Once upon a time, the _insert counter in the OpCounters class was only ever incremented when the single global writelock was held. In that world, the implementation that uses a non-atomic operation to increment the counter wouldn't lead to lost counts, though it could hypothetically lead to incorrect reads if the _insert counter weren't 4-byte aligned.

Today, the function is often called without a global lock held (only a per database lock), making these adds lossy.

It would be wise to fix this, perhaps by consolidating the implementation of AtomicUInt with the AtomicWord<T> or at least the AtomicIntrinsics<T> from mongo/platform/atomic_word.h. This would also allow us to make these counters 64-bit instead of 32-bit, in case we're ever worried about rollover.