Re: [PATCH net-next v8 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present

[Simon Schippers <simon.schippers@tu-dortmund.de>]

To summarize the discussion from my POV:

Open point: __ptr_ring_zero_tail() is only called after
            consuming ring.batch elements.
1) Consumer wakes up the producer but the slot is not cleaned.
--> I disagree, the consumer only wakes after consuming ring.size/2.
    Then __ptr_ring_zero_tail() was called at least once.
2) Producer is woken up but see the ring is full, so it need to
   drop the packet.
--> I disagree, because then NETDEV_TX_BUSY is returned. This is
    noticeable as qdisc requeue and only happens very rarely.

Points I will address:
- Minor nit on patch 2 by MST.
- Rebase patch 3 because of commit d748047
  ("ptr_ring: disable KCSAN warnings").
- Document the pair of the smp_mb__after_atomic() in tun_net_xmit
  with tun_ring_consume().
- Use 1 ptr_ring spinlock instead of 2 (currently used for consume
  and empty check), not sure how to implement it pretty rn.
- Run pktgen benchmarks with pg_set SHARED.