(In reply to MirceaKitsune from comment #39)
> I randomly decided to google parts of my dmesg output. I was surprised to
> discover that someone else has reported a very similar issue, which looks
> like it might have the same root as mine!
> 
> https://bugs.freedesktop.org/show_bug.cgi?id=101325
> 
> The dmesg output their provided almost perfectly matches my last log, and
> they also have a RadeonSI card which further narrows down the problem. The
> main difference is that they experience this with Unreal Engine 4 Editor,
> whereas for me the trigger is the Plasma desktop.
> 
> That report seems to contain a fair amount of logs, so hopefully bringing it
> and this together can help produce a solution at long last.

All of these bug reports are GPU lockups.  Whether they are the same root cause
or not remains to be seen.