[PATCH net-next] gve: Fix race condition on tx->dropped_pkt update

[Harshitha Ramamurthy <hramamurthy@google.com>]

From: Max Yuan <maxyuan@google.com>

The tx->dropped_pkt counter is a 64-bit integer that is incremented
directly. On 32-bit architectures, this operation is not atomic and
can lead to read/write tearing if a reader accesses the counter during
the update. This can result in incorrect values being reported for
dropped packets.

To prevent this potential data corruption, wrap the increment
operation with u64_stats_update_begin() and u64_stats_update_end().
This ensures that updates to the 64-bit counter are atomic, even on
32-bit systems, by using a sequence lock.

The u64_stats_sync API requires the writer to have exclusive access,
which is already provided in this context by the network stack's
serialization of the transmit path (net_device_ops::ndo_start_xmit
[1]) for a given queue.

[1]: https://www.kernel.org/doc/Documentation/networking/netdevices.txt

Signed-off-by: Max Yuan <maxyuan@google.com>
Reviewed-by: Jordan Rhee <jordanrhee@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
---
 drivers/net/ethernet/google/gve/gve_tx.c     | 2 ++
 drivers/net/ethernet/google/gve/gve_tx_dqo.c | 6 ++++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/net/ethernet/google/gve/gve_tx.c b/drivers/net/ethernet/google/gve/gve_tx.c
index c6ff0968929d..97efc8d27e6f 100644
--- a/drivers/net/ethernet/google/gve/gve_tx.c
+++ b/drivers/net/ethernet/google/gve/gve_tx.c
@@ -730,7 +730,9 @@ static int gve_tx_add_skb_no_copy(struct gve_priv *priv, struct gve_tx_ring *tx,
 		gve_tx_unmap_buf(tx->dev, &tx->info[idx & tx->mask]);
 	}
 drop:
+	u64_stats_update_begin(&tx->statss);
 	tx->dropped_pkt++;
+	u64_stats_update_end(&tx->statss);
 	return 0;
 }
 
diff --git a/drivers/net/ethernet/google/gve/gve_tx_dqo.c b/drivers/net/ethernet/google/gve/gve_tx_dqo.c
index 6f1d515673d2..40b89b3e5a31 100644
--- a/drivers/net/ethernet/google/gve/gve_tx_dqo.c
+++ b/drivers/net/ethernet/google/gve/gve_tx_dqo.c
@@ -1002,7 +1002,9 @@ static int gve_try_tx_skb(struct gve_priv *priv, struct gve_tx_ring *tx,
 	return 0;
 
 drop:
+	u64_stats_update_begin(&tx->statss);
 	tx->dropped_pkt++;
+	u64_stats_update_end(&tx->statss);
 	dev_kfree_skb_any(skb);
 	return 0;
 }
@@ -1324,7 +1326,11 @@ static void remove_miss_completions(struct gve_priv *priv,
 		/* This indicates the packet was dropped. */
 		dev_kfree_skb_any(pending_packet->skb);
 		pending_packet->skb = NULL;
+
+		u64_stats_update_begin(&tx->statss);
 		tx->dropped_pkt++;
+		u64_stats_update_end(&tx->statss);
+
 		net_err_ratelimited("%s: No reinjection completion was received for: %d.\n",
 				    priv->dev->name,
 				    (int)(pending_packet - tx->dqo.pending_packets));
-- 
2.52.0.487.g5c8c507ade-goog