Hard lockup on X when started glxgears.
Nouveau driver.

Backtrace:
0: X(xf86SigHandler+0x84) [0x80b4704]
1: [0xffffe420]
2: /usr/lib/xorg/modules/drivers/nouveau_drv.so(NVDmaKickoff+0x42) [0xb7c11b82]
3: /usr/lib/xorg/modules/drivers/nouveau_drv.so(NVDmaKickoffCallback+0x1d) [0xb7
c11bfa]
4: /usr/lib/xorg/modules/drivers/nouveau_drv.so [0xb7c16cee]
5: X [0x814edc6]
6: X [0x8192a5d]
7: X(BlockHandler+0x58) [0x808a918]
8: X(WaitForSomething+0x121) [0x8195511]
9: X(Dispatch+0x81) [0x8086b61]
10: X(main+0x489) [0x806e5f9]
11: /lib/libc.so.6(__libc_start_main+0xd8) [0xb7d17838]
12: X(FontFileCompleteXLFD+0xad) [0x806d931]

Fatal server error:
Caught signal 11.  Server aborting

----
When running under mmio-trace, no output is there, and get this error:
libGL error: open DRM failed (Operation not permitted)
libGL error: reverting to (slow) indirect rendering
2 frames in 13.3 seconds =  0.150 FPS

The whole system becomes very slow, to the point where you'd think it is frozen.