1 files changed, 8 insertions, 2 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index bcd635e42841..3f93033d3c61 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1305,10 +1305,16 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
          * believe that a grace period is in progress, then we must wait
          * for the one following, which is in "c".  Because our request
          * will be noticed at the end of the current grace period, we don't
-         * need to explicitly start one.
+         * need to explicitly start one.  We only do the lockless check
+         * of rnp_root's fields if the current rcu_node structure thinks
+         * there is no grace period in flight, and because we hold rnp->lock,
+         * the only possible change is when rnp_root's two fields are
+         * equal, in which case rnp_root->gpnum might be concurrently
+         * incremented.  But that is OK, as it will just result in our
+         * doing some extra useless work.
          */
         if (rnp->gpnum != rnp->completed ||
-            ACCESS_ONCE(rnp->gpnum) != ACCESS_ONCE(rnp->completed)) {
+            ACCESS_ONCE(rnp_root->gpnum) != ACCESS_ONCE(rnp_root->completed)) {
                 rnp->need_future_gp[c & 0x1]++;
                 trace_rcu_future_gp(rnp, rdp, c, TPS("Startedleaf"));
                 goto out;