smp_mb(); /* Write futex before read queue */
if (rcu_defer_num_callbacks()) {
smp_mb(); /* Read queue before write futex */
/* Callbacks are queued, don't wait. */
smp_mb(); /* Write futex before read queue */
if (rcu_defer_num_callbacks()) {
smp_mb(); /* Read queue before write futex */
/* Callbacks are queued, don't wait. */