I just upstreamed the bug report below (opening this bug just to track it in ubuntu).

************************************

Just now I started Firefox and my xserver froze. I grabbed a backtrace from gdb
and X was stucking waiting for some ioctl(). I also took a copy of xorg.lorg
which contains a "xorg is probably in an infinite loop backtrace" as such:

[mi] EQ overflowing. The server is probably stuck in an infinite loop.

Backtrace:
0: /usr/X11R6/bin/X(xorg_backtrace+0x26) [0x4ee1a6]
1: /usr/X11R6/bin/X(mieqEnqueue+0x291) [0x4cebd1]
2: /usr/X11R6/bin/X(xf86PostMotionEventP+0xc4) [0x498554]
3: /usr/X11R6/bin/X(xf86PostMotionEvent+0xb1) [0x498731]
4: /usr/lib/xorg/modules/input//evdev_drv.so [0x7f26a0a559b2]
5: /usr/X11R6/bin/X [0x481625]
6: /usr/X11R6/bin/X [0x472147]
7: /lib/libpthread.so.0 [0x7f26b9bbf080]
8: /lib/libc.so.6(ioctl+0x7) [0x7f26b8221d87]
9: /usr/lib/libdrm.so.2 [0x7f26b6dfe8d3]
10: /usr/lib/libdrm.so.2(drmWaitVBlank+0x20) [0x7f26b6dfed70]
11: /usr/lib/dri/i965_dri.so [0x7f26a5ccb85e]
12: /usr/lib/dri/i965_dri.so(driWaitForVBlank+0x110) [0x7f26a5ccbaf0]
13: /usr/lib/dri/i965_dri.so(intelSwapBuffers+0xe5) [0x7f26a5cd53d5]
14: /usr/lib/dri/i965_dri.so [0x7f26a5cccdef]
15: /usr/lib/xorg/modules/extensions//libglx.so [0x7f26b7659b5f]
16: /usr/lib/xorg/modules/extensions//libglx.so [0x7f26b764d936]
17: /usr/lib/xorg/modules/extensions//libglx.so [0x7f26b7650bd2]
18: /usr/X11R6/bin/X(Dispatch+0x364) [0x44d754]
19: /usr/X11R6/bin/X(main+0x45d) [0x43376d]
20: /lib/libc.so.6(__libc_start_main+0xe6) [0x7f26b8162586]
21: /usr/X11R6/bin/X [0x432b49]
[mi] mieqEnequeue: out-of-order valuator event; dropping.
[mi] EQ overflowing. The server is probably stuck in an infinite loop.
[mi] mieqEnequeue: out-of-order valuator event; dropping.
[mi] EQ overflowing. The server is probably stuck in an infinite loop.
[mi] mieqEnequeue: out-of-order valuator event; dropping.
[mi] EQ overflowing. The server is probably stuck in an infinite loop.

------- Comment #1 From martin 2008-12-07 06:43:30 PST [reply] -------

Created an attachment (id=20867) [details]
xorg_log with "x has gone into inf loop" backtrace (related to EQ overflow)

------- Comment #2 From martin 2008-12-07 06:50:21 PST [reply] -------

Previously when I used 2.6.27 kernel with intel 2.4.1 I never saw this
particular freeze at all.

When I upgraded to jaunty I got 2.6.28 kernel and intel 2.5.1 I run into this
problem.

This bug is not reproducible by specific steps though, it just happens at
random times out of the blue. If you have additional info you need I can write
it down on a post-it next to my machine and try to collect that data once the
bug happens to be triggered again.

My hardware is a x64 box with intel g45 (some lines from "lspci -nn" below):
00:00.0 Host bridge [0600]: Intel Corporation 4 Series Chipset DRAM Controller
[8086:2e20] (rev 03)
00:02.0 VGA compatible controller [0300]: Intel Corporation 4 Series Chipset
Integrated Graphics Controller [8086:2e22] (rev 03)
00:02.1 Display controller [0380]: Intel Corporation 4 Series Chipset
Integrated Graphics Controller [8086:2e23] (rev 03)

I have "current jaunty versions" which right now means:
ii  libdrm-intel1                             2.4.1-0ubuntu7
ii  xserver-xorg-video-intel                  2:2.5.1-1ubuntu5
ii  linux-image-2.6.28-2-generic              2.6.28-2.3
ii  xserver-xorg                              1:7.4~5ubuntu5
ii  libgl1-mesa-dri                           7.2-1ubuntu2