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

Question

Suppose I have a non-volatile int field, and a thread which Interlocked.Increments it. Can another thread safely read this directly, or does the read also need to be interlocked?

I previously thought that I had to use an interlocked read to guarantee that I'm seeing the current value, since, after all, the field isn't volatile. I've been using Interlocked.CompareExchange(int, 0, 0) to achieve that.

However, I've stumbled across this answer which suggests that actually plain reads will always see the current version of an Interlocked.Incremented value, and since int reading is already atomic, there's no need to do anything special. I've also found a request in which Microsoft rejects a request for Interlocked.Read(ref int), further suggesting that this is completely redundant.

So can I really safely read the most current value of such an int field without Interlocked?

Answer 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.

Answer 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.