marvell: Remove rcu_read_lock() around XDP program invocation

The mvneta and mvpp2 drivers have 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.
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Marcin Wojtas <mw@semihalf.com>
Link: https://lore.kernel.org/bpf/20210624160609.292325-13-toke@redhat.com

drivers/net/ethernet/marvell/mvneta.c

@@ -2373,7 +2373,6 @@ static int mvneta_rx_swbm(struct napi_struct *napi,
 	/* Get number of received packets */
 	rx_todo = mvneta_rxq_busy_desc_num_get(pp, rxq);
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(pp->xdp_prog);
 
 	/* Fairness NAPI loop */
@@ -2451,7 +2450,6 @@ static int mvneta_rx_swbm(struct napi_struct *napi,
 		xdp_buf.data_hard_start = NULL;
 		sinfo.nr_frags = 0;
 	}
-	rcu_read_unlock();
 
 	if (xdp_buf.data_hard_start)
 		mvneta_xdp_put_buff(pp, rxq, &xdp_buf, &sinfo, -1);

drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c

@@ -3881,8 +3881,6 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 	int rx_done = 0;
 	u32 xdp_ret = 0;
 
-	rcu_read_lock();
-
 	xdp_prog = READ_ONCE(port->xdp_prog);
 
 	/* Get number of received packets and clamp the to-do */
@@ -4028,8 +4026,6 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 			mvpp2_bm_pool_put(port, pool, dma_addr, phys_addr);
 	}
 
-	rcu_read_unlock();
-
 	if (xdp_ret & MVPP2_XDP_REDIR)
 		xdp_do_flush_map();