邻居子系统 之 邻居项查找neigh_lookup、___neigh_lookup_noref

概述

邻居项查找是通过neigh_lookup相关函数来进行的；

___neigh_lookup_noref，该函数根据输出设备和主键值(IPv4为下一跳ip地址)在邻居项hash表中查找，找到则返回该项；

neigh_lookup，该函数调用了___neigh_lookup_noref函数，并且在找到邻居项之后，进行引用计数的递增，然后返回该项；

源码分析

 struct neighbour *neigh_lookup(struct neigh_table *tbl, const void *pkey,
                    struct net_device *dev)
 {
     struct neighbour *n;

     NEIGH_CACHE_STAT_INC(tbl, lookups);

     rcu_read_lock_bh();

     /* 非引用查找邻居项 */
     n = __neigh_lookup_noref(tbl, pkey, dev);

     /* 增加引用计数 */
     if (n) {
         if (!atomic_inc_not_zero(&n->refcnt))
             n = NULL;
         NEIGH_CACHE_STAT_INC(tbl, hits);
     }

     rcu_read_unlock_bh();

     /* 返回邻居项 */
     return n;
 }

 static inline struct neighbour *__neigh_lookup_noref(struct neigh_table *tbl,
                              const void *pkey,
                              struct net_device *dev)
 {
     return ___neigh_lookup_noref(tbl, tbl->key_eq, tbl->hash, pkey, dev);
 }

 static inline struct neighbour *___neigh_lookup_noref(
     struct neigh_table *tbl,
     bool (*key_eq)(const struct neighbour *n, const void *pkey),
     __u32 (*hash)(const void *pkey,
               const struct net_device *dev,
               __u32 *hash_rnd),
     const void *pkey,
     struct net_device *dev)
 {
     /* 获取hash */
     struct neigh_hash_table *nht = rcu_dereference_bh(tbl->nht);
     struct neighbour *n;
     u32 hash_val;

     /* 计算hash值 */
     hash_val = hash(pkey, dev, nht->hash_rnd) >> ( - nht->hash_shift);

     /* 遍历hash表项 */
     for (n = rcu_dereference_bh(nht->hash_buckets[hash_val]);
          n != NULL;
          n = rcu_dereference_bh(n->next)) {
         /* 找到则返回该项 */
         if (n->dev == dev && key_eq(n, pkey))
             return n;
     }

     /* 未找到则返回NULL */
     return NULL;
 }