps, if you made it to the operation right after A you can be assured that all N steps happened in isolation from concurrent operations.

If you had two threads which executed the atomic operation A you are guaranteed you will see only the complete result of one of the two threads. If you want to coordinate the threads, atomic operations could be used to create the required synchronization. But atomic operations in and of themselves do not provide higher level synchronization. The Interlocked family of methods are made available to provide some fundamental atomic operations.

Synchronization is a broader kind of concurrency control, often built around atomic operations. Most processors include memory barriers which allow you to ensure all cache lines are flushed and you have a consistent view of memory. Volatile reads are a way to ensure consistent access to a given memory location.

While not immediately applicable to your problem, reading up on ACID (atomicity, consistency, isolation, and durability) with respect to databases may help you with the terminology.