This looks all inherently racy (which doesn't matter much as you suggest)
so I just wanted to suggest that if you used per-cpu counters you'd get
race-free and faster code at the cost of larger data structures and using
percpu_counter_add() instead of ++ (which doesn't seem like a big
complication to me).

No problem. I'll switch to per-cpu counters in next post.

Thanks,
Fengguang