drm/i915: Reinstate reservation_object zapping for batch_pool objects

I removed the zapping of the reservation_object->fence array of shared
fences prematurely. We don't yet have the code to zap that array when
retiring the object, and so currently it remains possible to continually
grow the shared array trapping requests when reusing the batch_pool
object across many timelines.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170518094638.5469-4-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_batch_pool.c

@@ -114,12 +114,27 @@ i915_gem_batch_pool_get(struct i915_gem_batch_pool *pool,
 	list_for_each_entry(obj, list, batch_pool_link) {
 		/* The batches are strictly LRU ordered */
 		if (i915_gem_object_is_active(obj)) {
-			if (!reservation_object_test_signaled_rcu(obj->resv,
-								  true))
+			struct reservation_object *resv = obj->resv;
+
+			if (!reservation_object_test_signaled_rcu(resv, true))
 				break;
 
 			i915_gem_retire_requests(pool->engine->i915);
 			GEM_BUG_ON(i915_gem_object_is_active(obj));
+
+			/*
+			 * The object is now idle, clear the array of shared
+			 * fences before we add a new request. Although, we
+			 * remain on the same engine, we may be on a different
+			 * timeline and so may continually grow the array,
+			 * trapping a reference to all the old fences, rather
+			 * than replace the existing fence.
+			 */
+			if (rcu_access_pointer(resv->fence)) {
+				reservation_object_lock(resv, NULL);
+				reservation_object_add_excl_fence(resv, NULL);
+				reservation_object_unlock(resv);
+			}
 		}
 
 		GEM_BUG_ON(!reservation_object_test_signaled_rcu(obj->resv,