Hi,

I have a Dell laptop (Vostro 3560). When I boot Fedora 20 with the
acpi_backlight=vendor option, the kernel locks up hard during the boot
proces, when systemd runs udevadm trigger. This is a hard lockup -
magic-sysrq doesn't work, and neither does caps lock/vt-change/etc.

I've bisected this to:

commit 81c0a2bb515fd4daae8cab64352877480792b515
Author: Johannes Weiner <hannes@cmpxchg.org>
Date:   Wed Sep 11 14:20:47 2013 -0700

    mm: page_alloc: fair zone allocator policy

which seemed really unrelated, but I've confirmed that:

 - the commit before this patch doesn't cause the problem, and the commit
afterwrads does
 - reverting that patch from 3.12.0 fixes the problem
 - reverting that patch (and the partial revert
fff4068cba484e6b0abe334ed6b15d5a215a3b25) from master also fixes the problem
 - reverting that patch from the fedora 3.12.5-302.fc20 kernel fixes the
problem
 - applying that patch to 3.11.0 causes the problem

so I'm pretty sure that that is the patch that causes (or at least
triggers) this issue

I'm using the acpi_backlight option to get the backlight working - without
this the backlight doesn't work at all. Removing 'acpi_backlight=vendor'
(or blacklisting the dell-laptop module, which is effectively the same
thing) fixes the issue.

The lockup happens when systemd runs "udevadm trigger", not when the module
is loaded - I can reproduce the issue by booting into emergency mode,
remounting the filesystem as rw, starting up systemd-udevd and running
udevadm trigger manually. It dies a few seconds after loading the
dell-laptop module.

This happens even if I don't boot into X (using
systemd.unit=multi-user.target)

Triggering udev individually for each item doesn't trigger the issue ie:

for i in `udevadm --debug trigger --type=devices --action=add --dry-run
--verbose`; do echo $i; udevadm --debug trigger --type=devices --action=add
--verbose --parent-match=$i; sleep 1; done

works, so I haven't been able to work out what specific combination of
actions are causing this.

With the acpi_backlight option, I can manually read/write to the sysfs
dell-laptop backlight file, and it works (and changes the backlight as
expected)

This is 100% reproducible. I've also tested by powering off the laptop and
pulling the battery just in case one of the previous boots with the bisect
left the hardware in a strange state - no change.

I did successfully boot a 3.12 kernel on F19 (before I upgraded to F20), so
there's presumably something that F20 is doing differently. It was only one
boot though.

I reported this to fedora (
https://bugzilla.redhat.com/show_bug.cgi?id=1045807) but it looks like this
is an upstream issue so I was asked to report it here.

This is an 8-core single i7 cpu (one numa node) - its a laptop, so nothing
fancy. DMI data is attached to the fedora bug.

Bradley