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

 1 struct neighbour *neigh_lookup(struct neigh_table *tbl, const void *pkey,
 2                    struct net_device *dev)
 3 {
 4     struct neighbour *n;
 5
 6     NEIGH_CACHE_STAT_INC(tbl, lookups);
 7
 8     rcu_read_lock_bh();
 9
10     /* 非引用查找邻居项 */
11     n = __neigh_lookup_noref(tbl, pkey, dev);
12
13     /* 增加引用计数 */
14     if (n) {
15         if (!atomic_inc_not_zero(&n->refcnt))
16             n = NULL;
17         NEIGH_CACHE_STAT_INC(tbl, hits);
18     }
19
20     rcu_read_unlock_bh();
21
22     /* 返回邻居项 */
23     return n;
24 }

1 static inline struct neighbour *__neigh_lookup_noref(struct neigh_table *tbl,
2                              const void *pkey,
3                              struct net_device *dev)
4 {
5     return ___neigh_lookup_noref(tbl, tbl->key_eq, tbl->hash, pkey, dev);
6 }

 1 static inline struct neighbour *___neigh_lookup_noref(
 2     struct neigh_table *tbl,
 3     bool (*key_eq)(const struct neighbour *n, const void *pkey),
 4     __u32 (*hash)(const void *pkey,
 5               const struct net_device *dev,
 6               __u32 *hash_rnd),
 7     const void *pkey,
 8     struct net_device *dev)
 9 {
10     /* 获取hash */
11     struct neigh_hash_table *nht = rcu_dereference_bh(tbl->nht);
12     struct neighbour *n;
13     u32 hash_val;
14
15     /* 计算hash值 */
16     hash_val = hash(pkey, dev, nht->hash_rnd) >> (32 - nht->hash_shift);
17
18     /* 遍历hash表项 */
19     for (n = rcu_dereference_bh(nht->hash_buckets[hash_val]);
20          n != NULL;
21          n = rcu_dereference_bh(n->next)) {
22         /* 找到则返回该项 */
23         if (n->dev == dev && key_eq(n, pkey))
24             return n;
25     }
26
27     /* 未找到则返回NULL */
28     return NULL;
29 }