Re: [PATCH v5] can: mcp251xfd: fix infinite loop when xmit fails

[Vitor Soares <ivitro@gmail.com>]

Hi Marc,

Appreciate your feedback.

On Tue, 2024-05-14 at 14:23 +0200, Marc Kleine-Budde wrote:
> On 14.05.2024 11:58:22, 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 starting a workqueue to write
> > the tx obj synchronously if err = -EBUSY. In case of another error,
> > it decrements tx_ring->head, removes skb from the echo stack, and
> > drops the message.
> 
> This looks quite good! Good work!
> 
> I think you better move the allocation/destroy of the wq into the open()
> and stop() callbacks. You have to destroy the workqueue before putting
> the interface to sleep.
> 
> > 
> > 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>
> > ---
> > 
> > V4->V5:
> >   - Start a workqueue to write tx obj with spi_sync() when spi_async() == -
> > EBUSY.
> > 
> > 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.
> > 
> >  .../net/can/spi/mcp251xfd/mcp251xfd-core.c    | 13 ++++-
> >  drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c  | 51 ++++++++++++++++---
> >  drivers/net/can/spi/mcp251xfd/mcp251xfd.h     |  5 ++
> >  3 files changed, 60 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c
> > b/drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c
> > index 1d9057dc44f2..6cca853f2b1e 100644
> > --- a/drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c
> > +++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c
> > @@ -2141,15 +2141,25 @@ static int mcp251xfd_probe(struct spi_device *spi)
> >         if (err)
> >                 goto out_free_candev;
> >  
> > +       priv->tx_work_obj = NULL;
> > +       priv->wq = alloc_workqueue("mcp251xfd_wq", WQ_FREEZABLE, 0);
> 
> The m_can driver uses a ordered workqueue and you can add the name of
> the spi device to the wq's name :)
> 
>         priv->wq = alloc_ordered_workqueue("%s-mcp251xfd_wq", WQ_FREEZABLE |
> WQ_MEM_RECLAIM, dev_name(&spi->dev));
> 
> > +       if (!priv->wq) {
> > +               err = -ENOMEM;
> > +               goto out_can_rx_offload_del;
> > +       }
> > +       INIT_WORK(&priv->tx_work, mcp251xfd_tx_obj_write_sync);
> > +
> >         err = mcp251xfd_register(priv);
> >         if (err) {
> >                 dev_err_probe(&spi->dev, err, "Failed to detect %s.\n",
> >                               mcp251xfd_get_model_str(priv));
> > -               goto out_can_rx_offload_del;
> > +               goto out_can_free_wq;
> 
> nitpick:
>                      out_destroy_workqueue;
> 
> to match the function call.
> 
> >         }
> >  
> >         return 0;
> >  
> > + out_can_free_wq:
> > +       destroy_workqueue(priv->wq);
> >   out_can_rx_offload_del:
> >         can_rx_offload_del(&priv->offload);
> >   out_free_candev:
> > @@ -2165,6 +2175,7 @@ static void mcp251xfd_remove(struct spi_device *spi)
> >         struct mcp251xfd_priv *priv = spi_get_drvdata(spi);
> >         struct net_device *ndev = priv->ndev;
> >  
> > +       destroy_workqueue(priv->wq);
> >         can_rx_offload_del(&priv->offload);
> >         mcp251xfd_unregister(priv);
> >         spi->max_speed_hz = priv->spi_max_speed_hz_orig;
> > diff --git a/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > b/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > index 160528d3cc26..1e7ddf316643 100644
> > --- a/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > +++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > @@ -131,6 +131,41 @@ mcp251xfd_tx_obj_from_skb(const struct mcp251xfd_priv
> > *priv,
> >         tx_obj->xfer[0].len = len;
> >  }
> >  
> > +static void mcp251xfd_tx_failure_drop(const struct mcp251xfd_priv *priv,
> > +                                     struct mcp251xfd_tx_ring *tx_ring,
> > +                                     int err)
> > +{
> > +       struct net_device *ndev = priv->ndev;
> > +       struct net_device_stats *stats = &ndev->stats;
> > +       unsigned int frame_len = 0;
> > +       u8 tx_head;
> > +
> > +       tx_ring->head--;
> > +       stats->tx_dropped++;
> > +       tx_head = mcp251xfd_get_tx_head(tx_ring);
> > +       can_free_echo_skb(ndev, tx_head, &frame_len);
> > +       netdev_completed_queue(ndev, 1, frame_len);
> > +       netif_wake_queue(ndev);
> > +
> > +       if (net_ratelimit())
> > +               netdev_err(priv->ndev, "ERROR in %s: %d\n", __func__, err);
> > +}
> > +
> > +void mcp251xfd_tx_obj_write_sync(struct work_struct *ws)
> > +{
> > +       struct mcp251xfd_priv *priv = container_of(ws, struct
> > mcp251xfd_priv,
> > +                                                  tx_work);
> > +       struct mcp251xfd_tx_obj *tx_obj = priv->tx_work_obj;
> > +       struct mcp251xfd_tx_ring *tx_ring = priv->tx;
> > +       int err;
> > +
> > +       err = spi_sync(priv->spi, &tx_obj->msg);
> > +       if (err)
> > +               mcp251xfd_tx_failure_drop(priv, tx_ring, err);
> > +
> > +       priv->tx_work_obj = NULL;
> 
> Race condition:
> - after spi_sync() the CAN frame is send
> - after the TX complete IRQ the TX queue is restarted
> - the xmit handler might get BUSY
> - fill the tx_work_obj again
> 
> > +}
> > +
> >  static int mcp251xfd_tx_obj_write(const struct mcp251xfd_priv *priv,
> >                                   struct mcp251xfd_tx_obj *tx_obj)
> >  {
> > @@ -175,7 +210,7 @@ netdev_tx_t mcp251xfd_start_xmit(struct sk_buff *skb,
> >         if (can_dev_dropped_skb(ndev, skb))
> >                 return NETDEV_TX_OK;
> >  
> > -       if (mcp251xfd_tx_busy(priv, tx_ring))
> > +       if (mcp251xfd_tx_busy(priv, tx_ring) || priv->tx_work_obj)
> 
> This should not happen, but better save than sorry.


As there is the race condition you mentioned above, on this condition:
priv->tx_work_obj = tx_obj --> xmit will return NETDEV_TX_BUSY

or

priv->tx_work_obj = NULL --> It goes through the rest of the code or the
workqueue may sleep after setting tx_work_obj to NULL. Should I use work_busy()
here instead or do you have another suggestion?


Everything else is clear to me and I'm addressing it for the v6.

Best regards,
Vitor Soares