kernel/sched/topology.c

490 	struct root_domain *old_rd = NULL;
496 		old_rd = rq->rd;
498 		if (cpumask_test_cpu(rq->cpu, old_rd->online))
501 		cpumask_clear_cpu(rq->cpu, old_rd->span);
504 		 * If we dont want to free the old_rd yet then
505 		 * set old_rd to NULL to skip the freeing later
508 		if (!atomic_dec_and_test(&old_rd->refcount))
509 			old_rd = NULL;
521 	if (old_rd)
522 		call_rcu(&old_rd->rcu, free_rootdomain);