Thinking more I believe we can do similar tricks for all TCP
transmit traffic.

Sounds reasonable.

Packets bound to sockets never outlive those sockets (and thus
their cached routes) unless we skb_orphan().

The only not covered case is where the socket cached route
is reset or changed.  We could defer the dst put until the
transmit queue reaches a certain point, kind of like a retransmit
queue RCU :-)

Just some ideas...

netfilter makes it somewhat tricky, for compatibility you would
need to reclone the route on the fly.

-Andi

-- 
ak@linux.intel.com