Re: [PATCH RFC v2 1/2] can: fix multiple delivery of a single CAN frame for overlapping CAN filters

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 03/29/2015 08:09 PM, Oliver Hartkopp wrote:
> The CAN_RAW socket can set multiple CAN identifier specific filters that lead
> to multiple filters in the af_can.c filter processing. These filters are
> indenpendent from each other which leads to logical OR'ed filters when applied.
> 
> This patch makes sure that every CAN frame which is filtered for a specific
> socket is only delivered once to the user space. This is independent from the
> number of matching CAN filters of this socket.
> 
> As the can_raw() function is executed from NET_RX softirq the introduced
> variables are implemented as per-CPU variables to avoid extensive locking at
> CAN frame reception time.
> 
> Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
> ---
>  net/can/raw.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/net/can/raw.c b/net/can/raw.c
> index 00c13ef..866a9b3 100644
> --- a/net/can/raw.c
> +++ b/net/can/raw.c
> @@ -86,6 +86,8 @@ struct raw_sock {
>  	struct can_filter dfilter; /* default/single filter */
>  	struct can_filter *filter; /* pointer to filter(s) */
>  	can_err_mask_t err_mask;
> +	struct sk_buff __percpu **uniq_skb;
> +	ktime_t __percpu *uniq_tstamp;
>  };
>  
>  /*
> @@ -123,6 +125,15 @@ static void raw_rcv(struct sk_buff *oskb, void *data)
>  	if (!ro->fd_frames && oskb->len != CAN_MTU)
>  		return;
>  
> +	/* eliminate multiple filter matches for the same skb */
> +	if (*this_cpu_ptr(ro->uniq_skb) == oskb &&
> +	    ktime_equal(*this_cpu_ptr(ro->uniq_tstamp), oskb->tstamp)) {
> +			return;
> +	} else {
> +		*this_cpu_ptr(ro->uniq_skb) = oskb;
> +		*this_cpu_ptr(ro->uniq_tstamp) = oskb->tstamp;
> +	}
> +

What happens if you're preempted somewhere in this code, it's not
atomic? I think, if we only have to take care about the skb, an atomic
compare exchange would work. But we have two variables....If you use a
struct (see previous mail), I think the usage of get_cpu_ptr(),
git_cpu_ptr() ensures that we're not preempted.

Marc

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