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

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Message-ID: <20240506-bronze-snake-of-imagination-1db027-mkl@pengutronix.de>
Date: Mon, 6 May 2024 17:14:17 +0200
From: Marc Kleine-Budde <mkl@...gutronix.de>
To: Vitor Soares <ivitro@...il.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>, 
	Thomas Kopp <thomas.kopp@...rochip.com>, Vincent Mailhol <mailhol.vincent@...adoo.fr>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Vitor Soares <vitor.soares@...adex.com>, linux-can@...r.kernel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v4] can: mcp251xfd: fix infinite loop when xmit fails

On 06.05.2024 15:49:18, 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 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@...r.kernel.org
> Signed-off-by: Vitor Soares <vitor.soares@...adex.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?

This is not an option. I think you need a echo_skb function that does
the necessary cleanup, something like:

void can_remove_echo_skb(struct net_device *dev, unsigned int idx)
{
	struct can_priv *priv = netdev_priv(dev);

        priv->echo_skb[idx] = NULL;
}

I think you can open-code the "priv->echo_skb[idx] = NULL;" directly in
the driver.

And you have to take care of calling netdev_completed_queue(priv->ndev,
1, frame_len);

Another option would be to start a workqueue and use spi_sync() in case
the spi_async() is busy.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde          |
Embedded Linux                   | https://www.pengutronix.de |
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