We may race the interrupt signaling with retirement, in which case the
order in which we acquire the reference inside the interrupt is vital to
provide the correct barrier against the request being freed in
retirement, i.e. we need to acquire our reference before marking the
breadcrumb as cancelled (as soon as the breadcrumb is cancelled
retirement may drop its reference to the request without serialisation
with the interrupt handler).

<3>[  683.372226] BUG i915_request (Tainted: G     U           ): Object already free
<3>[  683.372269] -----------------------------------------------------------------------------

<4>[  683.372323] Disabling lock debugging due to kernel taint
<3>[  683.372393] INFO: Allocated in i915_request_alloc+0x169/0x810 [i915] age=0 cpu=2 pid=1420
<3>[  683.372412] 	kmem_cache_alloc+0x21c/0x280
<3>[  683.372478] 	i915_request_alloc+0x169/0x810 [i915]
<3>[  683.372540] 	i915_gem_do_execbuffer+0x84e/0x1ae0 [i915]
<3>[  683.372603] 	i915_gem_execbuffer2_ioctl+0x11b/0x420 [i915]
<3>[  683.372617] 	drm_ioctl_kernel+0x83/0xf0
<3>[  683.372626] 	drm_ioctl+0x2f3/0x3b0
<3>[  683.372636] 	do_vfs_ioctl+0xa0/0x6e0
<3>[  683.372645] 	ksys_ioctl+0x35/0x60
<3>[  683.372654] 	__x64_sys_ioctl+0x11/0x20
<3>[  683.372664] 	do_syscall_64+0x55/0x190
<3>[  683.372675] 	entry_SYSCALL_64_after_hwframe+0x49/0xbe
<3>[  683.372740] INFO: Freed in i915_request_retire_upto+0xfb/0x2e0 [i915] age=0 cpu=0 pid=1419
<3>[  683.372807] 	i915_request_retire_upto+0xfb/0x2e0 [i915]
<3>[  683.372870] 	i915_request_add+0x3bd/0x9d0 [i915]
<3>[  683.372931] 	i915_gem_do_execbuffer+0x141c/0x1ae0 [i915]
<3>[  683.372991] 	i915_gem_execbuffer2_ioctl+0x11b/0x420 [i915]
<3>[  683.373001] 	drm_ioctl_kernel+0x83/0xf0
<3>[  683.373008] 	drm_ioctl+0x2f3/0x3b0
<3>[  683.373015] 	do_vfs_ioctl+0xa0/0x6e0
<3>[  683.373023] 	ksys_ioctl+0x35/0x60
<3>[  683.373030] 	__x64_sys_ioctl+0x11/0x20
<3>[  683.373037] 	do_syscall_64+0x55/0x190
<3>[  683.373045] 	entry_SYSCALL_64_after_hwframe+0x49/0xbe
<3>[  683.373054] INFO: Slab 0x0000000079bcdd71 objects=30 used=2 fp=0x000000006d77b8af flags=0x8000000000010201
<3>[  683.373069] INFO: Object 0x000000006d77b8af @offset=24000 fp=0x000000007b061eab

<3>[  683.373083] Redzone 00000000ee47ef28: bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
<3>[  683.373097] Redzone 000000000cb91471: bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
<3>[  683.373111] Redzone 00000000cf2b86ee: bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
<3>[  683.373125] Redzone 00000000f1f5a2cd: bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb  ................
<3>[  683.373139] Object 000000006d77b8af: 00 00 00 00 5a 5a 5a 5a 00 3c 49 c0 ff ff ff ff  ....ZZZZ.<I.....
<3>[  683.373153] Object 000000006f9b6204: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<3>[  683.373167] Object 0000000091410ffb: e0 dd 6b fa 87 9f ff ff e0 dd 6b fa 87 9f ff ff  ..k.......k.....
<3>[  683.373181] Object 000000004cdf799d: 20 de 6b fa 87 9f ff ff 3d 00 00 00 00 00 00 00   .k.....=.......
<3>[  683.373195] Object 00000000545afebc: aa b3 00 00 00 00 00 00 0f 00 00 00 00 00 00 00  ................
<3>[  683.373209] Object 00000000e4a394a8: 25 bd bd 1b 9f 00 00 00 00 00 00 00 5a 5a 5a 5a  %...........ZZZZ
<3>[  683.373223] Object 0000000029a7878a: 00 00 00 00 ad 4e ad de ff ff ff ff 5a 5a 5a 5a  .....N......ZZZZ
<3>[  683.373237] Object 00000000d37797b3: ff ff ff ff ff ff ff ff e8 6e 57 c0 ff ff ff ff  .........nW.....
<3>[  683.373251] Object 00000000d50414f6: 00 b3 c8 8e ff ff ff ff 80 b0 c8 8e ff ff ff ff  ................
<3>[  683.373265] Object 00000000c28e8847: 41 01 4b c0 ff ff ff ff 00 00 88 8e 88 9f ff ff  A.K.............
<3>[  683.373279] Object 00000000c74212ab: 38 c1 6d 8a 88 9f ff ff 58 21 74 8a 88 9f ff ff  8.m.....X!t.....
<3>[  683.373293] Object 000000000d8012cf: c0 c1 6d 8a 88 9f ff ff 58 79 dd d9 87 9f ff ff  ..m.....Xy......
<3>[  683.373306] Object 00000000c9900b91: 98 d0 4e 8a 88 9f ff ff 58 3c e8 9b 88 9f ff ff  ..N.....X<......
<3>[  683.373320] Object 0000000044bb8c3d: 58 3c e8 9b 88 9f ff ff 64 f5 04 00 00 00 00 00  X<......d.......
<3>[  683.373334] Object 00000000180c4cca: 00 00 00 00 ad 4e ad de ff ff ff ff 5a 5a 5a 5a  .....N......ZZZZ
<3>[  683.373348] Object 00000000c9044498: ff ff ff ff ff ff ff ff e0 6e 57 c0 ff ff ff ff  .........nW.....
<3>[  683.373362] Object 0000000072d0dfb3: 00 00 00 00 00 00 00 00 c0 b1 c8 8e ff ff ff ff  ................
<3>[  683.373376] Object 0000000081f198b9: 55 01 4b c0 ff ff ff ff d8 de 6b fa 87 9f ff ff  U.K.......k.....
<3>[  683.373390] Object 000000006a375a13: d8 de 6b fa 87 9f ff ff cc 05 39 c0 ff ff ff ff  ..k.......9.....
<3>[  683.373404] Object 00000000b8392dd1: ff ff ff ff 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ....ZZZZZZZZZZZZ
<3>[  683.373418] Object 00000000e5c1bbcb: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<3>[  683.373432] Object 00000000199feccd: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<3>[  683.373446] Object 0000000020f5e08b: 20 df 6b fa 87 9f ff ff 20 df 6b fa 87 9f ff ff   .k..... .k.....
<3>[  683.373460] Object 0000000090591b0f: 30 df 6b fa 87 9f ff ff 30 df 6b fa 87 9f ff ff  0.k.....0.k.....
<3>[  683.373473] Object 00000000232f7cd0: 40 df 6b fa 87 9f ff ff 40 df 6b fa 87 9f ff ff  @.k.....@.k.....
<3>[  683.373487] Object 0000000060458027: 50 df 6b fa 87 9f ff ff 50 df 6b fa 87 9f ff ff  P.k.....P.k.....
<3>[  683.373501] Object 00000000e3c82ce2: 06 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a  ........ZZZZZZZZ
<3>[  683.373515] Object 00000000ec804eb8: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<3>[  683.373529] Object 00000000ce7ccc08: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<3>[  683.373543] Object 000000002dbc575c: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<3>[  683.373557] Object 00000000b86d3417: 5a 5a 5a 5a 5a 5a 5a 5a 00 de 6b fa 87 9f ff ff  ZZZZZZZZ..k.....
<3>[  683.373571] Object 00000000d1e82276: b8 61 dd d9 87 9f ff ff a0 06 00 00 d0 06 00 00  .a..............
<3>[  683.373585] Object 00000000cc53f969: e8 06 00 00 20 07 00 00 28 07 00 00 00 00 00 00  .... ...(.......
<3>[  683.373599] Object 00000000ea2426d2: 40 0c 8c 7b 88 9f ff ff 00 00 00 00 00 00 00 00  @..{............
<3>[  683.373613] Object 00000000b860c1c3: 68 0d 8c 7b 88 9f ff ff 68 25 8c 7b 88 9f ff ff  h..{....h%.{....
<3>[  683.373627] Object 0000000016455ea0: 96 d5 05 00 01 00 00 00 00 5a 5a 5a 5a 5a 5a 5a  .........ZZZZZZZ
<3>[  683.373640] Object 00000000e66ede82: 00 e0 6b fa 87 9f ff ff 00 e0 6b fa 87 9f ff ff  ..k.......k.....
<3>[  683.373654] Object 0000000080964939: 10 e0 6b fa 87 9f ff ff 10 e0 6b fa 87 9f ff ff  ..k.......k.....
<3>[  683.373668] Object 00000000e7ffc5dd: 00 00 00 00 00 00 00 00 00 01 00 00 00 00 ad de  ................
<3>[  683.373682] Object 000000000ce9d6ca: 00 02 00 00 00 00 ad de 5a 5a 5a 5a 5a 5a 5a 5a  ........ZZZZZZZZ
<3>[  683.373696] Object 00000000386659d0: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<3>[  683.373710] Redzone 0000000075d2069d: bb bb bb bb bb bb bb bb                          ........
<3>[  683.373723] Padding 0000000054e14c6b: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<3>[  683.373737] Padding 00000000425e5b34: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<3>[  683.373751] Padding 00000000ad3d4db9: 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a 5a  ZZZZZZZZZZZZZZZZ
<4>[  683.373767] CPU: 1 PID: 151 Comm: kworker/1:2 Tainted: G    BU            5.0.0-rc8-g39139489403b-drmtip_236+ #1
<4>[  683.373769] Hardware name: Intel Corporation Ice Lake Client Platform/IceLake Y LPDDR4x T4 RVP TLC, BIOS ICLSFWR1.R00.3087.A00.1902250334 02/25/2019
<4>[  683.373773] Workqueue: events delayed_fput
<4>[  683.373775] Call Trace:
<4>[  683.373777]  <IRQ>
<4>[  683.373781]  dump_stack+0x67/0x9b
<4>[  683.373783]  free_debug_processing+0x344/0x370
<4>[  683.373832]  ? intel_engine_breadcrumbs_irq+0x2e4/0x380 [i915]
<4>[  683.373836]  __slab_free+0x337/0x4f0
<4>[  683.373840]  ? _raw_spin_unlock_irqrestore+0x39/0x60
<4>[  683.373844]  ? debug_check_no_obj_freed+0x132/0x210
<4>[  683.373889]  ? intel_engine_breadcrumbs_irq+0x2e4/0x380 [i915]
<4>[  683.373892]  ? kmem_cache_free+0x275/0x2e0
<4>[  683.373894]  kmem_cache_free+0x275/0x2e0
<4>[  683.373939]  intel_engine_breadcrumbs_irq+0x2e4/0x380 [i915]
<4>[  683.373984]  gen8_cs_irq_handler+0x4e/0xa0 [i915]
<4>[  683.374026]  gen11_irq_handler+0x24b/0x330 [i915]
<4>[  683.374032]  __handle_irq_event_percpu+0x41/0x2d0
<4>[  683.374034]  ? handle_irq_event+0x27/0x50
<4>[  683.374038]  handle_irq_event_percpu+0x2b/0x70
<4>[  683.374040]  handle_irq_event+0x2f/0x50
<4>[  683.374044]  handle_edge_irq+0xe7/0x190
<4>[  683.374048]  handle_irq+0x67/0x160
<4>[  683.374051]  do_IRQ+0x5e/0x130
<4>[  683.374054]  common_interrupt+0xf/0xf

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109827
Fixes: 52c0fdb2 ("drm/i915: Replace global breadcrumbs with per-context interrupt tracking")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190304114113.371-1-chris@chris-wilson.co.uk