https://bugs.freedesktop.org/show_bug.cgi?id=90873
--- Comment #6 from Tom neatnoise@gmail.com --- (In reply to Michel Dänzer from comment #5)
http://lists.freedesktop.org/archives/dri-devel/2015-June/084514.html fixes the hang for me (not a GPU hang but a kernel hang, because it tries to lock a mutex that's already locked).
[ 141.232226] radeon 0000:01:00.0: bo ffff8804236d3400 va 0x0000001000 conflict with (bo ffff88042a018800 0x0000001000 0x0000001563)
These still happen, but at least with GNOME it continues working for me somehow. I tried various workarounds for this in the Xorg driver, but I haven't found anything that avoids these. I think the proper solution for these would be for the kernel driver to track GPU VA ranges per GEM handle instead of per BO.
The latest patch applied to linux kernel rc8 fixed this issue, thanks.