As __active_retire() does it's final atomic_dec() under the
ref->tree_lock spinlock, in order to prevent ourselves from reusing the
ref->cache and ref->tree as they are being destroyed, we need to
serialise with the retirement during i915_active_acquire().

[  +0.000005] kernel BUG at drivers/gpu/drm/i915/i915_active.c:157!
[  +0.000011] invalid opcode: 0000 [#1] SMP
[  +0.000004] CPU: 7 PID: 188 Comm: kworker/u16:4 Not tainted 5.4.0-rc8-03070-gac5e57322614 #89
[  +0.000002] Hardware name: Razer Razer Blade Stealth 13 Late 2019/LY320, BIOS 1.02 09/10/2019
[  +0.000082] Workqueue: events_unbound active_work [i915]
[  +0.000059] RIP: 0010:__active_retire+0x115/0x120 [i915]
[  +0.000003] Code: 75 28 48 8b 3d 8c 6e 1a 00 48 89 ee e8 e4 5f a5 c0 48 8b 44 24 10 65 48 33 04 25 28 00 00 00 75 0f 48 83 c4 18 5b 5d 41 5c c3 <0f> 0b 0f 0b 0f 0b e8 a0 90 87 c0 0f 1f 44 00 00 48 8b 3d 54 6e 1a
[  +0.000002] RSP: 0018:ffffb833003f7e48 EFLAGS: 00010286
[  +0.000003] RAX: ffff8d6e8d726d00 RBX: ffff8d6f9db4e840 RCX: 0000000000000000
[  +0.000001] RDX: ffffffff82605930 RSI: ffff8d6f9adc4908 RDI: ffff8d6e96cefe28
[  +0.000002] RBP: ffff8d6e96cefe00 R08: 0000000000000000 R09: ffff8d6f9ffe9a50
[  +0.000002] R10: 0000000000000048 R11: 0000000000000018 R12: ffff8d6f9adc4930
[  +0.000001] R13: ffff8d6f9e04fb00 R14: 0000000000000000 R15: ffff8d6f9adc4988
[  +0.000002] FS:  0000000000000000(0000) GS:ffff8d6f9ffc0000(0000) knlGS:0000000000000000
[  +0.000002] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  +0.000002] CR2: 000055eb5a34cf10 CR3: 000000018d609002 CR4: 0000000000760ee0
[  +0.000002] PKRU: 55555554
[  +0.000001] Call Trace:
[  +0.000010]  process_one_work+0x1aa/0x350
[  +0.000004]  worker_thread+0x4d/0x3a0
[  +0.000004]  kthread+0xfb/0x130
[  +0.000004]  ? process_one_work+0x350/0x350
[  +0.000003]  ? kthread_park+0x90/0x90
[  +0.000005]  ret_from_fork+0x1f/0x40
Reported-by: Kenneth Graunke <kenneth@whitecape.org>
Fixes: c9ad602f ("drm/i915: Split i915_active.mutex into an irq-safe spinlock for the rbtree")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Matthew Auld <matthew.auld@intel.com>
Tested-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205183332.801237-1-chris@chris-wilson.co.uk