@DLP (ii3osshogg) Thanks for putting me straight. I had been happily using 2.79b but thought it was about time to get with the flow and try out 2.91. When it had so many issues with GPU rendering I tried using 2.83 and found the same behaviour. I saw the posts about 2.83 and thought that must have been fixed by now so assumed it was the amdgpu-pro driver, especially as the whole machine was hung not just Blender. I'm pretty sure I couldn't get Lux to work before I uninstalled amdgpu-pro in despair but I'll give that another go.

Have this same issue with Radeon rx 5700 xt using Blender 2.91 with cycles on Ubuntu 18.04. With one scene, rendering stalls after compiling the kernels. At that stage Blender can still be killed using top or kill command in a terminal window. If I click a few buttons on Blender the machine becomes unresponsive to mouse and keyboard but I can ssh in and kill Blender and reboot. I thought for a while it was the file itself as it was started in 2.79, however I've also reproduced this with a fresh blend file with, say, 20 copies of the default cube rendering the preview with gpu.