netdev - [PATCH bpf-next v2 13/16] sfc: remove rcu_read_lock() around XDP program invocation

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Message-Id: <20210615145455.564037-14-toke@redhat.com>
Date:   Tue, 15 Jun 2021 16:54:52 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     bpf@...r.kernel.org, netdev@...r.kernel.org
Cc:     Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Edward Cree <ecree.xilinx@...il.com>,
        Martin Habets <habetsm.xilinx@...il.com>
Subject: [PATCH bpf-next v2 13/16] 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.

Cc: Edward Cree <ecree.xilinx@...il.com>
Cc: Martin Habets <habetsm.xilinx@...il.com>
Acked-by: Edward Cree <ecree.xilinx@...il.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 drivers/net/ethernet/sfc/rx.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
index 17b8119c48e5..3e5b88ab42db 100644
--- a/drivers/net/ethernet/sfc/rx.c
+++ b/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())
@@ -295,8 +291,10 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
 	xdp_prepare_buff(&xdp, *ehp - EFX_XDP_HEADROOM, EFX_XDP_HEADROOM,
 			 rx_buf->len, false);
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
-	rcu_read_unlock();
 
 	offset = (u8 *)xdp.data - *ehp;
 
-- 
2.31.1