When config preempt RCU, if task switch happened in rcu read critical region. the current task be add to "rnp->blkd_tasks" link list, these rnp is leaf node in RCU tree. In "rcu_gp_fqs_loop" func,
static void rcu_gp_fqs_loop(void)
{
struct rcu_node *rnp = rcu_get_root();
}
