drm/i915: Prune the reservation shared fence array

The shared fence array is not autopruning and may continue to grow as an
object is shared between new timelines. Take the opportunity when we
think the object is idle (we have to confirm that any external fence is
also signaled) to decouple all the fences.

We apply a similar trick after waiting on an object, see commit
e54ca977 ("drm/i915: Remove completed fences after a wait")

v2: No longer need to handle the batch pool as a special case.
v3: Need to trylock from within i915_vma_retire as this may be called
form the shrinker - and we may later try to allocate underneath the
reservation lock, so a deadlock is possible.

References: https://bugs.freedesktop.org/show_bug.cgi?id=102936
Fixes: d07f0e59 ("drm/i915: Move GEM activity tracking into a common struct reservation_object")
Fixes: 80b204bc ("drm/i915: Enable multiple timelines")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107220656.5020-1-chris@chris-wilson.co.ukReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_vma.c

@@ -54,6 +54,13 @@ i915_vma_retire(struct i915_gem_active *active,
 	if (--obj->active_count)
 		return;
 
+	/* Prune the shared fence arrays iff completely idle (inc. external) */
+	if (reservation_object_trylock(obj->resv)) {
+		if (reservation_object_test_signaled_rcu(obj->resv, true))
+			reservation_object_add_excl_fence(obj->resv, NULL);
+		reservation_object_unlock(obj->resv);
+	}
+
 	/* Bump our place on the bound list to keep it roughly in LRU order
 	 * so that we don't steal from recently used but inactive objects
 	 * (unless we are forced to ofc!)