(Sorry for the previous uncompleted comment.  I will continue.)

I'm hitting it in Gnome_shell
(https://bugzilla.redhat.com/show_bug.cgi?id=924076).

I'm hitting it in Cinnamon (https://bugzilla.redhat.com/show_bug.cgi?id=926036)

I'm hitting it in KWin (https://bugs.kde.org/show_bug.cgi?id=315089)

In each case, I get a SIGBUS in __memset_sse2, called from r600 code.  I'm not
used to getting SIGBUSes since I migrated from the PDP-11 :-).

This makes me think that it is a radeon driver bug.

One KDE developer pointed in another direction.  Since it is reported as an
intel driver bug, I don't think that it is right, but it might be relevant:
https://bugs.freedesktop.org/show_bug.cgi?id=58834

I think that I still have crash dumps for each of these.  If something from
them would be useful, ask soon before I recover the space.  But I think that I
can regenerate one all too quickly.