linux-kernel - Re: [PATCH] can: mcp251xfd: fix infinite loop when xmit fails

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Date: Thu, 7 Mar 2024 21:40:33 +0100
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: vitor <ivitro@...il.com>
Cc: manivannan.sadhasivam@...aro.org, thomas.kopp@...rochip.com, 
	wg@...ndegger.com, linux-can@...r.kernel.org, linux-kernel@...r.kernel.org, 
	vitor.soares@...adex.com, stable@...r.kernel.org
Subject: Re: [PATCH] can: mcp251xfd: fix infinite loop when xmit fails

On 07.03.2024 16:54:13, vitor wrote:
> > On 07.03.2024 12:04:42, Vitor Soares wrote:
> > > From: Vitor Soares <vitor.soares@...adex.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 attempt to
> > > trigger
> > > the issue again, the driver prints an error and discard the
> > > message.
> > 
> > What about returning NETDEV_TX_BUSY, then the networking stack will
> > retry.
> 
> Do you mean when err == -EBUSY?

ACK

> > 
> > > Fixes: 55e5b97f003e ("can: mcp25xxfd: add driver for Microchip
> > > MCP25xxFD SPI CAN")
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Vitor Soares <vitor.soares@...adex.com>
> > > ---
> > >  drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c | 27 ++++++++++------
> > > ----
> > >  1 file changed, 14 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..a8eb941c1b95 100644
> > > --- a/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > > +++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd-tx.c
> > > @@ -181,25 +181,26 @@ 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);
> > > +       can_put_echo_skb(skb, ndev, tx_head, frame_len);
> > > +
> > > +       tx_ring->head++;
> > >  
> > >         err = mcp251xfd_tx_obj_write(priv, tx_obj);
> > > -       if (err)
> > > -               goto out_err;
> > > +       if (err) {
> > > +               can_free_echo_skb(ndev, tx_head, NULL);
> > >  
> > > -       return NETDEV_TX_OK;
> > > +               tx_ring->head--;
> > > +
> 
> I'm testing returning here when err = -EBUSY, but can_put_echo_skb()
> should be invoked after mcp251xfd_tx_obj_write(). Otherwise, I get a
> Kernel NULL pointer dereference error.

I mean if mcp251xfd_tx_obj_write() returns -EBUSY don't print an error
message, on any other error it's ok to print the error.

> > 
> > I'm not sure, if we want an error message for -EBUSY. We could add
> > proper ethtool statistics.
> > 
> > > +               netdev_err(priv->ndev, "ERROR in %s: %d\n",
> > > __func__, err);
> > > +       } else {
> > > +               /* Stop queue if we occupy the complete TX FIFO */
> > > +               if (mcp251xfd_get_tx_free(tx_ring) == 0)
> > > +                       netif_stop_queue(ndev);
> > >  
> > > - out_err:
> > > -       netdev_err(priv->ndev, "ERROR in %s: %d\n", __func__, err);
> > > +               netdev_sent_queue(priv->ndev, frame_len);
> > > +       }
> > >  
> > >         return NETDEV_TX_OK;
> > >  }

Marc

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