netdev - [PATCH bpf-next v2 12/16] qede: remove rcu_read

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Message-Id: <20210615145455.564037-13-toke@redhat.com>
Date:   Tue, 15 Jun 2021 16:54:51 +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>,
        Ariel Elior <aelior@...vell.com>,
        GR-everest-linux-l2@...vell.com
Subject: [PATCH bpf-next v2 12/16] qede: remove rcu_read_lock() around XDP program invocation

The qede 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: Ariel Elior <aelior@...vell.com>
Cc: GR-everest-linux-l2@...vell.com
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 drivers/net/ethernet/qlogic/qede/qede_fp.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/drivers/net/ethernet/qlogic/qede/qede_fp.c b/drivers/net/ethernet/qlogic/qede/qede_fp.c
index 8e150dd4f899..d806ab925cee 100644
--- a/drivers/net/ethernet/qlogic/qede/qede_fp.c
+++ b/drivers/net/ethernet/qlogic/qede/qede_fp.c
@@ -1089,13 +1089,10 @@ static bool qede_rx_xdp(struct qede_dev *edev,
 	xdp_prepare_buff(&xdp, page_address(bd->data), *data_offset,
 			 *len, false);
 
-	/* Queues always have a full reset currently, so for the time
-	 * being until there's atomic program replace just mark read
-	 * side for map helpers.
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
 	 */
-	rcu_read_lock();
 	act = bpf_prog_run_xdp(prog, &xdp);
-	rcu_read_unlock();
 
 	/* Recalculate, as XDP might have changed the headers */
 	*data_offset = xdp.data - xdp.data_hard_start;
-- 
2.31.1