Re: [PATCH] drivers/net/can/spi/mcp251x.c: Fix race condition on receive interrupt

[Marc Kleine-Budde]

On 03.08.2022 17:32:59, 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, an 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.

Nitpick: On mcp2515 the buffer is freed automatically.
With this change the interrupt flags are _always_ checked a 2nd time,
even if buffer 0 is not serviced. See below for a change proposal.

> 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
> 
> Signed-off-by: Sebastian Würl <sebastian.wuerl@xxxxxxxxxxxxx>
> ---
>  drivers/net/can/spi/mcp251x.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/can/spi/mcp251x.c b/drivers/net/can/spi/mcp251x.c
> index 666a4505a55a..687aafef4717 100644
> --- a/drivers/net/can/spi/mcp251x.c
> +++ b/drivers/net/can/spi/mcp251x.c
> @@ -1063,17 +1063,14 @@ 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);
> -
> -		/* 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.)
> @@ -1083,14 +1080,24 @@ static irqreturn_t mcp251x_can_ist(int irq, void *dev_id)
>  						   CANINTF_RX0IF, 0x00);
>  		}
>  
> +		/* intf needs to be read again to avoid a race condition */
> +		mcp251x_read_2regs(spi, CANINTF, &intf1, &eflag1);

I think we only have to re-read the interrupt flag if we actually read
data from buffer 0. So what about moving this into the if () { } above?

> +
>  		/* 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);

Marc

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