netdev - Re: [PATCH] can: rx-offload: Break loop on queue full

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Date:   Thu, 11 Aug 2022 10:30:39 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
Cc:     Wolfgang Grandegger <wg@...ndegger.com>, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH] can: rx-offload: Break loop on queue full

On 10.08.2022 16:45:36, Uwe Kleine-König wrote:
> The following happend on an i.MX25 using flexcan with many packets on
> the bus:
> 
> The rx-offload queue reached a length more than skb_queue_len_max. So in
> can_rx_offload_offload_one() the drop variable was set to true which
> made the call to .mailbox_read() (here: flexcan_mailbox_read()) just
> return ERR_PTR(-ENOBUFS) (plus some irrelevant hardware interaction) and
> so can_rx_offload_offload_one() returned ERR_PTR(-ENOBUFS), too.
> 
> Now can_rx_offload_irq_offload_fifo() looks as follows:
> 
> 	while (1) {
> 		skb = can_rx_offload_offload_one(offload, 0);
> 		if (IS_ERR(skb))
> 			continue;
> 		...
> 	}
> 
> As the i.MX25 is a single core CPU while the rx-offload processing is
> active there is no thread to process packets from the offload queue and
> so it doesn't get shorter.
> 
> The result is a tight loop: can_rx_offload_offload_one() does nothing
> relevant and returns an error code and so
> can_rx_offload_irq_offload_fifo() calls can_rx_offload_offload_one()
> again.
> 
> To break that loop don't continue calling can_rx_offload_offload_one()
> after it reported an error.
> 
> Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
> ---
> Hello,
> 
> this patch just implements the obvious change to break the loop. I'm not
> 100% certain that there is no corner case where the break is wrong. The
> problem exists at least since v5.6, didn't go back further to check.
> 
> This fixes a hard hang on said i.MX25.

As Uwe suggested in an IRC conversation, the correct fix for the flexcan
driver is to return NULL if there is no CAN frame pending.

I'll send a -v2.

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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