SDDM crashes with nixos kernel: QQmlThread[2129]: segfault at 0 ip 00007fb27bef93ea sp 00007fb2622494b0 error 4 in libQt5Qml.so.5.9.1[7fb27bdec000+3e0000]

[jirkadanek]

Issue description

When booting into upgraded system, I was welcomed by an empty black nonresponisive screen on vt7 with single white underscore in the top left corner. Instead, I was expecting SDDM to start there. I was able to switch to vt1 and rescue the system from there.

I "resolved" this by switching from services.xserver.displayManager.sddm.enable to services.xserver.displayManager.lightdm.enable = true.

Steps to reproduce

Happens on every reboot.

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

 - system: `"x86_64-linux"`
 - host os: `Linux 4.9.65, NixOS, 18.03pre121253.2f1a818d00 (Impala)`
 - multi-user?: `yes`
 - sandbox: `no`
 - version: `nix-env (Nix) 1.11.15`
 - channels(root): `"nixos-18.03pre121253.2f1a818d00"`
 - channels(jdanek): `"nixos-18.03pre121253.2f1a818d00"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs

journalctl -xe provides this information, which seems to be the cause of my problem

-- Startup of the manager took 10714 microseconds.
Nov 26 11:36:20 nixos systemd[1]: Started User Manager for UID 175.
-- Subject: Unit user@175.service has finished start-up
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit user@175.service has finished starting up.
-- 
-- The start-up result is done.
Nov 26 11:36:20 nixos sddm[2088]: Greeter session started successfully
Nov 26 11:36:20 nixos sddm-greeter[2123]: High-DPI autoscaling not Enabled
Nov 26 11:36:20 nixos sddm-greeter[2123]: Loading theme configuration from "/run/current-system/sw/share/sddm/themes/breeze/theme.conf"
Nov 26 11:36:20 nixos sddm-greeter[2123]: Reading from "/nix/store/z8i5446rlkbhl1bvdsibzp07q7xqjlgz-desktops/plasma5.desktop"
Nov 26 11:36:20 nixos sddm-greeter[2123]: Reading from "/nix/store/z8i5446rlkbhl1bvdsibzp07q7xqjlgz-desktops/xterm.desktop"
Nov 26 11:36:20 nixos sddm-greeter[2123]: inotify_add_watch("/usr/share/wayland-sessions") failed: "No such file or directory"
Nov 26 11:36:20 nixos sddm-greeter[2123]: Connected to the daemon.
Nov 26 11:36:20 nixos sddm[2088]: Message received from greeter: Connect
Nov 26 11:36:20 nixos sddm-greeter[2123]: Loading file:///run/current-system/sw/share/sddm/themes/breeze/Main.qml...
Nov 26 11:36:20 nixos sddm-greeter[2123]: QObject: Cannot create children for a parent that is in a different thread.
                                          (Parent is SDDM::GreeterApp(0x7fff56858ef0), parent's thread is QThread(0x24b5620), current thread is QThread(0x2515ea0)
Nov 26 11:36:20 nixos sddm-greeter[2123]: QObject: Cannot create children for a parent that is in a different thread.
                                          (Parent is SDDM::GreeterApp(0x7fff56858ef0), parent's thread is QThread(0x24b5620), current thread is QThread(0x2515ea0)
Nov 26 11:36:20 nixos sddm-greeter[2123]: QObject: Cannot create children for a parent that is in a different thread.
                                          (Parent is SDDM::GreeterApp(0x7fff56858ef0), parent's thread is QThread(0x24b5620), current thread is QThread(0x2515ea0)
Nov 26 11:36:20 nixos sddm-greeter[2123]: QObject: Cannot create children for a parent that is in a different thread.
                                          (Parent is SDDM::GreeterApp(0x7fff56858ef0), parent's thread is QThread(0x24b5620), current thread is QThread(0x2515ea0)
Nov 26 11:36:20 nixos sddm-greeter[2123]: QObject::installEventFilter(): Cannot filter events for objects in a different thread.
Nov 26 11:36:20 nixos sddm-greeter[2123]: QObject: Cannot create children for a parent that is in a different thread.
                                          (Parent is SDDM::GreeterApp(0x7fff56858ef0), parent's thread is QThread(0x24b5620), current thread is QThread(0x2515ea0)
Nov 26 11:36:20 nixos kernel: QQmlThread[2129]: segfault at 0 ip 00007fb27bef93ea sp 00007fb2622494b0 error 4 in libQt5Qml.so.5.9.1[7fb27bdec000+3e0000]
Nov 26 11:36:20 nixos sddm-helper[2104]: [PAM] Closing session
Nov 26 11:36:20 nixos sddm-helper[2104]: [PAM] Ended.
Nov 26 11:36:20 nixos sddm[2088]: Auth: sddm-helper exited with 11
Nov 26 11:36:20 nixos sddm[2088]: Greeter stopped.
Nov 26 11:36:20 nixos systemd-logind[1170]: Removed session c2.
-- Subject: Session c2 has been terminated
-- Defined-By: systemd
-- Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- Documentation: https://www.freedesktop.org/wiki/Software/systemd/multiseat

I will be happy to provide more debugging information if specifically asked.

[bkchr]

Do you use a nixpkgs checkout? Or on which channel are you?

I thought that the bug should be fixed... Did you updated regularly or did that bug occur after an recent update?

Could you please enable sddm again and try the following:
sudo rm -fr /var/lib/sddm/.cache/sddm-greeter/qmlcache

[jirkadanek]

I use nixos-unstable. I have some software from a nixpkgs checkout (IntelliJ IDE updates, the moment they come out) and I have older SpiderOaks Groups installed that I build myself from RPM which my company provided. I am on 18.03pre121253.2f1a818d00, as stated in the first post.

The bug appeared right after the update I did today. I do update regularly, but infrequently, once every two months, or so.

That qmlcache directory is rather full of stuff. I will try purging later tomorrow and let you know.

[jirkadanek]

@bkchr Purging the qmlcache has made SLIM work for me again. I guess you could close the ticket.

Anything wrong or problematic about my update strategy (as outlined in my previous comment)?

[bkchr]

Nice that it worked. But that is weird, as that should be done automatically :/
It is not your fault, it is/was a problem of Qt that broke the caching stuff.

You can close the ticket by yourself (I can not do that).

[MHOOO]

Had a similar issue today, where sddm (& lightdm) were unable to be used together with plasma 5 (sddm greeter stayed black, whereas lightdm simply fallback to none+xmonad) .

Deleting the qmlcache dir solved this for me.