The intent here was that we would be listening to
i915_gem_request_unsubmit in order to cancel the signaler quickly and
release the reference on the request. Cancelling the signaler is done
directly via intel_engine_cancel_signaling (called from unsubmit), but
that does not directly wake up the signaling thread, and neither does
setting the request->global_seqno back to zero wake up listeners to the
request->execute waitqueue. So the only time that listening to the
request->execute waitqueue would wake up the signaling kthread would be
on the request resubmission, during which time we would already receive
wake ups from rejoining the global breadcrumbs wait rbtree.

Trying to wake up to release the request remains an issue. If the
signaling was cancelled and no other request required signaling, then it
is possible for us to shutdown with the reference on the request still
held. To ensure that we do not try to shutdown, leaking that request, we
kick the signaling threads whenever we disarm the breadcrumbs, i.e. on
parking the engine when idle.

v2: We do need to be sure to release the last reference on stopping the
kthread; asserting that it has been dropped already is insufficient.

Fixes: d6a2289d ("drm/i915: Remove the preempted request from the execution queue")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171208121033.5236-1-chris@chris-wilson.co.ukAcked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>