When I run glxgears as me I get this

libGL error: failed to open DRM: Operation not permitted
libGL error: reverting to (slow) indirect rendering

If I run it as root, I don't get this message, and I get much faster
rates. I know the solution to this is supposed to be to set
permissions correctly, but I already have this in my XF86Config:

Section "DRI"
Mode 0666
EndSection

So I'm mystified. Any advice?