netdev - [PATCH RFC bpf-next 4/4] i40e: remove rcu_read_lock() around XDP program invocation

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Date:   Fri, 23 Apr 2021 13:05:20 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH RFC bpf-next 4/4] i40e: remove rcu_read_lock() around XDP
 program invocation

From: Toke Høiland-Jørgensen <toke@...hat.com>

The i40e 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.

Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 drivers/net/ethernet/intel/i40e/i40e_txrx.c |    2 --
 drivers/net/ethernet/intel/i40e/i40e_xsk.c  |    6 +-----
 2 files changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.c b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
index fc20afc23bfa..3f4c947a5185 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_txrx.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
@@ -2303,7 +2303,6 @@ static struct sk_buff *i40e_run_xdp(struct i40e_ring *rx_ring,
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
 	xdp_prog = READ_ONCE(rx_ring->xdp_prog);
 
 	if (!xdp_prog)
@@ -2334,7 +2333,6 @@ static struct sk_buff *i40e_run_xdp(struct i40e_ring *rx_ring,
 		break;
 	}
 xdp_out:
-	rcu_read_unlock();
 	return ERR_PTR(-result);
 }
 
diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
index d89c22347d9d..93b349f11d3b 100644
--- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
+++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
@@ -153,7 +153,6 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 	struct bpf_prog *xdp_prog;
 	u32 act;
 
-	rcu_read_lock();
 	/* NB! xdp_prog will always be !NULL, due to the fact that
 	 * this path is enabled by setting an XDP program.
 	 */
@@ -162,9 +161,7 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 
 	if (likely(act == XDP_REDIRECT)) {
 		err = xdp_do_redirect(rx_ring->netdev, xdp, xdp_prog);
-		result = !err ? I40E_XDP_REDIR : I40E_XDP_CONSUMED;
-		rcu_read_unlock();
-		return result;
+		return !err ? I40E_XDP_REDIR : I40E_XDP_CONSUMED;
 	}
 
 	switch (act) {
@@ -184,7 +181,6 @@ static int i40e_run_xdp_zc(struct i40e_ring *rx_ring, struct xdp_buff *xdp)
 		result = I40E_XDP_CONSUMED;
 		break;
 	}
-	rcu_read_unlock();
 	return result;
 }