static void neigh_periodic_work(struct work_struct *work)
{
	struct neigh_table *tbl = container_of(work, struct neigh_table, gc_work.work);
	struct neighbour *n;
	struct neighbour __rcu **np;
	unsigned int i;
	struct neigh_hash_table *nht;

	NEIGH_CACHE_STAT_INC(tbl, periodic_gc_runs);

	write_lock_bh(&tbl->lock);
	nht = rcu_dereference_protected(tbl->nht,
					lockdep_is_held(&tbl->lock));

	/*
	 *	periodically recompute ReachableTime from random function
	 */

	if (time_after(jiffies, tbl->last_rand + 300 * HZ)) {
		struct neigh_parms *p;
		tbl->last_rand = jiffies;
		list_for_each_entry(p, &tbl->parms_list, list)
			p->reachable_time =
				neigh_rand_reach_time(NEIGH_VAR(p, BASE_REACHABLE_TIME));
	}

	if (atomic_read(&tbl->entries) < tbl->gc_thresh1)   //如果这个值没有满足，就永远不会扫描arp表清理失效的记录。
		goto out;

	for (i = 0 ; i < (1 << nht->hash_shift); i++) {   // 扫描清理 arp的hash表
		np = &nht->hash_buckets[i];

		while ((n = rcu_dereference_protected(*np,
				lockdep_is_held(&tbl->lock))) != NULL) {
			unsigned int state;

			write_lock(&n->lock);

			state = n->nud_state;
			if ((state & (NUD_PERMANENT | NUD_IN_TIMER)) ||
			    (n->flags & NTF_EXT_LEARNED)) {
				write_unlock(&n->lock);
				goto next_elt;
			}

			if (time_before(n->used, n->confirmed))
				n->used = n->confirmed;

			if (refcount_read(&n->refcnt) == 1 &&
			    (state == NUD_FAILED ||
			     time_after(jiffies, n->used + NEIGH_VAR(n->parms, GC_STALETIME)))) {
				*np = n->next;
				n->dead = 1;
				write_unlock(&n->lock);
				neigh_cleanup_and_release(n);
				continue;
			}
			write_unlock(&n->lock);

next_elt:
			np = &n->next;
		}
		/*
		 * It's fine to release lock here, even if hash table
		 * grows while we are preempted.
		 */
		write_unlock_bh(&tbl->lock);
		cond_resched();
		write_lock_bh(&tbl->lock);
		nht = rcu_dereference_protected(tbl->nht,
						lockdep_is_held(&tbl->lock));
	}
out:
	/* Cycle through all hash buckets every BASE_REACHABLE_TIME/2 ticks.
	 * ARP entry timeouts range from 1/2 BASE_REACHABLE_TIME to 3/2
	 * BASE_REACHABLE_TIME.
	 */
	queue_delayed_work(system_power_efficient_wq, &tbl->gc_work,
			      NEIGH_VAR(&tbl->parms, BASE_REACHABLE_TIME) >> 1);
	write_unlock_bh(&tbl->lock);
}