[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <55192872.7000108@hartkopp.net>
Date: Mon, 30 Mar 2015 12:41:54 +0200
From: Oliver Hartkopp <socketcan@...tkopp.net>
To: Marc Kleine-Budde <mkl@...gutronix.de>, linux-can@...r.kernel.org
CC: netdev@...r.kernel.org
Subject: Re: [PATCH RFC v2 1/2] can: fix multiple delivery of a single CAN
frame for overlapping CAN filters
On 30.03.2015 12:10, Marc Kleine-Budde wrote:
>>
>> + /* 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.
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).
Putting a struct into these percpu handling can be done - but does it increase
the readability in this case?
Regards,
Oliver
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Powered by blists - more mailing lists