As both the hotplug event and fbdev configuration run asynchronously, it
is possible for them to run concurrently. If configuration fails, we were
freeing the fbdev causing a use-after-free in the hotplug event.

<7>[ 3069.935211] [drm:intel_fb_initial_config [i915]] Not using firmware configuration
<7>[ 3069.935225] [drm:drm_setup_crtcs] looking for cmdline mode on connector 77
<7>[ 3069.935229] [drm:drm_setup_crtcs] looking for preferred mode on connector 77 0
<7>[ 3069.935233] [drm:drm_setup_crtcs] found mode 3200x1800
<7>[ 3069.935236] [drm:drm_setup_crtcs] picking CRTCs for 8192x8192 config
<7>[ 3069.935253] [drm:drm_setup_crtcs] desired mode 3200x1800 set on crtc 43 (0,0)
<7>[ 3069.935323] [drm:intelfb_create [i915]] no BIOS fb, allocating a new one
<4>[ 3069.967737] general protection fault: 0000 [#1] PREEMPT SMP
<0>[ 3069.977453] ---------------------------------
<4>[ 3069.977457] Modules linked in: i915(+) vgem snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic x86_pkg_temp_thermal intel_powerclamp coretemp crct10dif_pclmul crc32_pclmul ghash_clmulni_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm r8169 mei_me mii prime_numbers mei i2c_hid pinctrl_geminilake pinctrl_intel [last unloaded: i915]
<4>[ 3069.977492] CPU: 1 PID: 15414 Comm: kworker/1:0 Tainted: G     U          4.14.0-CI-CI_DRM_3388+ #1
<4>[ 3069.977497] Hardware name: Intel Corp. Geminilake/GLK RVP1 DDR4 (05), BIOS GELKRVPA.X64.0062.B30.1708222146 08/22/2017
<4>[ 3069.977508] Workqueue: events output_poll_execute
<4>[ 3069.977512] task: ffff880177734e40 task.stack: ffffc90001fe4000
<4>[ 3069.977519] RIP: 0010:__lock_acquire+0x109/0x1b60
<4>[ 3069.977523] RSP: 0018:ffffc90001fe7bb0 EFLAGS: 00010002
<4>[ 3069.977526] RAX: 6b6b6b6b6b6b6b6b RBX: 0000000000000282 RCX: 0000000000000000
<4>[ 3069.977530] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff880170d4efd0
<4>[ 3069.977534] RBP: ffffc90001fe7c70 R08: 0000000000000001 R09: 0000000000000000
<4>[ 3069.977538] R10: 0000000000000000 R11: ffffffff81899609 R12: ffff880170d4efd0
<4>[ 3069.977542] R13: ffff880177734e40 R14: 0000000000000001 R15: 0000000000000000
<4>[ 3069.977547] FS:  0000000000000000(0000) GS:ffff88017fc80000(0000) knlGS:0000000000000000
<4>[ 3069.977551] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4>[ 3069.977555] CR2: 00007f7e8b7bcf04 CR3: 0000000003e0f000 CR4: 00000000003406e0
<4>[ 3069.977559] Call Trace:
<4>[ 3069.977565]  ? mark_held_locks+0x64/0x90
<4>[ 3069.977571]  ? _raw_spin_unlock_irq+0x24/0x50
<4>[ 3069.977575]  ? _raw_spin_unlock_irq+0x24/0x50
<4>[ 3069.977579]  ? trace_hardirqs_on_caller+0xde/0x1c0
<4>[ 3069.977583]  ? _raw_spin_unlock_irq+0x2f/0x50
<4>[ 3069.977588]  ? finish_task_switch+0xa5/0x210
<4>[ 3069.977592]  ? lock_acquire+0xaf/0x200
<4>[ 3069.977596]  lock_acquire+0xaf/0x200
<4>[ 3069.977600]  ? __mutex_lock+0x5e9/0x9b0
<4>[ 3069.977604]  _raw_spin_lock+0x2a/0x40
<4>[ 3069.977608]  ? __mutex_lock+0x5e9/0x9b0
<4>[ 3069.977612]  __mutex_lock+0x5e9/0x9b0
<4>[ 3069.977616]  ? drm_fb_helper_hotplug_event.part.19+0x16/0xa0
<4>[ 3069.977621]  ? drm_fb_helper_hotplug_event.part.19+0x16/0xa0
<4>[ 3069.977625]  drm_fb_helper_hotplug_event.part.19+0x16/0xa0
<4>[ 3069.977630]  output_poll_execute+0x8d/0x180
<4>[ 3069.977635]  process_one_work+0x22e/0x660
<4>[ 3069.977640]  worker_thread+0x48/0x3a0
<4>[ 3069.977644]  ? _raw_spin_unlock_irqrestore+0x4c/0x60
<4>[ 3069.977649]  kthread+0x102/0x140
<4>[ 3069.977653]  ? process_one_work+0x660/0x660
<4>[ 3069.977657]  ? kthread_create_on_node+0x40/0x40
<4>[ 3069.977662]  ret_from_fork+0x27/0x40
<4>[ 3069.977666] Code: 8d 62 f8 c3 49 81 3c 24 e0 fa 3c 82 41 be 00 00 00 00 45 0f 45 f0 83 fe 01 77 86 89 f0 49 8b 44 c4 08 48 85 c0 0f 84 76 ff ff ff <f0> ff 80 38 01 00 00 8b 1d 62 f9 e8 01 45 8b 85 b8 08 00 00 85
<1>[ 3069.977707] RIP: __lock_acquire+0x109/0x1b60 RSP: ffffc90001fe7bb0
<4>[ 3069.977712] ---[ end trace 4ad012eb3af62df7 ]---

In order to keep the dev_priv->ifbdev alive after failure, we have to
avoid the free and leave it empty until we unload the module (which is
less than ideal, but a necessary evil for simplicity). Then we can use
intel_fbdev_sync() to serialise the hotplug event with the configuration.
The serialisation between the two was removed in commit 934458c2
("Revert "drm/i915: Fix races on fbdev""), but the use after free is much
older, commit 366e39b4 ("drm/i915: Tear down fbdev if initialization
fails")

Fixes: 366e39b4 ("drm/i915: Tear down fbdev if initialization fails")
Fixes: 934458c2 ("Revert "drm/i915: Fix races on fbdev"")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lukas Wunner <lukas@wunner.de>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20171125194155.355-1-chris@chris-wilson.co.uk
(cherry picked from commit ad88d7fc)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>