NETIF_F_LLTX breaks iwlwifi

[Jean Delvare <jdelvare@suse.de>]

Hi Felix, Toke, Johannes,

After updating to kernel 5.2, I started losing wireless network on my
workstation a few minutes after boot. I could restart the network
service to get it back, but it would go away again a few minutes later.
No error message logged, but somehow the network traffic was no long
being processed.

My hardware is:

05:00.0 Network controller [0280]: Intel Corporation Wireless 8265 / 8275 [8086:24fd] (rev 78)

This is an Intel 8265 PCIe WiFI adapter by Gigabyte, model GC-WB867D-I,
which worked flawlessly for me until then.

I bisected it down to:

commit 8dbb000ee73be2c05e34756739ce308885312a29 (refs/bisect/bad)
Author: Felix Fietkau
Date:   Sat Mar 16 18:06:34 2019 +0100

    mac80211: set NETIF_F_LLTX when using intermediate tx queues

So whatever the commit message says, it is apparently not safe to run
TX handlers on multiple CPUs in parallel for this specific driver /
device.

Unless someone has an immediate explanation as to why it broke the
iwlwifi driver and the actual bug is in iwlwifi and it can be fixed
quickly and easily there, I would suggest that the above commit is
reverted for the time being, as apparently it wasn't fixing anything
but was just a performance optimization.

I am available to do any amount of tests or debugging, given the
guidance.

Thanks,
-- 
Jean Delvare
SUSE L3 Support