From: Oliver Hartkopp <socketcan@hartkopp•net>
To: Marc Kleine-Budde <mkl@pengutronix•de>, linux-can@vger•kernel.org
Cc: netdev@vger•kernel.org
Subject: Re: [PATCH RFC v2 1/2] can: fix multiple delivery of a single CAN frame for overlapping CAN filters
Date: Mon, 30 Mar 2015 12:41:54 +0200 [thread overview]
Message-ID: <55192872.7000108@hartkopp.net> (raw)
In-Reply-To: <5519210E.40005@pengutronix.de>
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
next prev parent reply other threads:[~2015-03-30 10:41 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-29 18:09 [PATCH RFC v2 0/2] can: join filters with per-CPU variables Oliver Hartkopp
2015-03-29 18:09 ` [PATCH RFC v2 1/2] can: fix multiple delivery of a single CAN frame for overlapping CAN filters Oliver Hartkopp
2015-03-30 9:50 ` Marc Kleine-Budde
2015-03-30 10:29 ` Oliver Hartkopp
2015-03-30 10:36 ` Marc Kleine-Budde
2015-03-30 10:10 ` Marc Kleine-Budde
2015-03-30 10:16 ` Marc Kleine-Budde
2015-03-30 10:41 ` Oliver Hartkopp [this message]
2015-03-31 12:32 ` Marc Kleine-Budde
2015-03-31 20:24 ` Oliver Hartkopp
2015-03-30 12:33 ` Sergei Shtylyov
2015-03-30 15:49 ` Oliver Hartkopp
2015-03-30 17:14 ` Sergei Shtylyov
2015-03-30 17:25 ` Oliver Hartkopp
2015-03-29 18:09 ` [PATCH RFC v2 2/2] can: introduce new raw socket option to join the given " Oliver Hartkopp
2015-03-31 12:36 ` Marc Kleine-Budde
2015-03-31 20:30 ` Oliver Hartkopp
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