mlx4: Remove rcu_read_lock() around XDP program invocation

The mlx4 driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
program invocations. However, the actual lifetime of the objects referred
by the XDP program invocation is longer, all the way through to the call to
xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
turns out to be harmless because it all happens in a single NAPI poll
cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
misleading.

Rather than extend the scope of the rcu_read_lock(), just get rid of it
entirely. With the addition of RCU annotations to the XDP_REDIRECT map
types that take bh execution into account, lockdep even understands this to
be safe, so there's really no reason to keep it around. Also switch the RCU
dereferences in the driver loop itself to the _bh variants.
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/bpf/20210624160609.292325-14-toke@redhat.com

drivers/net/ethernet/mellanox/mlx4/en_rx.c

@@ -679,9 +679,7 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
 
 	ring = priv->rx_ring[cq_ring];
 
-	/* Protect accesses to: ring->xdp_prog, priv->mac_hash list */
-	rcu_read_lock();
-	xdp_prog = rcu_dereference(ring->xdp_prog);
+	xdp_prog = rcu_dereference_bh(ring->xdp_prog);
 	xdp_init_buff(&xdp, priv->frag_info[0].frag_stride, &ring->xdp_rxq);
 	doorbell_pending = false;
 
@@ -744,7 +742,7 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
 				/* Drop the packet, since HW loopback-ed it */
 				mac_hash = ethh->h_source[MLX4_EN_MAC_HASH_IDX];
 				bucket = &priv->mac_hash[mac_hash];
-				hlist_for_each_entry_rcu(entry, bucket, hlist) {
+				hlist_for_each_entry_rcu_bh(entry, bucket, hlist) {
 					if (ether_addr_equal_64bits(entry->mac,
 								    ethh->h_source))
 						goto next;
@@ -899,8 +897,6 @@ int mlx4_en_process_rx_cq(struct net_device *dev, struct mlx4_en_cq *cq, int bud
 			break;
 	}
 
-	rcu_read_unlock();
-
 	if (likely(polled)) {
 		if (doorbell_pending) {
 			priv->tx_cq[TX_XDP][cq_ring]->xdp_busy = true;