If we drop the engine lock, we may run execlists_dequeue which may free
the priolist. Therefore if we ever drop the execution lock on the
engine, we have to discard our cache and refetch the priolist to ensure
we do not use a stale pointer.

[  506.418935] [IGT] gem_exec_whisper: starting subtest contexts-priority
[  593.240825] general protection fault: 0000 [#1] SMP
[  593.240863] CPU: 1 PID: 494 Comm: gem_exec_whispe Tainted: G     U            5.0.0-rc6+ #100
[  593.240879] Hardware name:  /NUC6CAYB, BIOS AYAPLCEL.86A.0029.2016.1124.1625 11/24/2016
[  593.240965] RIP: 0010:__i915_schedule+0x1fe/0x320 [i915]
[  593.240981] Code: 48 8b 0c 24 48 89 c3 49 8b 45 28 49 8b 75 20 4c 89 3c 24 48 89 46 08 48 89 30 48 8b 43 08 48 89 4b 08 49 89 5d 20 49 89 45 28 <48> 89 08 45 39 a7 b8 03 00 00 7d 44 45 89 a7 b8 03 00 00 49 8b 85
[  593.240999] RSP: 0018:ffffc90000057a60 EFLAGS: 00010046
[  593.241013] RAX: 6b6b6b6b6b6b6b6b RBX: ffff8882582d7870 RCX: ffff88826baba6f0
[  593.241026] RDX: 0000000000000000 RSI: ffff8882582d6e70 RDI: ffff888273482194
[  593.241049] RBP: ffffc90000057a68 R08: ffff8882582d7680 R09: ffff8882582d7840
[  593.241068] R10: 0000000000000000 R11: ffffea00095ebe08 R12: 0000000000000728
[  593.241105] R13: ffff88826baba6d0 R14: ffffc90000057a40 R15: ffff888273482158
[  593.241120] FS:  00007f4613fb3900(0000) GS:ffff888277a80000(0000) knlGS:0000000000000000
[  593.241133] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  593.241146] CR2: 00007f57d3c66a84 CR3: 000000026e2b6000 CR4: 00000000001406e0
[  593.241158] Call Trace:
[  593.241233]  i915_schedule+0x1f/0x30 [i915]
[  593.241326]  i915_request_add+0x1a9/0x290 [i915]
[  593.241393]  i915_gem_do_execbuffer+0x45f/0x1150 [i915]
[  593.241411]  ? init_object+0x49/0x80
[  593.241425]  ? ___slab_alloc.constprop.91+0x4b8/0x4e0
[  593.241491]  ? i915_gem_execbuffer2_ioctl+0x99/0x380 [i915]
[  593.241563]  ? i915_gem_execbuffer_ioctl+0x270/0x270 [i915]
[  593.241629]  i915_gem_execbuffer2_ioctl+0x1bb/0x380 [i915]
[  593.241705]  ? i915_gem_execbuffer_ioctl+0x270/0x270 [i915]
[  593.241724]  drm_ioctl_kernel+0x81/0xd0
[  593.241738]  drm_ioctl+0x1a7/0x310
[  593.241803]  ? i915_gem_execbuffer_ioctl+0x270/0x270 [i915]
[  593.241819]  ? __update_load_avg_se+0x1c9/0x240
[  593.241834]  ? pick_next_entity+0x7e/0x120
[  593.241851]  do_vfs_ioctl+0x88/0x5d0
[  593.241880]  ksys_ioctl+0x35/0x70
[  593.241894]  __x64_sys_ioctl+0x11/0x20
[  593.241907]  do_syscall_64+0x44/0xf0
[  593.241924]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  593.241940] RIP: 0033:0x7f4615ffe757
[  593.241952] Code: 00 00 90 48 8b 05 39 a7 0c 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 09 a7 0c 00 f7 d8 64 89 01 48
[  593.241970] RSP: 002b:00007ffc1030ddf8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[  593.241984] RAX: ffffffffffffffda RBX: 00007ffc10324420 RCX: 00007f4615ffe757
[  593.241997] RDX: 00007ffc1030e220 RSI: 0000000040406469 RDI: 0000000000000003
[  593.242010] RBP: 00007ffc1030e220 R08: 00007f46160c9208 R09: 00007f46160c9240
[  593.242022] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000040406469
[  593.242038] R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000000
[  593.242058] Modules linked in: i915 intel_gtt drm_kms_helper prime_numbers

v2: Track the local engine cache and explicitly clear it when switching
engine locks.

Fixes: a02eb975 ("drm/i915/execlists: Cache the priolist when rescheduling")
Testcase: igt/gem_exec_whisper/contexts-priority # rare!
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190211204647.26723-1-chris@chris-wilson.co.uk