netdev - [BUG] pfifo_fast may cause out-of-order CAN frame transmission

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Date:   Wed, 8 Jan 2020 15:55:34 +0100
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     linux-can@...r.kernel.org, netdev@...r.kernel.org,
        pabeni@...hat.com
Cc:     Sascha Hauer <kernel@...gutronix.de>
Subject: [BUG] pfifo_fast may cause out-of-order CAN frame transmission

Hello,

I've run into an issue of CAN frames being sent out-of-order on an i.MX6 Dual
with Linux v5.5-rc5. Bisecting has lead me down to this commit:

ba27b4cdaaa ("net: dev: introduce support for sch BYPASS for lockless qdisc")

With it, using pfifo_fast, every few hundred frames, FlexCAN's .ndo_start_xmit is
passed frames in an order different from how userspace stuffed them into the same
socket.

Reverting it fixes the issue as does booting with maxcpus=1 or using pfifo
instead of pfifo_fast.

According to [1], such reordering shouldn't be happening.

Details on my setup:
Kernel version: v5.5-rc5, (occurs much more often with LOCKDEP turned on)
CAN-Bitrate: 250 kbit/s
CAN frames are generated with:
cangen canX -I2 -L1 -Di -i -g0.12 -p 100
which keeps polling after ENOBUFS until socket is writable, sends out a CAN
frame with one incrementing payload byte and then waits 120 usec before repeating.

Please let me know if any additional info is needed.

Cheers
Ahmad

[1]: http://linux-tc-notes.sourceforge.net/tc/doc/sch_pfifo_fast.txt

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