reservation: add reservation_object_get_excl_rcu()

In the atomic modesetting path, each driver simply wants to grab a ref
to the exclusive fence from a reservation object to store in the incoming
drm_plane_state, without doing the whole RCU dance.  Since each driver
will need to do this, lets make a helper.

v2: rename to _rcu instead of _unlocked to be more consistent
Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>

include/linux/reservation.h

@@ -120,6 +120,24 @@ reservation_object_get_excl(struct reservation_object *obj)
 					 reservation_object_held(obj));
 }
 
+static inline struct fence *
+reservation_object_get_excl_rcu(struct reservation_object *obj)
+{
+	struct fence *fence;
+	unsigned seq;
+retry:
+	seq = read_seqcount_begin(&obj->seq);
+	rcu_read_lock();
+	fence = rcu_dereference(obj->fence_excl);
+	if (read_seqcount_retry(&obj->seq, seq)) {
+		rcu_read_unlock();
+		goto retry;
+	}
+	fence = fence_get(fence);
+	rcu_read_unlock();
+	return fence;
+}
+
 int reservation_object_reserve_shared(struct reservation_object *obj);
 void reservation_object_add_shared_fence(struct reservation_object *obj,
 					 struct fence *fence);