netdev - [PATCH bpf-next 05/17] ena: remove rcu_read_lock() around XDP program invocation

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Date:   Wed,  9 Jun 2021 12:33:14 +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>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Guy Tzalik <gtzalik@...zon.com>,
        Saeed Bishara <saeedb@...zon.com>
Subject: [PATCH bpf-next 05/17] ena: remove rcu_read_lock() around XDP program invocation

The ena 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: Guy Tzalik <gtzalik@...zon.com>
Cc: Saeed Bishara <saeedb@...zon.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 drivers/net/ethernet/amazon/ena/ena_netdev.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/net/ethernet/amazon/ena/ena_netdev.c b/drivers/net/ethernet/amazon/ena/ena_netdev.c
index 881f88754bf6..a4378b14af4c 100644
--- a/drivers/net/ethernet/amazon/ena/ena_netdev.c
+++ b/drivers/net/ethernet/amazon/ena/ena_netdev.c
@@ -385,7 +385,6 @@ static int ena_xdp_execute(struct ena_ring *rx_ring, struct xdp_buff *xdp)
 	u64 *xdp_stat;
 	int qid;

-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_bpf_prog);

 	if (!xdp_prog)
@@ -443,8 +442,6 @@ static int ena_xdp_execute(struct ena_ring *rx_ring, struct xdp_buff *xdp)

 	ena_increase_stat(xdp_stat, 1, &rx_ring->syncp);
 out:
-	rcu_read_unlock();
-
 	return verdict;
 }

-- 
2.31.1