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Date:   Thu, 4 Aug 2022 10:28:01 +0200
From:   Marc Kleine-Budde <>
To:     Sebastian Würl <>
Cc:     Wolfgang Grandegger <>,
        "David S. Miller" <>,
        Eric Dumazet <>,
        Jakub Kicinski <>,
        Paolo Abeni <>,
        Vincent Mailhol <>,
        Andy Shevchenko <>,
        Oliver Hartkopp <>,
        Stefan Mätje <>,
        Uwe Kleine-König 
        Sebastian Andrzej Siewior <>,
        Christian Pellegrin <>,,,
Subject: Re: [PATCH v4] can: mcp251x: Fix race condition on receive interrupt

On 04.08.2022 10:14:11, Sebastian Würl wrote:
> The mcp251x driver uses both receiving mailboxes of the CAN controller
> chips. For retrieving the CAN frames from the controller via SPI, it checks
> once per interrupt which mailboxes have been filled and will retrieve the
> messages accordingly.
> This introduces a race condition, as another CAN frame can enter mailbox 1
> while mailbox 0 is emptied. If now another CAN frame enters mailbox 0 until
> the interrupt handler is called next, mailbox 0 is emptied before
> mailbox 1, leading to out-of-order CAN frames in the network device.
> This is fixed by checking the interrupt flags once again after freeing
> mailbox 0, to correctly also empty mailbox 1 before leaving the handler.
> For reproducing the bug I created the following setup:
>  - Two CAN devices, one Raspberry Pi with MCP2515, the other can be any.
>  - Setup CAN to 1 MHz
>  - Spam bursts of 5 CAN-messages with increasing CAN-ids
>  - Continue sending the bursts while sleeping a second between the bursts
>  - Check on the RPi whether the received messages have increasing CAN-ids
>  - Without this patch, every burst of messages will contain a flipped pair
> Fixes: bf66f3736a94 ("can: mcp251x: Move to threaded interrupts instead of workqueues.")
> Signed-off-by: Sebastian Würl <>

I've reduced the scope of intf1, eflag1 while applying the patch to


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

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