For synchronization mechanisms similar to RCU, there could be no "grace
period" concept (e.g. hazard pointers), therefore allow
rcu_scale_ops::get_gp_seq to be a NULL pointer for these cases, and
simply treat started and finished grace period as 0.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 kernel/rcu/rcuscale.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
index b521d0455992..45413a73d61e 100644
--- a/kernel/rcu/rcuscale.c
+++ b/kernel/rcu/rcuscale.c
@@ -568,8 +568,10 @@ rcu_scale_writer(void *arg)
 		if (gp_exp) {
 			b_rcu_gp_test_started =
 				cur_ops->exp_completed() / 2;
-		} else {
+		} else if (cur_ops->get_gp_seq) {
 			b_rcu_gp_test_started = cur_ops->get_gp_seq();
+		} else {
+			b_rcu_gp_test_started = 0;
 		}
 	}
 
@@ -625,9 +627,11 @@ rcu_scale_writer(void *arg)
 				if (gp_exp) {
 					b_rcu_gp_test_finished =
 						cur_ops->exp_completed() / 2;
-				} else {
+				} else if (cur_ops->get_gp_seq) {
 					b_rcu_gp_test_finished =
 						cur_ops->get_gp_seq();
+				} else {
+					b_rcu_gp_test_finished = 0;
 				}
 				if (shutdown) {
 					smp_mb(); /* Assign before wake. */
-- 
2.39.5 (Apple Git-154)