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 03/30/2015 12:10 PM, Marc Kleine-Budde wrote:
> 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.
       ^
independent

>> 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>

>> +	/* 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.
  ^^^^^^^^^^^^^
put_cpu_ptr()

Marc

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