netdev - [PATCH bpf-next v3 14/16] netsec: remove rcu_read

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Message-Id: <20210617212748.32456-15-toke@redhat.com>
Date:   Thu, 17 Jun 2021 23:27:46 +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>,
        Jassi Brar <jaswinder.singh@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>
Subject: [PATCH bpf-next v3 14/16] netsec: remove rcu_read_lock() around XDP program invocation

The netsec driver has a rcu_read_lock()/rcu_read_unlock() pair around the
full RX loop, covering everything up to and including xdp_do_flush(). This
is actually the correct behaviour, but because it all happens in a single
NAPI poll cycle (and thus under local_bh_disable()), it is also technically
redundant.

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 the rcu_read_lock() around anymore, so
let's just remove it.

Cc: Jassi Brar <jaswinder.singh@...aro.org>
Cc: Ilias Apalodimas <ilias.apalodimas@...aro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@...aro.org>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 drivers/net/ethernet/socionext/netsec.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/socionext/netsec.c b/drivers/net/ethernet/socionext/netsec.c
index dfc85cc68173..e07b7c870046 100644
--- a/drivers/net/ethernet/socionext/netsec.c
+++ b/drivers/net/ethernet/socionext/netsec.c
@@ -958,7 +958,6 @@ static int netsec_process_rx(struct netsec_priv *priv, int budget)
 
 	xdp_init_buff(&xdp, PAGE_SIZE, &dring->xdp_rxq);
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(priv->xdp_prog);
 	dma_dir = page_pool_get_dma_dir(dring->page_pool);
 
@@ -1019,6 +1018,10 @@ static int netsec_process_rx(struct netsec_priv *priv, int budget)
 				 pkt_len, false);
 
 		if (xdp_prog) {
+			/* This code is invoked within a single NAPI poll cycle
+			 * and thus under local_bh_disable(), which provides the
+			 * needed RCU protection.
+			 */
 			xdp_result = netsec_run_xdp(priv, xdp_prog, &xdp);
 			if (xdp_result != NETSEC_XDP_PASS) {
 				xdp_act |= xdp_result;
@@ -1069,8 +1072,6 @@ static int netsec_process_rx(struct netsec_priv *priv, int budget)
 	}
 	netsec_finalize_xdp_rx(priv, xdp_act, xdp_xmit);
 
-	rcu_read_unlock();
-
 	return done;
 }
 
-- 
2.32.0