netdev - [PATCH bpf-next v2 07/16] freescale: remove rcu_read

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Message-Id: <20210615145455.564037-8-toke@redhat.com>
Date:   Tue, 15 Jun 2021 16:54:46 +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>,
        Madalin Bucur <madalin.bucur@....com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Ioana Radulescu <ruxandra.radulescu@....com>
Subject: [PATCH bpf-next v2 07/16] freescale: remove rcu_read_lock() around XDP program invocation

The dpaa and dpaa2 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: Madalin Bucur <madalin.bucur@....com>
Cc: Ioana Ciornei <ioana.ciornei@....com>
Cc: Ioana Radulescu <ruxandra.radulescu@....com>
Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
---
 drivers/net/ethernet/freescale/dpaa/dpaa_eth.c   | 11 ++++-------
 drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c |  6 +++---
 2 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
index 177c020bf34a..98fdcbde687a 100644
--- a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
+++ b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
@@ -2558,13 +2558,9 @@ static u32 dpaa_run_xdp(struct dpaa_priv *priv, struct qm_fd *fd, void *vaddr,
 	u32 xdp_act;
 	int err;
 
-	rcu_read_lock();
-
 	xdp_prog = READ_ONCE(priv->xdp_prog);
-	if (!xdp_prog) {
-		rcu_read_unlock();
+	if (!xdp_prog)
 		return XDP_PASS;
-	}
 
 	xdp_init_buff(&xdp, DPAA_BP_RAW_SIZE - DPAA_TX_PRIV_DATA_SIZE,
 		      &dpaa_fq->xdp_rxq);
@@ -2585,6 +2581,9 @@ static u32 dpaa_run_xdp(struct dpaa_priv *priv, struct qm_fd *fd, void *vaddr,
 	}
 #endif
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
 
 	/* Update the length and the offset of the FD */
@@ -2638,8 +2637,6 @@ static u32 dpaa_run_xdp(struct dpaa_priv *priv, struct qm_fd *fd, void *vaddr,
 		break;
 	}
 
-	rcu_read_unlock();
-
 	return xdp_act;
 }
 
diff --git a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
index 8433aa730c42..964d85c9e37d 100644
--- a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
+++ b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
@@ -352,8 +352,6 @@ static u32 dpaa2_eth_run_xdp(struct dpaa2_eth_priv *priv,
 	u32 xdp_act = XDP_PASS;
 	int err, offset;
 
-	rcu_read_lock();
-
 	xdp_prog = READ_ONCE(ch->xdp.prog);
 	if (!xdp_prog)
 		goto out;
@@ -363,6 +361,9 @@ static u32 dpaa2_eth_run_xdp(struct dpaa2_eth_priv *priv,
 	xdp_prepare_buff(&xdp, vaddr + offset, XDP_PACKET_HEADROOM,
 			 dpaa2_fd_get_len(fd), false);
 
+	/* This code is invoked within a single NAPI poll cycle and thus under
+	 * local_bh_disable(), which provides the needed RCU protection.
+	 */
 	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
 
 	/* xdp.data pointer may have changed */
@@ -414,7 +415,6 @@ static u32 dpaa2_eth_run_xdp(struct dpaa2_eth_priv *priv,
 
 	ch->xdp.res |= xdp_act;
 out:
-	rcu_read_unlock();
 	return xdp_act;
 }
 
-- 
2.31.1