netdev - [PATCH bpf-next v2 09/16] marvell: remove rcu_read

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Message-Id: <20210615145455.564037-10-toke@redhat.com>
Date:   Tue, 15 Jun 2021 16:54:48 +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>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Russell King <linux@...linux.org.uk>
Subject: [PATCH bpf-next v2 09/16] 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.

Cc: Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Cc: Marcin Wojtas <mw@...ihalf.com>
Cc: Russell King <linux@...linux.org.uk>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 drivers/net/ethernet/marvell/mvneta.c           | 6 ++++--
 drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c | 8 ++++----
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
index 7d5cd9bc6c99..c2e9cbebc8d1 100644
--- a/drivers/net/ethernet/marvell/mvneta.c
+++ b/drivers/net/ethernet/marvell/mvneta.c
@@ -2370,7 +2370,6 @@ 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 */
@@ -2421,6 +2420,10 @@ static int mvneta_rx_swbm(struct napi_struct *napi,
 			goto next;
 		}
 
+		/* This code is invoked within a single NAPI poll cycle and thus
+		 * under local_bh_disable(), which provides the needed RCU
+		 * protection.
+		 */
 		if (xdp_prog &&
 		    mvneta_run_xdp(pp, rxq, xdp_prog, &xdp_buf, frame_sz, &ps))
 			goto next;
@@ -2448,7 +2451,6 @@ 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);
diff --git a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
index b2259bf1d299..658db1720826 100644
--- a/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
+++ b/drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c
@@ -3852,8 +3852,6 @@ 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 */
@@ -3925,6 +3923,10 @@ static int mvpp2_rx(struct mvpp2_port *port, struct napi_struct *napi,
 					 MVPP2_MH_SIZE + MVPP2_SKB_HEADROOM,
 					 rx_bytes, false);
 
+			/* This code is invoked within a single NAPI poll cycle
+			 * and thus under local_bh_disable(), which provides the
+			 * needed RCU protection.
+			 */
 			ret = mvpp2_run_xdp(port, xdp_prog, &xdp, pp, &ps);
 
 			if (ret) {
@@ -3988,8 +3990,6 @@ 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();
 
-- 
2.31.1