So here is a patch to make sure that there is a barrier between the
reading of dev->*_ptr and *dev->neigh_parms.

With these barriers in place, it's clear that *dev->neigh_parms can no
longer be NULL since once the parms are allocated, that pointer is never
reset to NULL again.  Therefore I've also removed the parms check in
these paths.

They were bogus to begin with since if they ever triggered then we'll
have dead neigh entries stuck in the hash table.

Unfortunately I couldn't arrange for this to happen with DECnet due
to the dn_db->parms.up() call that's sandwiched between the assignment
of dev->dn_ptr and dn_db->neigh_parms.  So I've kept the parms check
there but it will now fail instead of continuing.  I've also added an
smp_wmb() there so that at least we won't be reading garbage from
dn_db->neigh_parms.

DECnet is also buggy since there is no locking at all in the destruction
path.  It either needs locking or RCU like IPv4.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>