sfc: Remove rcu_read_lock() around XDP program invocation

The sfc 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.

Signed-off-by: Toke Høiland-Jørgensen <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Edward Cree <[email protected]>
Cc: Martin Habets <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

drivers/net/ethernet/sfc/rx.c

@@ -260,18 +260,14 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
 	s16 offset;
 	int err;
 
-	rcu_read_lock();
-	xdp_prog = rcu_dereference(efx->xdp_prog);
-	if (!xdp_prog) {
-		rcu_read_unlock();
+	xdp_prog = rcu_dereference_bh(efx->xdp_prog);
+	if (!xdp_prog)
 		return true;
-	}
 
 	rx_queue = efx_channel_get_rx_queue(channel);
 
 	if (unlikely(channel->rx_pkt_n_frags > 1)) {
 		/* We can't do XDP on fragmented packets - drop. */
-		rcu_read_unlock();
 		efx_free_rx_buffers(rx_queue, rx_buf,
 				    channel->rx_pkt_n_frags);
 		if (net_ratelimit())
@@ -296,7 +292,6 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
 			 rx_buf->len, false);
 
 	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
-	rcu_read_unlock();
 
 	offset = (u8 *)xdp.data - *ehp;