Re: [PATCH v2] can: mcp251xfd: fix infinite loop when xmit fails

[vitor <ivitro@gmail.com>]

On Fri, 2024-03-08 at 15:15 +0000, Vitor Soares wrote:
> 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 if
> mcp251xfd_start_xmit() fails. With the fix, if we trigger the issue
> and
> the err = -EBUSY, the driver returns NETDEV_TX_BUSY. The network
> stack
> retries to transmit the message.
> Otherwise, it prints an error and discards the message.
> 
> 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>
> ---
> 
> 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 | 29 +++++++++++-------
> --
>  1 file changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> b/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> index 160528d3cc26..0fdaececebdd 100644
> --- a/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> +++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> @@ -181,25 +181,28 @@ netdev_tx_t mcp251xfd_start_xmit(struct sk_buff
> *skb,
>         tx_obj = mcp251xfd_get_tx_obj_next(tx_ring);
>         mcp251xfd_tx_obj_from_skb(priv, tx_obj, skb, tx_ring->head);
>  
> -       /* Stop queue if we occupy the complete TX FIFO */
>         tx_head = mcp251xfd_get_tx_head(tx_ring);
> -       tx_ring->head++;
> -       if (mcp251xfd_get_tx_free(tx_ring) == 0)
> -               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)
> -               netdev_sent_queue(priv->ndev, frame_len);
> +
> +       tx_ring->head++;
>  
>         err = mcp251xfd_tx_obj_write(priv, tx_obj);
> -       if (err)
> -               goto out_err;
> +       if (err) {
> +               tx_ring->head--;
>  
> -       return NETDEV_TX_OK;
> +               if (err == -EBUSY)
> +                       return NETDEV_TX_BUSY;

Missing the stats for dropped packages. I will add on v3.

>  
> - out_err:
> -       netdev_err(priv->ndev, "ERROR in %s: %d\n", __func__, err);
> +               netdev_err(priv->ndev, "ERROR in %s: %d\n", __func__,
> err);
> +       } else {
> +               can_put_echo_skb(skb, ndev, tx_head, frame_len);

Not sure if it is save to call this here.

> +
> +               /* Stop queue if we occupy the complete TX FIFO */
> +               if (mcp251xfd_get_tx_free(tx_ring) == 0)
> +                       netif_stop_queue(ndev);
> +
> +               netdev_sent_queue(priv->ndev, frame_len);

This is not correct. Should be called only if can_put_echo_skb()
succeed. I will fix this in v3.

> +       }
>  
>         return NETDEV_TX_OK;
>  }

Best regards,
Vitor Soares