Comment # 5 on bug 79756 from Michel Dänzer

I can't reproduce this with glamor from xserver Git, can you try that?

If not, or if it still happens with that, please get a backtrace with gdb, and
make sure /usr/lib/xorg/modules/dri/radeonsi_dri.so and
/usr/lib/xorg/modules/extensions/libglx.so have debugging symbols.