from dmesg:

[76223.077212] radeon 0000:01:00.0: bo ffff88035b4d3c00 va 0x00000284cc conflict with (bo ffff8803ed46e800 0x00000284cc 0x00000284db)
[76437.621968] INFO: task LoversInADanger:8897 blocked for more than 120 seconds.
[76437.621973] Tainted: P OE 3.19.0-16-generic #16-Ubuntu
[76437.621974] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[76437.621976] LoversInADanger D ffff88009c323c28 0 8897 8880 0x00000004
[76437.621979] ffff88009c323c28 ffff8800c0962740 0000000000014200 ffff88009c323fd8
[76437.621982] 0000000000014200 ffff88041d6789d0 ffff8800c0962740 000000006ee92010
[76437.621983] ffff88035b4d3df0 ffff88035b4d3df4 ffff8800c0962740 00000000ffffffff
[76437.621985] Call Trace:
[76437.621992] [<ffffffff817c4f49>] schedule_preempt_disabled+0x29/0x70
[76437.621995] [<ffffffff817c6e05>] __mutex_lock_slowpath+0x95/0x100
[76437.621997] [<ffffffff817c6e93>] mutex_lock+0x23/0x40
[76437.622027] [<ffffffffc02aa070>] radeon_gem_object_close+0x70/0x120 [radeon]
[76437.622042] [<ffffffffc0134a6c>] drm_gem_handle_delete+0xac/0x100 [drm]
[76437.622049] [<ffffffffc0135125>] drm_gem_close_ioctl+0x25/0x30 [drm]
[76437.622056] [<ffffffffc0135a6f>] drm_ioctl+0x1df/0x680 [drm]
[76437.622060] [<ffffffff812335de>] ? fsnotify+0x37e/0x530
[76437.622068] [<ffffffffc027404c>] radeon_drm_ioctl+0x4c/0x80 [radeon]
[76437.622071] [<ffffffff81207b70>] do_vfs_ioctl+0x2e0/0x4e0
[76437.622073] [<ffffffff811f5ae5>] ? __sb_end_write+0x35/0x70
[76437.622075] [<ffffffff811f3552>] ? vfs_write+0x1b2/0x1f0
[76437.622077] [<ffffffff81207df1>] SyS_ioctl+0x81/0xa0
[76437.622079] [<ffffffff817c934d>] system_call_fastpath+0x16/0x1b

Any clues?

Forgot to mention that the game freezes apparently in driver kernel code. Cannot even kill it, but I can start a new instance and play that :) Other than that (probably radeon driver bug anyway) game runs fine.