I'm forwarding this bug from an Ubuntu reporter:
https://bugs.edge.launchpad.net/bugs/568138

[Problem]
Reporter gets occasional X freezes with no obvious trigger.  The system remains responsive to SSH.  Attempting to restart X fails.

There are no errors recorded in Xorg.0.log when these freezes occur, but dmesg contains hung task backtraces ending in __mutex_lock_slowpath.

Reporter is using libdrm 2.4.18, drm 2.6.33 and xf86-video-intel 2.9.1.
I have also experienced freezes with the same dmesg backtrace using libdrm 2.4.20, drm 2.6.35-rc3 and xf86-video-intel 2.11.0 on a GM45 card.

[lspci]
00:02.0 VGA compatible controller: Intel Corporation Core Processor Integrated Graphics Controller (rev 02)

Xorg.0.log: http://launchpadlibrarian.net/44988804/Xorg.0.log
Example dmesg outputs containing blocked task backtrace: 
http://launchpadlibrarian.net/45481306/dmesg
http://launchpadlibrarian.net/44988795/dmesg
Blocked task & held locks output from lock-debugging kernel: http://launchpadlibrarian.net/47665797/locks.txt
intel_gpu_dump output: http://launchpadlibrarian.net/44988870/gpudump

[Original report]
Occasionally X hangs completely, not even SAK will kill it, however SAK followed by ctrl-alt-del successfully switches to the shutdown splash.

When this happens, the following shows up in dmesg:

[ 5169.457964] INFO: task i915:302 blocked for more than 120 seconds.
[ 5169.457970] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 5169.457974] i915 D 00000000ffffffff 0 302 2 0x00000000
[ 5169.457981] ffff88022e0d9d20 0000000000000046 0000000000015bc0 0000000000015bc0
[ 5169.457988] ffff88022f791ab0 ffff88022e0d9fd8 0000000000015bc0 ffff88022f7916f0
[ 5169.457994] 0000000000015bc0 ffff88022e0d9fd8 0000000000015bc0 ffff88022f791ab0
[ 5169.458000] Call Trace:
[ 5169.458015] [<ffffffff8153f867>] __mutex_lock_slowpath+0xe7/0x170
[ 5169.458021] [<ffffffff8153f75b>] mutex_lock+0x2b/0x50
[ 5169.458063] [<ffffffffa00d5550>] intel_idle_update+0x60/0x1d0 [i915]
[ 5169.458085] [<ffffffffa00d54f0>] ? intel_idle_update+0x0/0x1d0 [i915]
[ 5169.458095] [<ffffffff81080777>] run_workqueue+0xc7/0x1a0
[ 5169.458101] [<ffffffff810808f3>] worker_thread+0xa3/0x110
[ 5169.458107] [<ffffffff81085320>] ? autoremove_wake_function+0x0/0x40
[ 5169.458113] [<ffffffff81080850>] ? worker_thread+0x0/0x110
[ 5169.458117] [<ffffffff81084fa6>] kthread+0x96/0xa0
[ 5169.458124] [<ffffffff810141ea>] child_rip+0xa/0x20
[ 5169.458129] [<ffffffff81084f10>] ? kthread+0x0/0xa0
[ 5169.458133] [<ffffffff810141e0>] ? child_rip+0x0/0x20
[ 5169.458154] INFO: task Xorg:1364 blocked for more than 120 seconds.
[ 5169.458157] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 5169.458160] Xorg D 0000000000000000 0 1364 1263 0x00400004
[ 5169.458166] ffff88020bd13ca8 0000000000000082 0000000000015bc0 0000000000015bc0
[ 5169.458172] ffff880208c74890 ffff88020bd13fd8 0000000000015bc0 ffff880208c744d0
[ 5169.458177] 0000000000015bc0 ffff88020bd13fd8 0000000000015bc0 ffff880208c74890
[ 5169.458183] Call Trace:
[ 5169.458189] [<ffffffff8153f867>] __mutex_lock_slowpath+0xe7/0x170
[ 5169.458195] [<ffffffff8153f75b>] mutex_lock+0x2b/0x50
[ 5169.458214] [<ffffffffa00c3435>] i915_gem_madvise_ioctl+0x55/0x160 [i915]
[ 5169.458231] [<ffffffffa006de2a>] drm_ioctl+0x27a/0x480 [drm]
[ 5169.458250] [<ffffffffa00c33e0>] ? i915_gem_madvise_ioctl+0x0/0x160 [i915]
[ 5169.458256] [<ffffffff81085320>] ? autoremove_wake_function+0x0/0x40
[ 5169.458264] [<ffffffff81152a92>] vfs_ioctl+0x22/0xa0
[ 5169.458269] [<ffffffff81152d41>] do_vfs_ioctl+0x81/0x380
[ 5169.458274] [<ffffffff811431df>] ? vfs_read+0x12f/0x1a0
[ 5169.458279] [<ffffffff811530c1>] sys_ioctl+0x81/0xa0
[ 5169.458286] [<ffffffff810131b2>] system_call_fastpath+0x16/0x1b