My system (Threadripper, Vega 64) started exhibiting the same issue on 4.17. 
It will lock hard for me under IO.  I have a custom python script that I run
that does NFS IO off my X550 network card and then invokes imagemagick/convert
to generate thumbnails.  I wasn't experiencing issues on 4.16 personally. 
4.17.5 locks 100% on running of the script with the attached ciri example
dmesg.

The only other system change I have made recently is the addition of the
opencl-amd package in a failed attempt to make Divinci Resolve run
(https://aur.archlinux.org/packages/opencl-amd/).

My linux-firmware package is linux-firmware-git 20171125.17e6288-1.  There
might be a better way to get the firmware version from the card itself, but I
don't possess such knowledge (yet).

Barry