Alexander Duyck reported high false sharing on dst refcount in tcp stack
when prequeue is used. prequeue is the mechanism used when a thread is
blocked in recvmsg()/read() on a TCP socket, using a blocking model
rather than select()/poll()/epoll() non blocking one.

We already try to use RCU in input path as much as possible, but we were
forced to take a refcount on the dst when skb escaped RCU protected
region. When/if the user thread runs on different cpu, dst_release()
will then touch dst refcount again.

Commit 09316255 (tcp: force a dst refcount when prequeue packet)
was an example of a race fix.

It turns out the only remaining usage of skb->dst for a packet stored
in a TCP socket prequeue is IP early demux.

We can add a logic to detect when IP early demux is probably going
to use skb->dst. Because we do an optimistic check rather than duplicate
existing logic, we need to guard inet_sk_rx_dst_set() and
inet6_sk_rx_dst_set() from using a NULL dst.

Many thanks to Alexander for providing a nice bug report, git bisection,
and reproducer.

Tested using Alexander script on a 40Gb NIC, 8 RX queues.
Hosts have 24 cores, 48 hyper threads.

echo 0 >/proc/sys/net/ipv4/tcp_autocorking

for i in `seq 0 47`
do
  for j in `seq 0 2`
  do
     netperf -H $DEST -t TCP_STREAM -l 1000 \
             -c -C -T $i,$i -P 0 -- \
             -m 64 -s 64K -D &
  done
done

Before patch : ~6Mpps and ~95% cpu usage on receiver
After patch : ~9Mpps and ~35% cpu usage on receiver.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>