netdev - [PATCH bpf-next v2 11/16] nfp: remove rcu_read_lock() around XDP program invocation

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Message-Id: <20210615145455.564037-12-toke@redhat.com>
Date:   Tue, 15 Jun 2021 16:54:50 +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>,
        Simon Horman <simon.horman@...ronome.com>,
        oss-drivers@...ronome.com
Subject: [PATCH bpf-next v2 11/16] nfp: remove rcu_read_lock() around XDP program invocation

The nfp 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.

While this is not actually an issue for the nfp driver because it doesn't
support XDP_REDIRECT (and thus doesn't call xdp_do_flush()), the
rcu_read_lock() is still unneeded. And 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: Simon Horman <simon.horman@...ronome.com>
Cc: oss-drivers@...ronome.com
Reviewed-by: Simon Horman <simon.horman@...ronome.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 drivers/net/ethernet/netronome/nfp/nfp_net_common.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
index eeb30680b4dc..a3d59abed6ae 100644
--- a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
+++ b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
@@ -1819,7 +1819,6 @@ static int nfp_net_rx(struct nfp_net_rx_ring *rx_ring, int budget)
 	struct xdp_buff xdp;
 	int idx;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(dp->xdp_prog);
 	true_bufsz = xdp_prog ? PAGE_SIZE : dp->fl_bufsz;
 	xdp_init_buff(&xdp, PAGE_SIZE - NFP_NET_RX_BUF_HEADROOM,
@@ -1919,6 +1918,10 @@ static int nfp_net_rx(struct nfp_net_rx_ring *rx_ring, int budget)
 					 pkt_off - NFP_NET_RX_BUF_HEADROOM,
 					 pkt_len, true);
 
+			/* This code is invoked within a single NAPI poll cycle
+			 * and thus under local_bh_disable(), which provides the
+			 * needed RCU protection.
+			 */
 			act = bpf_prog_run_xdp(xdp_prog, &xdp);
 
 			pkt_len = xdp.data_end - xdp.data;
@@ -2036,7 +2039,6 @@ static int nfp_net_rx(struct nfp_net_rx_ring *rx_ring, int budget)
 			if (!nfp_net_xdp_complete(tx_ring))
 				pkts_polled = budget;
 	}
-	rcu_read_unlock();
 
 	return pkts_polled;
 }
-- 
2.31.1