[no subject]

[Zhang, Qiang]

 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();
}