[PATCH net v2 5/5] dpaa2-eth: execute xdp_do_flush() before napi_complete_done()

[Magnus Karlsson <magnus.karlsson@gmail.com>]

From: Magnus Karlsson <magnus.karlsson@intel.com>

Make sure that xdp_do_flush() is always executed before
napi_complete_done(). This is important for two reasons. First, a
redirect to an XSKMAP assumes that a call to xdp_do_redirect() from
napi context X on CPU Y will be followed by a xdp_do_flush() from the
same napi context and CPU. This is not guaranteed if the
napi_complete_done() is executed before xdp_do_flush(), as it tells
the napi logic that it is fine to schedule napi context X on another
CPU. Details from a production system triggering this bug using the
veth driver can be found following the first link below.

The second reason is that the XDP_REDIRECT logic in itself relies on
being inside a single NAPI instance through to the xdp_do_flush() call
for RCU protection of all in-kernel data structures. Details can be
found in the second link below.

Fixes: d678be1dc1ec ("dpaa2-eth: add XDP_REDIRECT support")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
Link: https://lore.kernel.org/all/20210624160609.292325-1-toke@redhat.com/
---
 drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
index 0c35abb7d065..2e79d18fc3c7 100644
--- a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
+++ b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
@@ -1993,10 +1993,15 @@ static int dpaa2_eth_poll(struct napi_struct *napi, int budget)
 		if (rx_cleaned >= budget ||
 		    txconf_cleaned >= DPAA2_ETH_TXCONF_PER_NAPI) {
 			work_done = budget;
+			if (ch->xdp.res & XDP_REDIRECT)
+				xdp_do_flush();
 			goto out;
 		}
 	} while (store_cleaned);
 
+	if (ch->xdp.res & XDP_REDIRECT)
+		xdp_do_flush();
+
 	/* Update NET DIM with the values for this CDAN */
 	dpaa2_io_update_net_dim(ch->dpio, ch->stats.frames_per_cdan,
 				ch->stats.bytes_per_cdan);
@@ -2032,9 +2037,7 @@ static int dpaa2_eth_poll(struct napi_struct *napi, int budget)
 		txc_fq->dq_bytes = 0;
 	}
 
-	if (ch->xdp.res & XDP_REDIRECT)
-		xdp_do_flush_map();
-	else if (rx_cleaned && ch->xdp.res & XDP_TX)
+	if (rx_cleaned && ch->xdp.res & XDP_TX)
 		dpaa2_eth_xdp_tx_flush(priv, ch, &priv->fq[flowid]);
 
 	return work_done;
-- 
2.34.1