https://bugs.freedesktop.org/show_bug.cgi?id=104806
--- Comment #2 from Alexandr Paliy alexandrpaliy@gmail.com --- Also gentoo, also radeon card (r600).
Posting this just to confirm there is an issue. Can't provide additional info because I needed to restore access to my PC as fast as possible.
dmesg pointed segfaults in plasmashell and sddm-helper processes. First I thought it is somehow related to updating kernel 4.14.12->4.14.15, but reverting it did not help.
Then, I saw mentions of r600g_dri.so Switched from mesa-18.0.0-rc2 to mesa-17.3.3, and issue is gone.