As we unwind the requests for a preemption event, we return a virtual
request back to its original virtual engine (so that it is available for
execution on any of its siblings). In the process, this means that its
breadcrumb should no longer be associated with the original physical
engine, and so we are forced to decouple it. Previously, as the request
could not complete without our awareness, we would move it to the next
real engine without any danger. However, preempt-to-busy allowed for
requests to continue on the HW and complete in the background as we
unwound, which meant that we could end up retiring the request before
fixing up the breadcrumb link.

[51679.517943] INFO: trying to register non-static key.
[51679.517956] the code is fine but needs lockdep annotation.
[51679.517960] turning off the locking correctness validator.
[51679.517966] CPU: 0 PID: 3270 Comm: kworker/u8:0 Tainted: G     U            5.2.0+ #717
[51679.517971] Hardware name: Intel Corporation NUC7i5BNK/NUC7i5BNB, BIOS BNKBL357.86A.0052.2017.0918.1346 09/18/2017
[51679.518012] Workqueue: i915 retire_work_handler [i915]
[51679.518017] Call Trace:
[51679.518026]  dump_stack+0x67/0x90
[51679.518031]  register_lock_class+0x52c/0x540
[51679.518038]  ? find_held_lock+0x2d/0x90
[51679.518042]  __lock_acquire+0x68/0x1800
[51679.518047]  ? find_held_lock+0x2d/0x90
[51679.518073]  ? __i915_sw_fence_complete+0xff/0x1c0 [i915]
[51679.518079]  lock_acquire+0x90/0x170
[51679.518105]  ? i915_request_cancel_breadcrumb+0x29/0x160 [i915]
[51679.518112]  _raw_spin_lock+0x27/0x40
[51679.518138]  ? i915_request_cancel_breadcrumb+0x29/0x160 [i915]
[51679.518165]  i915_request_cancel_breadcrumb+0x29/0x160 [i915]
[51679.518199]  i915_request_retire+0x43f/0x530 [i915]
[51679.518232]  retire_requests+0x4d/0x60 [i915]
[51679.518263]  i915_retire_requests+0xdf/0x1f0 [i915]
[51679.518294]  retire_work_handler+0x4c/0x60 [i915]
[51679.518301]  process_one_work+0x22c/0x5c0
[51679.518307]  worker_thread+0x37/0x390
[51679.518311]  ? process_one_work+0x5c0/0x5c0
[51679.518316]  kthread+0x116/0x130
[51679.518320]  ? kthread_create_on_node+0x40/0x40
[51679.518325]  ret_from_fork+0x24/0x30
[51679.520177] ------------[ cut here ]------------
[51679.520189] list_del corruption, ffff88883675e2f0->next is LIST_POISON1 (dead000000000100)

Fixes: 22b7a426 ("drm/i915/execlists: Preempt-to-busy")
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/20190716124931.5870-4-chris@chris-wilson.co.uk