drm/i915: Clear breadcrumb node when cancelling signaling

drm/i915: Clear breadcrumb node when cancelling signaling

When we call intel_engine_cancel_signaling() to stop reporting when
a request is completed via an asynchronous signal, we remove that request
from the breadcrumb wait queue. However, we may be concurrently
processing that request in the signaler itself, the actual operations on
the request's node itself are serialised but we do not actually clear the
waiter after removing it from the tree allowing both parties to attempt
to do so and corrupting the rbtree. (Previously removing from the
breadcrumb wait queue could only be done on behalf of i915_wait_request,
so this race could not happen).

Reported-by: "He, Bo" <[email protected]>
Fixes: 9eb143bbec7d ("drm/i915: Allow a request to be cancelled")
Signed-off-by: Chris Wilson <[email protected]>
Cc: "He, Bo" <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Michał Winiarski <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Joonas Lahtinen <[email protected]>
(cherry picked from commit c534612e780c4a2c8ef5bfc11583c7d58436baca)
Signed-off-by: Jani Nikula <[email protected]>