By the way, this isn't just theoretical -- I'm not smart enough to
realize this except that I just saw:
ib1: TX ring full, stopping kernel net queue
NETDEV WATCHDOG: ib1: transmit timed out
ib1: transmit timeout: latency 1240 msecs
ib1: queue stopped 1, tx_head 5291313, tx_tail 5291255
and of course it never recovers.