Re: [PATCH] can: mcp251x: Fix race condition on receive interrupt

[Marc Kleine-Budde]

On 04.08.2022 08:48:03, 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 <sebastian.wuerl@xxxxxxxxxxxxx>

Thanks for your patch! I think we're almost there. If you send a new
version of the patch, please increase the reroll count, i.e. add a -v3
to the patch subject, this can be done with the parameter "-v3" to git
send-email or git format-patch.

> ---
>  drivers/net/can/spi/mcp251x.c | 26 +++++++++++++++++++-------
>  1 file changed, 19 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/can/spi/mcp251x.c b/drivers/net/can/spi/mcp251x.c
> index 89897a2d41fa..ca462868141c 100644
> --- a/drivers/net/can/spi/mcp251x.c
> +++ b/drivers/net/can/spi/mcp251x.c
> @@ -1068,17 +1068,14 @@ static irqreturn_t mcp251x_can_ist(int irq, void *dev_id)
>  	mutex_lock(&priv->mcp_lock);
>  	while (!priv->force_quit) {
>  		enum can_state new_state;
> -		u8 intf, eflag;
> +		u8 intf, intf0, intf1, eflag, eflag0, eflag1;
>  		u8 clear_intf = 0;
>  		int can_id = 0, data1 = 0;
>  
> -		mcp251x_read_2regs(spi, CANINTF, &intf, &eflag);

Keep the read into "&intf, &eflag" here....

> -
> -		/* mask out flags we don't care about */
> -		intf &= CANINTF_RX | CANINTF_TX | CANINTF_ERR;
> +		mcp251x_read_2regs(spi, CANINTF, &intf0, &eflag0);
>  
>  		/* receive buffer 0 */
> -		if (intf & CANINTF_RX0IF) {
> +		if (intf0 & CANINTF_RX0IF) {
>  			mcp251x_hw_rx(spi, 0);
>  			/* Free one buffer ASAP
>  			 * (The MCP2515/25625 does this automatically.)
> @@ -1086,16 +1083,31 @@ static irqreturn_t mcp251x_can_ist(int irq, void *dev_id)
>  			if (mcp251x_is_2510(spi))
>  				mcp251x_write_bits(spi, CANINTF,
>  						   CANINTF_RX0IF, 0x00);
> +
> +			if (intf0 & CANINTF_RX1IF) {
> +				/* buffer 1 is already known to be full, no need to re-read */

Nice! I haven't thought about this optimization.

> +				intf1 = intf0;

...no need to assign intf1.

> +			} else {

Move intf1 into this scope...

> +				/* intf needs to be read again to avoid a race condition */
> +				mcp251x_read_2regs(spi, CANINTF, &intf1, &eflag1);

...and "or" it to intf here:

intf |= intf1;

Another optimization idea: Do we need to re-read the eflag1? "eflag" is
for error handling only and you're optimizing the good path.

> +			}
>  		}
>  
>  		/* receive buffer 1 */
> -		if (intf & CANINTF_RX1IF) {
> +		if (intf1 & CANINTF_RX1IF) {
>  			mcp251x_hw_rx(spi, 1);
>  			/* The MCP2515/25625 does this automatically. */
>  			if (mcp251x_is_2510(spi))
>  				clear_intf |= CANINTF_RX1IF;
>  		}
>  
> +		/* combine flags from both operations for error handling */
> +		intf = intf0 | intf1;
> +		eflag = eflag0 | eflag1;
> +
> +		/* mask out flags we don't care about */
> +		intf &= CANINTF_RX | CANINTF_TX | CANINTF_ERR;
> +
>  		/* any error or tx interrupt we need to clear? */
>  		if (intf & (CANINTF_ERR | CANINTF_TX))
>  			clear_intf |= intf & (CANINTF_ERR | CANINTF_TX);
> -- 
> 2.30.2
> 
> 

regards,
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 |

Attachment: signature.asc
Description: PGP signature