Comment 25 for bug 1434396

I saw similar issue when adding a 2nd screen to my 15.10 systemd + lightdm system, which had been happy with single screen.
Also happy when systemd not used, even with 2 displays

Booted OK, but on resume from suspend (usually after going to lock screen) things were completely hung with per-second repeats of
Dec 1 01:13:00 grunt kernel: [ 591.465083] nouveau E[compiz[5704]] fail ttm_va
lidate
Dec 1 01:13:00 grunt kernel: [ 591.465092] nouveau E[compiz[5704]] validating bo list
Dec 1 01:13:00 grunt kernel: [ 591.465118] nouveau E[compiz[5704]] validate: -12
...

Nouveau is kernel driver, but the issue seemed to come from compiz paging.
Yet compiz behavior is directed by its clients.

When I noticed 'service lightdm status' had warnings about libpam_kwallet5, I looked at how that was used.
Found I had no pam-kwallet5, because I don't use kde/plasma.
I did have kwallet4, which gnome depends on.
So what was bitching about kwallet5???

Still not sure where the missing dpkg dependency on pam-kwallet5 should go,
but it's definately needed -- I added pam-kwallet5 over ssh session and the hung displays
immediately came back to life.

Lightdm's pam usage of kwallet5 is allegedly always optional, as in
/etc/pam.d/lightdm-greeter:132:auth optional pam_kwallet5.so
/etc/pam.d/lightdm-greeter:543:session optional pam_kwallet5.so auto_start
/etc/pam.d/lightdm:223:auth optional pam_kwallet5.so
/etc/pam.d/lightdm:675:session optional pam_kwallet5.so auto_start
But some defect makes it mandatory in a multi-display systemd setup.

Perhaps it's a sequential overspecification in restore logic revealed by multi-display and system event handling.
Don't yet understand, but adding the allegedly optional pam-kwallet5 fixed it