David found another issue with priority bumping from mmioflips, where we
are accessing the requests concurrently to them being retired and freed.
Whilst we are skipping the dependency if has been submitted, that is not
sufficient to stop the dependency from disappearing if another thread
retires that request. To prevent we can either employ the struct_mutex (or a
request mutex in the future) to serialise retiring before it is freed.
Alternatively, we need to keep the dependencies alive using RCU whilst
they are being accessed via the DFS.

[ 1746.698111] general protection fault: 0000 [#1] PREEMPT SMP
[ 1746.698305] Modules linked in: snd_hda_intel snd_hda_codec snd_hwdep x86_pkg_temp_thermal snd_hda_core coretemp crct10dif_pclmul crc32_pclmul snd_pcm ghash_clmulni_intel mei_me mei i915 e1000e ptp pps_core i2c_hid
[ 1746.698750] CPU: 1 PID: 6716 Comm: kworker/u8:2 Not tainted 4.9.0-rc6-CI-Nightly_816+ #1
[ 1746.698871] Hardware name: GIGABYTE GB-BKi7A-7500/MFLP7AP-00, BIOS F1 07/27/2016
[ 1746.699125] Workqueue: events_unbound intel_mmio_flip_work_func [i915]
[ 1746.699266] task: ffff880260a5e800 task.stack: ffffc90000f6c000
[ 1746.699361] RIP: 0010:[<ffffffffa006595d>]  [<ffffffffa006595d>] execlists_schedule+0x8d/0x300 [i915]
[ 1746.699632] RSP: 0018:ffffc90000f6fcd8  EFLAGS: 00010206
[ 1746.699724] RAX: dead0000000000f8 RBX: ffff8801f64b2bf0 RCX: ffff8801f64b2c10
[ 1746.699842] RDX: dead000000000100 RSI: 0000000000000000 RDI: ffff8801f64b0458
[ 1746.699972] RBP: ffffc90000f6fd68 R08: ffff88026488dc00 R09: 0000000000000002
[ 1746.700090] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000400
[ 1746.700195] R13: ffffc90000f6fcf0 R14: ffff88020955aa40 R15: ffff88020955aa68
[ 1746.700307] FS:  0000000000000000(0000) GS:ffff88026dc80000(0000) knlGS:0000000000000000
[ 1746.700435] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1746.700532] CR2: 0000000002a69e90 CR3: 0000000002c07000 CR4: 00000000003406e0
[ 1746.700635] Stack:
[ 1746.700682]  ffff880260a5e880 ffffc90000f6fd50 ffffffff810af69a ffffc90000f6fd28
[ 1746.700827]  ffff88020955a628 ffff8801e1eaebf0 0000000000000020 0000000000000000
[ 1746.700947]  00000196af1edc96 ffff88025dfa4000 ffff8801f0b030a8 ffffc90000f6fcf0
[ 1746.701071] Call Trace:
[ 1746.701117]  [<ffffffff810af69a>] ? dequeue_entity+0x25a/0xb50
[ 1746.701260]  [<ffffffffa00516be>] fence_set_priority+0x7e/0x80 [i915]
[ 1746.701406]  [<ffffffffa0051a15>] i915_gem_object_wait_priority+0x85/0x160 [i915]
[ 1746.701599]  [<ffffffffa008ccd7>] intel_mmio_flip_work_func+0x47/0x2b0 [i915]
[ 1746.701717]  [<ffffffff81094c4d>] process_one_work+0x14d/0x470
[ 1746.701809]  [<ffffffff81094fb3>] worker_thread+0x43/0x4e0
[ 1746.701888]  [<ffffffff81094f70>] ? process_one_work+0x470/0x470
[ 1746.701969]  [<ffffffff81094f70>] ? process_one_work+0x470/0x470
[ 1746.702072]  [<ffffffff8109a4d5>] kthread+0xc5/0xe0
[ 1746.702152]  [<ffffffff81771c59>] ? _raw_spin_unlock_irq+0x9/0x10
[ 1746.702234]  [<ffffffff8109a410>] ? kthread_park+0x60/0x60
[ 1746.702318]  [<ffffffff81772272>] ret_from_fork+0x22/0x30
[ 1746.702387] Code: 89 42 08 48 8b 45 88 48 89 55 c0 4c 89 6d c8 4c 8d 70 d8 4d 8d 7e 28 4d 39 ef 74 72 49 8b 1e 48 8b 13 48 39 d3 48 8d 42 f8 74 3e <48> 8b 10 8b 52 38 41 39 d4 7e 26 48 8b 50 30 48 8b 78 28 48 8d
[ 1746.702921] RIP  [<ffffffffa006595d>] execlists_schedule+0x8d/0x300 [i915]
Nov 25 21:42:54 kbl-gbbki7 kernel: [ 1746.703027]  RSP <ffffc90000f6fcd8>

Fixes: 27745e82 ("drm/i915/execlists: Use a local lock for dfs_link access")
Fixes: 9a151987 ("drm/i915: Add execution priority boosting for mmioflips")
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161128143649.4289-1-chris@chris-wilson.co.ukReviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>