X.org hang with [drm:i915_gem_do_execbuffer] *ERROR* in dmesg
  http://bugzilla.kernel.org/show_bug.cgi?id=15114

Matej found he was hitting an error path within i915_gem_do_execbuffer()
that led to the attempt to dereference an uninitialised pointer during
cleanup. This path used to be safe as we used to calloc the object
lists, but this was changed in c8e0f93a. Daniel Vetter had also spotted
this error and proposed a similar patch.

[ 6379.732892] [drm:i915_gem_do_execbuffer] *ERROR* Object ffff880098cd6540 appears more than once in object list
[ 6379.740976] [drm:i915_gem_do_execbuffer] *ERROR* Object ffff880098cd6540 appears more than once in object list
[ 6379.740995] BUG: unable to handle kernel NULL pointer dereference at 00000000000000a0
[ 6379.740998] IP: [<ffffffff8122ddb5>] i915_gem_do_execbuffer+0xba5/0x1260
[ 6379.741006] PGD babab067 PUD bb435067 PMD 0
[ 6379.741010] Oops: 0002 [#1] PREEMPT SMP
[ 6379.741014] last sysfs file: /sys/devices/pci0000:00/0000:00:1c.2/0000:06:00.0/ieee80211/phy0/rfkill0/state
[ 6379.741017] CPU 1
[ 6379.741021] Pid: 2186, comm: X Not tainted 2.6.33-rc4-00399-g24bc7347 #142 M11D/ESPRIMO Mobile M9400
[ 6379.741023] RIP: 0010:[<ffffffff8122ddb5>] [<ffffffff8122ddb5>] i915_gem_do_execbuffer+0xba5/0x1260
[ 6379.741027] RSP: 0018:ffff8800b9047b78  EFLAGS: 00213206
[ 6379.741029] RAX: 0000000000000000 RBX: 000000000000004f RCX: ffff880098cac800
[ 6379.741032] RDX: ffff880098caca78 RSI: ffff8800b9047c98 RDI: ffff880098cd6540
[ 6379.741034] RBP: ffff8800b9047c78 R08: ffffffff814b96b5 R09: 0000000000000006
[ 6379.741036] R10: 0000000000000000 R11: 0000000000000003 R12: 000000000000004e
[ 6379.741038] R13: 00000000fffffff7 R14: 0000000000000000 R15: 0000000000000001
[ 6379.741041] FS:  0000000000000000(0000) GS:ffff880001900000(0063) knlGS:00000000f72636c0
[ 6379.741043] CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
[ 6379.741041] FS:  0000000000000000(0000) GS:ffff880001900000(0063) knlGS:00000000f72636c0
[ 6379.741043] CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
[ 6379.741045] CR2: 00000000000000a0 CR3: 00000000b9000000 CR4: 00000000000006e0
[ 6379.741048] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 6379.741050] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 6379.741052] Process X (pid: 2186, threadinfo ffff8800b9046000, task ffff8800bb5d8000)
[ 6379.741054] Stack:
[ 6379.741055]  ffffc90023f57000 ffffc90023f56fff ffffc90023f56fff ffffc90023f55000
[ 6379.741059] <0> ffff8800b9047c98 ffff8800bb43c840 ffff8800bf1de800 ffff8800bf1de820
[ 6379.741063] <0> ffff8800b9047bd8 ffff880098cac800 0000000000000000 0000000000000002
[ 6379.741068] Call Trace:
[ 6379.741072]  [<ffffffff8122e6cb>] ?  i915_gem_execbuffer+0x6b/0x370
[ 6379.741077]  [<ffffffff810a5f52>] ? __vmalloc_node+0xa2/0xb0
[ 6379.741080]  [<ffffffff8122e6cb>] ?  i915_gem_execbuffer+0x6b/0x370
[ 6379.741083]  [<ffffffff8122e816>] i915_gem_execbuffer+0x1b6/0x370
[ 6379.741086]  [<ffffffff8120cd55>] drm_ioctl+0x1d5/0x460
[ 6379.741089]  [<ffffffff8122e660>] ?  i915_gem_execbuffer+0x0/0x370
[ 6379.741093]  [<ffffffff81248c35>] i915_compat_ioctl+0x45/0x50
[ 6379.741097]  [<ffffffff810f1659>] compat_sys_ioctl+0xa9/0x1570
[ 6379.741102]  [<ffffffff810b1d5c>] ? vfs_read+0x13c/0x1a0
[ 6379.741106]  [<ffffffff81028424>] sysenter_dispatch+0x7/0x2b
[ 6379.741108] Code: 08 85 c0 74 52 31 db 0f 1f 80 00 00 00 00 48 63 c3 48 8b
8d 68 ff ff ff 48 8d 14 c1 48 8b 02 48 85 c0 74 25 48 8b 80 80 00 00 00 <c7> 80
a0 00 00 00 00 00 00 00 48 8b 3a 48 85 ff 74 0c 48 c7 c6
[ 6379.741142] RIP  [<ffffffff8122ddb5>] i915_gem_do_execbuffer+0xba5/0x1260
[ 6379.741145]  RSP <ffff8800b9047b78>
[ 6379.741147] CR2: 00000000000000a0
[ 6379.741159] ---[ end trace 0598809afa4c31db ]---
Reported-by: Matej Laitl <strohel@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>