Original bug in Gentoo Bugzilla with Linux kernel hang: https://bugs.gentoo.org/649298
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On my PC, I use default/linux/amd64/17.1/systemd profile and:
- sys-devel/gcc-7.3.0
- sys-firmware/intel-microcode-20171117-r1
- x11-base/xorg-server-1.19.6-r1
- x11-misc/lightdm-1.24.0-r1
- sys-boot/plymouth-0.9.3-r1
- sys-apps/systemd-237-r2
- x11-wm/awesome-4.2

I always use NVIDIA GeForce GTX 960 and 2 monitors connected to this card using 2 DisplayPorts. In UEFI, I always have Intel CPU graphics disabled. So NVIDIA is my the only graphics card. In my initramfs, I have the following /etc/modprobe.d/xxx.conf:

> blacklist i915
> blacklist nouveau
> options nvidia_drm modeset=1

In my lightdm.conf, I have:

> display-setup-script=/etc/do_setup_for_lightdm.sh

In my /etc/do_setup_for_lightdm.sh executable, I have:

> #!/usr/bin/env bash
> xrandr --output DP-4 --off
> xdotool mousemove 1280 600
> numlockx

I tried x11-drivers/nvidia-drivers-390.25 with sys-kernel/gentoo-sources-4.14.22 and all worked properly (as usual).

But when I switched to sys-kernel/gentoo-sources-4.15.7-r1, I faced an issue. System started, showed the Plymouth logo, then displayed systemd agetty greeting (as usual), then tried to launch systemd. But lightdm did not start. I faced system hang. Keyboard didn't work. Then I connected to my PC from my notebook via ssh and obtained dmesg.txt (see attachment). I tried to run "sync && systemctl reboot" but my PC did not reboot for some reason. I had to reboot it manually using reset button. After that, I rebooted to my 4.14.22 kernel, performed fsck.ext4, fixed some errors, launched Gentoo Linux on PC and executed "journalctl -a -b -1" (see attachment).

Please help.
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systemd journald log, dmesg output, my configs of 4.15.7 kernel and 4.14.22 kernel can be found here:
https://bugs.gentoo.org/649298