Re: [PATCH net v3 2/4] net: core: take the reuseport migration reference with acquire ordering

[Hang Nan <2122295973@qq.com>]

Thanks, agreed - the patch is dropped in v4, and the series is now three
patches: the two mptcp token patches and the iterator one, unchanged.

One correction to the reasoning: TCP sockets are allocated from a
SLAB_TYPESAFE_BY_RCU slab too (tcp_prot.slab_flags in net/ipv4/tcp_ipv4.c),
so "not a SLAB_TYPESAFE_BY_RCU lookup" is not what separates this site
from the ones in 5b5d8f351dd9 ("net: restore refcount ordering for
lockless socket access").  What separates it is the second half of your
argument, which I agree with: the acquire only orders a validation
performed after the increment, and reuseport_migrate_sock() performs
none.  It takes the reference, unlocks and returns the listener; the
callers go straight to tcp_filter() and tcp_check_req(), and the state
they actually depend on is re-checked under queue->rskq_lock in
inet_csk_reqsk_queue_add() (net/ipv4/inet_connection_sock.c:1403), which
drops the child when sk->sk_state is not TCP_LISTEN.  Ordering the
increment changes nothing about what any of those paths observe, so the
change was cosmetic and I should not have included it.

I also considered adding the missing post-acquire validation instead
(re-checking sk_state or membership in the reuseport group), but that
would duplicate the check that already exists in inet_csk_reqsk_queue_add
under the queue lock, so dropping the patch seemed the honest fix.

pw-bot: cr