Rpi0: Kernel 4.9 [RT] freezes under heavy load with usb hub

[pinoverclock]

Hi everyone, i'm using a rpi0/archlinuxarm for drone control that's why i need a real time kernel.
I've been manually making the linux-raspberrypi package (https://github.com/archlinuxarm/PKGBUILDs/tree/master/core/linux-raspberrypi) so i could patch it with the corresponding RT patches. (https://www.kernel.org/pub/linux/kernel/projects/rt/).

This have been working perfectly for me until the 4.9 kernel brach that freezes instantly under heavy cpu load only if a usb hub is connected, no matter the number of usb devices conected to it or if it is a powered hub or not, the rpi0 just freezes without printing a line so i'm unable to provide any logs.
I can confirm it's happening the latest kernel 4.9.13-2 too

I'm stuck in the last 4.4 brach kernel (4.4.49-1) until that problem is fixed

[WarheadsSE]

Have you reported this to their kernel tree? github.com/raspberrypi/linux/issues/

[pinoverclock]

Thank you for the tip, just did:
https://github.com/raspberrypi/linux/issues/1870

Greetins

[pinoverclock]

Looks like i have found something:

$this->bbcode_second_pass_quote('', 'F')ix lockup in dwc_otg driver that leads to a system freeze of the
4-core Raspberry Pi board when RT Preempt kernel is in use or when
interrupts are threaded in general.
The lockup occurs when the irq handler thread gets preempted while it
holds the fiq spin lock.
The patch makes sure to disable local irq while fiq spin lock is held
irrespective of whether the interrupt is threaded or not.
The patch also unifies the use of the fiq spin lock outside the fiq
handler by introducing two function-like macros fiq_fsm_spin_lock_irqsave
and fiq_fsm_spin_unlock_irqrestore.

Under RT kernel, the bug can be reproduced in a few minutes by running
hackbench and cyclictest in this way
$ ( while true; do nice hackbench 30 >/dev/null; done )&
$ echo "run 'kill $!' to stop hackbench"
$ cyclictest -a -t -n -p 80