On 03/30/2015 12:41 PM, Oliver Hartkopp wrote:

On 30.03.2015 12:10, Marc Kleine-Budde wrote:

quoted

quoted

+	/* 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.

Please check out

https://www.kernel.org/pub/linux/kernel/people/rusty/kernel-locking/

And especially 
https://www.kernel.org/pub/linux/kernel/people/rusty/kernel-locking/x173.html#LOCK-SOFTIRQS-SAME

When a softirq processes an incoming skb this remains on that selected CPU.

Okay, I was not sure about this. What about preempt_rt?

The mutithread-test from Andre just lead to the problem that the (former 
single instance) variables ro->uniq_skb and ro->uniq_tstamp have been used by 
different CPUs which made the checks unreliable.

So following the documentation and other examples in kernel source you can

- use spinlocks in can_receive() in af_can.c (instead of rcu_read_lock())
- use per-CPU variables to allow the softirq to run in parallel

Just make the variables atomic (as you suggested) is as bad as introduce 
spinlocks in can_receive() as you reduce the skb processing to just one 
thread. So at least percpu is the best for performance but needs to create a 
vector of variables (percpu).

Ack, lockless atomic-compare-exchange is only possbile for a single
variable.

Putting a struct into these percpu handling can be done - but does it increase 
the readability in this case?

It saves ressources, 1 pointer instead of 3 (considering both of your
patches) and only 1 allocation.

Marc

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