Reading an int that's updated by Interlocked on other threads

Question

(This is a repeat of: How to correctly read an Interlocked.Increment'ed int field? but, after reading the answers and comments, I\'m still not sure of the right answer.)

Answer 1

Will both work (in the sense of delivering the latest value possible regardless of optimizations, re-orderings, caching, etc.)?

No, the value you get is always stale. How stale the value might be is entirely unpredictable. The vast majority of the time it will be stale by a few nanoseconds, give or take, depending how quickly you act on the value. But there is no reasonable upper-bound:

• your thread can lose the processor when it context-switches another thread onto the core. Typical delays are around 45 msec with no firm upper-bound. This does not mean that another thread in your process also gets switched-out, it can keep motoring and continue to mutate the value.
• just like any user-mode code, your code is subjected to page-faults as well. Incurred when the processor needs RAM for another process. On a heavily loaded machine that can and will page-out active code. As sometimes happens to the mouse driver code for example, leaving a frozen mouse cursor.
• managed threads are subject to near-random garbage collection pauses. Tends to be the lesser problem since it is likely that another thread that's mutating the value will be paused as well.

Whatever you do with the value needs to take this into account. Needless to say perhaps, that's very, very difficult. Practical examples are hard to come by. The .NET Framework is a very large chunk of battle-scarred code. You can see the cross-reference to usage of VolatileRead from the Reference Source. Number of hits: 0.