https://bugs.freedesktop.org/show_bug.cgi?id=103370
--- Comment #27 from Michel Dänzer michel@daenzer.net --- Thanks for bisecting, but I don't think that commit can be directly responsible for a GPU hang. Before that commit, the DRI3 code in Mesa would only use one back buffer for glxgears, which means that the GPU could only start rendering a new frame after the previous one had finished presenting. Maybe that somehow prevented the hang.
A possible test for this theory is running
vblank_mode=0 DRI_PRIME=1 glxgears
with Mesa 12.0.3; does that also trigger the hang?