[PATCH v4] can: mcp251xfd: fix infinite loop when xmit fails

[Vitor Soares <ivitro@gmail.com>]

From: Vitor Soares <vitor.soares@toradex.com>

When the mcp251xfd_start_xmit() function fails, the driver stops
processing messages, and the interrupt routine does not return,
running indefinitely even after killing the running application.

Error messages:
[  441.298819] mcp251xfd spi2.0 can0: ERROR in mcp251xfd_start_xmit: -16
[  441.306498] mcp251xfd spi2.0 can0: Transmit Event FIFO buffer not empty. (seq=0x000017c7, tef_tail=0x000017cf, tef_head=0x000017d0, tx_head=0x000017d3).
... and repeat forever.

The issue can be triggered when multiple devices share the same
SPI interface. And there is concurrent access to the bus.

The problem occurs because tx_ring->head increments even if
mcp251xfd_start_xmit() fails. Consequently, the driver skips one
TX package while still expecting a response in
mcp251xfd_handle_tefif_one().

This patch resolves the issue by decreasing tx_ring->head and removing
the skb from the echo stack if mcp251xfd_start_xmit() fails.
Consequently, the package is dropped not been possible to re-transmit.

Fixes: 55e5b97f003e ("can: mcp25xxfd: add driver for Microchip MCP25xxFD SPI CAN")
Cc: stable@vger.kernel.org
Signed-off-by: Vitor Soares <vitor.soares@toradex.com>
---
With this approach, some packages get lost in concurrent SPI bus access
due to can_put_echo_skb() being called before mcp251xfd_tx_obj_write().
The can_put_echo_skb() calls can_create_echo_skb() that consumes the original skb
resulting in a Kernel NULL pointer dereference error if return NETDEV_TX_BUSY on
mcp251xfd_tx_obj_write() failure.
A potential solution would be to change the code to use spi_sync(), which would
wait for SPI bus to be unlocked. Any thoughts about this?

V3->V4:
  - Leave can_put_echo_skb() and stop the queue if needed, before mcp251xfd_tx_obj_write().
  - Re-sync head and remove echo skb if mcp251xfd_tx_obj_write() fails.
  - Revert -> return NETDEV_TX_BUSY if mcp251xfd_tx_obj_write() == -EBUSY.

V2->V3:
  - Add tx_dropped stats.
  - netdev_sent_queue() only if can_put_echo_skb() succeed.

V1->V2:
  - Return NETDEV_TX_BUSY if mcp251xfd_tx_obj_write() == -EBUSY.
  - Rework the commit message to address the change above.
  - Change can_put_echo_skb() to be called after mcp251xfd_tx_obj_write() succeed.
    Otherwise, we get Kernel NULL pointer dereference error.

 drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c | 26 ++++++++++++++------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c b/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
index 160528d3cc26..9909e23de7b9 100644
--- a/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
+++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
@@ -166,11 +166,12 @@ netdev_tx_t mcp251xfd_start_xmit(struct sk_buff *skb,
 				 struct net_device *ndev)
 {
 	struct mcp251xfd_priv *priv = netdev_priv(ndev);
+	struct net_device_stats *stats = &ndev->stats;
 	struct mcp251xfd_tx_ring *tx_ring = priv->tx;
 	struct mcp251xfd_tx_obj *tx_obj;
 	unsigned int frame_len;
+	int err, echo_err;
 	u8 tx_head;
-	int err;
 
 	if (can_dev_dropped_skb(ndev, skb))
 		return NETDEV_TX_OK;
@@ -188,18 +189,27 @@ netdev_tx_t mcp251xfd_start_xmit(struct sk_buff *skb,
 		netif_stop_queue(ndev);
 
 	frame_len = can_skb_get_frame_len(skb);
-	err = can_put_echo_skb(skb, ndev, tx_head, frame_len);
-	if (!err)
+	echo_err = can_put_echo_skb(skb, ndev, tx_head, frame_len);
+	if (!echo_err)
 		netdev_sent_queue(priv->ndev, frame_len);
 
 	err = mcp251xfd_tx_obj_write(priv, tx_obj);
-	if (err)
-		goto out_err;
+	if (err) {
+		tx_ring->head--;
 
-	return NETDEV_TX_OK;
+		if (!echo_err) {
+			can_free_echo_skb(ndev, tx_head, &frame_len);
+			netdev_completed_queue(ndev, 1, frame_len);
+		}
+
+		if (mcp251xfd_get_tx_free(tx_ring))
+			netif_wake_queue(ndev);
 
- out_err:
-	netdev_err(priv->ndev, "ERROR in %s: %d\n", __func__, err);
+		stats->tx_dropped++;
+		if (net_ratelimit())
+			netdev_err(priv->ndev,
+				   "ERROR in %s: %d\n", __func__, err);
+	}
 
 	return NETDEV_TX_OK;
 }
-- 
2.34.1