Style, the || should be on the previous line.

So we don't purge any addresses if a non-autoconf address belongs to the prefix, but isn't that kind of surprising behaviour? I thought the main purpose of ndp -P is to flush all autoconfigured addresses.

Call it ndpr_autoconf_addrcnt, to be more consistent with ndpr_addrcnt?

The if-statement condition is always true here.

In the other places, ndpr_num_autoconf_addrs was incremented/decremented w/o holding any lock.

Maybe, doing per-prefix wlock on increment/decrement and checking the condition under wlock here as well could work.

Also, a very dumb question: how does it help preventing panic?

Is this needed given M_ZERO above?