Currently, the vma is being unlink from the object lookup on destroy.
However, we are meant to be decoupling it upon close so that the user
cannot access the closed vma whilst it remains active on the GPU.

[   34.074858] kernel BUG at drivers/gpu/drm/i915/i915_gem_gtt.c:3561!
[   34.074875] invalid opcode: 0000 [#1] PREEMPT SMP
[   34.074888] Modules linked in: snd_hda_intel i915 x86_pkg_temp_thermal coretemp crct10dif_pclmul crc32_pclmul ghash_clmulni_intel lpc_ich mei_me mei snd_hda_codec_realtek snd_hda_codec_generic snd_hda_codec_hdmi snd_hda_codec snd_hwdep snd_hda_core i2c_designware_platform i2c_designware_core snd_pcm e1000e ptp pps_core sdhci_acpi sdhci mmc_core i2c_hid [last unloaded: i915]
[   34.075010] CPU: 1 PID: 6224 Comm: gem_close_race Tainted: G     U          4.9.0-rc3-CI-CI_DRM_1800+ #1
[   34.075034] Hardware name:                  /NUC5i7RYB, BIOS RYBDWi35.86A.0355.2016.0224.1501 02/24/2016
[   34.075057] task: ffff8802459a8040 task.stack: ffffc90000524000
[   34.075074] RIP: 0010:[<ffffffffa0392cbc>]  [<ffffffffa0392cbc>] i915_gem_obj_lookup_or_create_vma+0x8c/0xc0 [i915]
[   34.075118] RSP: 0018:ffffc90000527b68  EFLAGS: 00010202
[   34.075135] RAX: ffff8802426c5e40 RBX: 0000000000000000 RCX: ffff8802447fc2a8
[   34.075158] RDX: 0000000000000000 RSI: ffff8802447fc2a8 RDI: ffff880248a4a880
[   34.075181] RBP: ffffc90000527b88 R08: 0000000000000008 R09: 0000000000000000
[   34.075203] R10: 0000000000000001 R11: 0000000000000000 R12: ffff880248a4a880
[   34.075225] R13: ffff8802447fc2a8 R14: ffff880243e9afa8 R15: ffff880248a4a9c8
[   34.075248] FS:  00007f9b43e59740(0000) GS:ffff880256c80000(0000) knlGS:0000000000000000
[   34.075273] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   34.075292] CR2: 00007f9b43419140 CR3: 000000024455d000 CR4: 00000000003406e0
[   34.075314] Stack:
[   34.075323]  0000000000000000 ffffc90000527bd0 ffff880243cb8008 ffff880243e9afa8
[   34.075353]  ffffc90000527c08 ffffffffa03874c7 ffffc90000527bb8 ffff880243e9afa8
[   34.075383]  ffff880243e9afb0 ffffc90000527e10 ffff8802447fc2a8 ffff880243cb8040
[   34.075414] Call Trace:
[   34.075435]  [<ffffffffa03874c7>] eb_lookup_vmas.isra.7+0x247/0x330 [i915]
[   34.075468]  [<ffffffffa0388c34>] i915_gem_do_execbuffer.isra.15+0x604/0x1a10 [i915]
[   34.075507]  [<ffffffffa039c957>] ? i915_gem_object_get_sg+0x347/0x380 [i915]
[   34.075532]  [<ffffffff811a69ce>] ? __might_fault+0x3e/0x90
[   34.075562]  [<ffffffffa038a430>] i915_gem_execbuffer2+0xc0/0x250 [i915]
[   34.075585]  [<ffffffff81552926>] drm_ioctl+0x1f6/0x480
[   34.075604]  [<ffffffff8100107a>] ? trace_hardirqs_on_thunk+0x1a/0x1c
[   34.075635]  [<ffffffffa038a370>] ? i915_gem_execbuffer+0x330/0x330 [i915]
[   34.075658]  [<ffffffff81202d2e>] do_vfs_ioctl+0x8e/0x690
[   34.075677]  [<ffffffff8181582d>] ? _raw_spin_unlock_irqrestore+0x3d/0x60
[   34.075700]  [<ffffffff810fcd51>] ? SyS_timer_settime+0x141/0x1e0
[   34.075721]  [<ffffffff810d6de2>] ? trace_hardirqs_on_caller+0x122/0x1b0
[   34.075742]  [<ffffffff8120336c>] SyS_ioctl+0x3c/0x70
[   34.075760]  [<ffffffff8181602e>] entry_SYSCALL_64_fastpath+0x1c/0xb1
[   34.075781] Code: 44 a0 48 c7 c2 9a 7e 43 a0 be e0 0d 00 00 48 c7 c7 a0 45 44 a0 e8 55 b8 ce e0 48 85 db 74 a3 49 83 bd f8 03 00 00 00 74 99 0f 0b <0f> 0b 48 89 da 4c 89 ee 4c 89 e7 e8 04 a9 ff ff 48 89 da 49 89
[   34.075955] RIP  [<ffffffffa0392cbc>] i915_gem_obj_lookup_or_create_vma+0x8c/0xc0 [i915]
[   34.075994]  RSP <ffffc90000527b68>

Testcase: igt/gem_close_race/basic-threads
Fixes: db6c2b41 ("drm/i915: Store the vma in an rbtree...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161104161241.25871-1-chris@chris-wilson.co.ukReviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>