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Date: Mon, 6 May 2024 17:14:17 +0200
From: Marc Kleine-Budde <mkl@pengutronix.de>
To: Vitor Soares <ivitro@gmail.com>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>, 
	Thomas Kopp <thomas.kopp@microchip.com>, Vincent Mailhol <mailhol.vincent@wanadoo.fr>, 
	"David S. Miller" <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>, 
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>, 
	Vitor Soares <vitor.soares@toradex.com>, linux-can@vger.kernel.org, netdev@vger.kernel.org, 
	linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH v4] can: mcp251xfd: fix infinite loop when xmit fails
Message-ID: <20240506-bronze-snake-of-imagination-1db027-mkl@pengutronix.de>
References: <20240506144918.419536-1-ivitro@gmail.com>
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On 06.05.2024 15:49:18, Vitor Soares wrote:
> From: Vitor Soares <vitor.soares@toradex.com>
>=20
> 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.
>=20
> 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 empt=
y. (seq=3D0x000017c7, tef_tail=3D0x000017cf, tef_head=3D0x000017d0, tx_head=
=3D0x000017d3).
> ... and repeat forever.
>=20
> The issue can be triggered when multiple devices share the same
> SPI interface. And there is concurrent access to the bus.
>=20
> 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().
>=20
> 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.
>=20
> 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 orig=
inal 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 =3D netdev_priv(dev);

        priv->echo_skb[idx] =3D NULL;
}

I think you can open-code the "priv->echo_skb[idx] =3D 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

--=20
Pengutronix e.K.                 | Marc Kleine-Budde          |
Embedded Linux                   | https://www.pengutronix.de |
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