"RIP radeon_gem_va_ioctl+0x35/0x650", "Userspace still has active objects", and "trying to unbind memory from uninitialized GART !" when unbinding from radeon

[Monopoly Man]

I've been having this issue for awhile now and I can't seem to come up 
with the solution. I'm rather inexperienced when it comes to debugging 
these sort of issues, so I posted a thread on StackExchange and the Arch 
forums but never got a response. I just recently made a bug report on 
bugzilla, but thought I would also check here and see if anyone would be 
able to help me out. Here is a link to the bug report I made: 
https://bugzilla.kernel.org/show_bug.cgi?id=116101

In case it's easier, I'll also paste it below:

I am attempting to unbind my R9 290 from radeon and rebind it to vfio. However, the entire system hangs when "echo "0000:01:00.0" > /sys/bus/pci/devices/0000:01:00.0/driver/unbind" is ran as root.

Steps to Reproduce:
Run the following script as root

#!/bin/bash
set -x
echo "1002 67b1" > /sys/bus/pci/drivers/vfio-pci/new_id
echo "0000:01:00.0" > /sys/bus/pci/devices/0000:01:00.0/driver/unbind
echo "0000:01:00.0" > /sys/bus/pci/drivers/vfio-pci/bind
echo "1002 67b1" > /sys/bus/pci/drivers/vfio-pci/remove_id

echo "1002 aac8" > /sys/bus/pci/drivers/vfio-pci/new_id
echo "0000:01:00.1" > /sys/bus/pci/devices/0000:01:00.1/driver/unbind
echo "0000:01:00.1" > /sys/bus/pci/drivers/vfio-pci/bind
echo "1002 aac8" > /sys/bus/pci/drivers/vfio-pci/remove_id

set +x

Actual Results:
System hangs on "echo "0000:01:00.0" > /sys/bus/pci/devices/0000:01:00.0/driver/unbind"

Journalctl shows "Apr 09 16:10:26 joey-arch-pc kernel: radeon 0000:01:00.0: Userspace still has active objects !" followed by numerous "Apr 09 16:10:26 joey-arch-pc kernel: trying to unbind memory from uninitialized GART !"

Expected Results:

The GPU should be unbinding from radeon and rebinding to vfio-pci without hanging.

Additional information:
I experienced the same issue on 4.1.20-1.

I know of instances where the same script results in the expected outcome with a Cayman PRO graphics card.

Attached is the output of journalctl for the entire boot. Go to 16:10:26 to see relevant call traces.

Here is the journalctl output: 
https://bugzilla.kernel.org/attachment.cgi?id=212291