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SubjectRe: [bug, bisected] pfifo_fast causes packet reordering
On Tue, Mar 13, 2018 at 11:24 AM, Jakob Unterwurzacher
<> wrote:
> During stress-testing our "ucan" USB/CAN adapter SocketCAN driver on Linux
> v4.16-rc4-383-ged58d66f60b3 we observed that a small fraction of packets are
> delivered out-of-order.
> We have tracked the problem down to the driver interface level, and it seems
> that the driver's net_device_ops.ndo_start_xmit() function gets the packets
> handed over in the wrong order.
> This behavior was not observed on Linux v4.15 and I have bisected the
> problem down to this patch:
>> commit c5ad119fb6c09b0297446be05bd66602fa564758
>> Author: John Fastabend <>
>> Date: Thu Dec 7 09:58:19 2017 -0800
>> net: sched: pfifo_fast use skb_array
>> This converts the pfifo_fast qdisc to use the skb_array data structure
>> and set the lockless qdisc bit. pfifo_fast is the first qdisc to
>> support
>> the lockless bit that can be a child of a qdisc requiring locking. So
>> we add logic to clear the lock bit on initialization in these cases
>> when
>> the qdisc graft operation occurs.
>> This also removes the logic used to pick the next band to dequeue from
>> and instead just checks a per priority array for packets from top
>> priority
>> to lowest. This might need to be a bit more clever but seems to work
>> for now.
>> Signed-off-by: John Fastabend <>
>> Signed-off-by: David S. Miller <>
> The patch does not revert cleanly, but moving to one commit earlier makes
> the problem go away.
> Selecting the "fq" scheduler instead of "pfifo_fast" makes the problem go
> away as well.

I am of course, a fan of obsoleting pfifo_fast. There's no good reason
for it anymore.

> Is this an unintended side-effect of the patch or is there something the
> driver has to do to request in-order delivery?
> Thanks,
> Jakob


Dave Täht
CEO, TekLibre, LLC
Tel: 1-669-226-2619

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