When we retire a signaled fence, we free the dependency tree. However,
we skip clearing the list so that if we then try to adjust the priority
of the signaled fence, we may walk the list of freed dependencies.

[ 3083.156757] ==================================================================
[ 3083.156806] BUG: KASAN: use-after-free in execlists_schedule+0x199/0x660 [i915]
[ 3083.156810] Read of size 8 at addr ffff8806bf20f400 by task Xorg/831

[ 3083.156815] CPU: 0 PID: 831 Comm: Xorg Not tainted 4.15.0-rc6-no-psn+ #1
[ 3083.156817] Hardware name: Notebook                         N24_25BU/N24_25BU, BIOS 5.12 02/17/2017
[ 3083.156818] Call Trace:
[ 3083.156823]  dump_stack+0x5c/0x7a
[ 3083.156827]  print_address_description+0x6b/0x290
[ 3083.156830]  kasan_report+0x28f/0x380
[ 3083.156872]  ? execlists_schedule+0x199/0x660 [i915]
[ 3083.156914]  execlists_schedule+0x199/0x660 [i915]
[ 3083.156956]  ? intel_crtc_atomic_check+0x146/0x4e0 [i915]
[ 3083.156997]  ? execlists_submit_request+0xe0/0xe0 [i915]
[ 3083.157038]  ? i915_vma_misplaced.part.4+0x25/0xb0 [i915]
[ 3083.157079]  ? __i915_vma_do_pin+0x7c8/0xc80 [i915]
[ 3083.157121]  ? intel_atomic_state_alloc+0x44/0x60 [i915]
[ 3083.157130]  ? drm_atomic_helper_page_flip+0x3e/0xb0 [drm_kms_helper]
[ 3083.157145]  ? drm_mode_page_flip_ioctl+0x7d2/0x850 [drm]
[ 3083.157159]  ? drm_ioctl_kernel+0xa7/0xf0 [drm]
[ 3083.157172]  ? drm_ioctl+0x45b/0x560 [drm]
[ 3083.157211]  i915_gem_object_wait_priority+0x14c/0x2c0 [i915]
[ 3083.157251]  ? i915_gem_get_aperture_ioctl+0x150/0x150 [i915]
[ 3083.157290]  ? i915_vma_pin_fence+0x1d8/0x320 [i915]
[ 3083.157331]  ? intel_pin_and_fence_fb_obj+0x175/0x250 [i915]
[ 3083.157372]  ? intel_rotation_info_size+0x60/0x60 [i915]
[ 3083.157413]  ? intel_link_compute_m_n+0x80/0x80 [i915]
[ 3083.157428]  ? drm_dev_printk+0x1b0/0x1b0 [drm]
[ 3083.157443]  ? drm_dev_printk+0x1b0/0x1b0 [drm]
[ 3083.157485]  intel_prepare_plane_fb+0x2f8/0x5a0 [i915]
[ 3083.157527]  ? intel_crtc_get_vblank_counter+0x80/0x80 [i915]
[ 3083.157536]  drm_atomic_helper_prepare_planes+0xa0/0x1c0 [drm_kms_helper]
[ 3083.157587]  intel_atomic_commit+0x12e/0x4e0 [i915]
[ 3083.157605]  drm_atomic_helper_page_flip+0xa2/0xb0 [drm_kms_helper]
[ 3083.157621]  drm_mode_page_flip_ioctl+0x7d2/0x850 [drm]
[ 3083.157638]  ? drm_mode_cursor2_ioctl+0x10/0x10 [drm]
[ 3083.157652]  ? drm_lease_owner+0x1a/0x30 [drm]
[ 3083.157668]  ? drm_mode_cursor2_ioctl+0x10/0x10 [drm]
[ 3083.157681]  drm_ioctl_kernel+0xa7/0xf0 [drm]
[ 3083.157696]  drm_ioctl+0x45b/0x560 [drm]
[ 3083.157711]  ? drm_mode_cursor2_ioctl+0x10/0x10 [drm]
[ 3083.157725]  ? drm_getstats+0x20/0x20 [drm]
[ 3083.157729]  ? timerqueue_del+0x49/0x80
[ 3083.157732]  ? __remove_hrtimer+0x62/0xb0
[ 3083.157735]  ? hrtimer_try_to_cancel+0x173/0x210
[ 3083.157738]  do_vfs_ioctl+0x13b/0x880
[ 3083.157741]  ? ioctl_preallocate+0x140/0x140
[ 3083.157744]  ? _raw_spin_unlock_irq+0xe/0x30
[ 3083.157746]  ? do_setitimer+0x234/0x370
[ 3083.157750]  ? SyS_setitimer+0x19e/0x1b0
[ 3083.157752]  ? SyS_alarm+0x140/0x140
[ 3083.157755]  ? __rcu_read_unlock+0x66/0x80
[ 3083.157757]  ? __fget+0xc4/0x100
[ 3083.157760]  SyS_ioctl+0x74/0x80
[ 3083.157763]  entry_SYSCALL_64_fastpath+0x1a/0x7d
[ 3083.157765] RIP: 0033:0x7f6135d0c6a7
[ 3083.157767] RSP: 002b:00007fff01451888 EFLAGS: 00003246 ORIG_RAX: 0000000000000010
[ 3083.157769] RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 00007f6135d0c6a7
[ 3083.157771] RDX: 00007fff01451950 RSI: 00000000c01864b0 RDI: 000000000000000c
[ 3083.157772] RBP: 00007f613076f600 R08: 0000000000000001 R09: 0000000000000000
[ 3083.157773] R10: 0000000000000060 R11: 0000000000003246 R12: 0000000000000000
[ 3083.157774] R13: 0000000000000060 R14: 000000000000001b R15: 0000000000000060

[ 3083.157779] Allocated by task 831:
[ 3083.157783]  kmem_cache_alloc+0xc0/0x200
[ 3083.157822]  i915_gem_request_await_dma_fence+0x2c4/0x5d0 [i915]
[ 3083.157861]  i915_gem_request_await_object+0x321/0x370 [i915]
[ 3083.157900]  i915_gem_do_execbuffer+0x1165/0x19c0 [i915]
[ 3083.157937]  i915_gem_execbuffer2+0x1ad/0x550 [i915]
[ 3083.157950]  drm_ioctl_kernel+0xa7/0xf0 [drm]
[ 3083.157962]  drm_ioctl+0x45b/0x560 [drm]
[ 3083.157964]  do_vfs_ioctl+0x13b/0x880
[ 3083.157966]  SyS_ioctl+0x74/0x80
[ 3083.157968]  entry_SYSCALL_64_fastpath+0x1a/0x7d

[ 3083.157971] Freed by task 831:
[ 3083.157973]  kmem_cache_free+0x77/0x220
[ 3083.158012]  i915_gem_request_retire+0x72c/0xa70 [i915]
[ 3083.158051]  i915_gem_request_alloc+0x1e9/0x8b0 [i915]
[ 3083.158089]  i915_gem_do_execbuffer+0xa96/0x19c0 [i915]
[ 3083.158127]  i915_gem_execbuffer2+0x1ad/0x550 [i915]
[ 3083.158140]  drm_ioctl_kernel+0xa7/0xf0 [drm]
[ 3083.158153]  drm_ioctl+0x45b/0x560 [drm]
[ 3083.158155]  do_vfs_ioctl+0x13b/0x880
[ 3083.158156]  SyS_ioctl+0x74/0x80
[ 3083.158158]  entry_SYSCALL_64_fastpath+0x1a/0x7d

[ 3083.158162] The buggy address belongs to the object at ffff8806bf20f400
                which belongs to the cache i915_dependency of size 64
[ 3083.158166] The buggy address is located 0 bytes inside of
                64-byte region [ffff8806bf20f400, ffff8806bf20f440)
[ 3083.158168] The buggy address belongs to the page:
[ 3083.158171] page:00000000d43decc4 count:1 mapcount:0 mapping:          (null) index:0x0
[ 3083.158174] flags: 0x17ffe0000000100(slab)
[ 3083.158179] raw: 017ffe0000000100 0000000000000000 0000000000000000 0000000180200020
[ 3083.158182] raw: ffffea001afc16c0 0000000500000005 ffff880731b881c0 0000000000000000
[ 3083.158184] page dumped because: kasan: bad access detected

[ 3083.158187] Memory state around the buggy address:
[ 3083.158190]  ffff8806bf20f300: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[ 3083.158192]  ffff8806bf20f380: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[ 3083.158195] >ffff8806bf20f400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[ 3083.158196]                    ^
[ 3083.158199]  ffff8806bf20f480: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[ 3083.158201]  ffff8806bf20f500: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[ 3083.158203] ==================================================================
Reported-by: Alexandru Chirvasitu <achirvasub@gmail.com>
Reported-by: Mike Keehan <mike@keehan.net>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104436
Fixes: 1f181225 ("drm/i915/execlists: Keep request->priority for its lifetime")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Alexandru Chirvasitu <achirvasub@gmail.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Tested-by: Alexandru Chirvasitu <achirvasub@gmail.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180106105618.13532-1-chris@chris-wilson.co.uk