内核中hash表（以net_device为例）

　　下边函数实现将新的 net_device 设备插入到内核链表中

/*
 * Device list insertion
 */
static void list_netdevice(struct net_device *dev)
{
    struct net *net = dev_net(dev);

    ASSERT_RTNL();

    write_lock_bh(&dev_base_lock);
    list_add_tail_rcu(&dev->dev_list, &net->dev_base_head);
    hlist_add_head_rcu(&dev->name_hlist, dev_name_hash(net, dev->name));
    hlist_add_head_rcu(&dev->index_hlist,
               dev_index_hash(net, dev->ifindex));
    write_unlock_bh(&dev_base_lock);

    dev_base_seq_inc(net);
}

　　如上所示，一共有一个链表，两个hash表，其中一个是name hash，另一个是ifindex hash。

#define NETDEV_HASHBITS    8
#define NETDEV_HASHENTRIES (1 << NETDEV_HASHBITS)    

static struct hlist_head * __net_init netdev_create_hash(void)
{
    int i;
    struct hlist_head *hash;

    hash = kmalloc(sizeof(*hash) * NETDEV_HASHENTRIES, GFP_KERNEL);
    if (hash != NULL)
        for (i = ; i < NETDEV_HASHENTRIES; i++)
            INIT_HLIST_HEAD(&hash[i]);

    return hash;
}

　　如上所示，一共有(1<<8 = 256) 个hash entry， key值为 8 bits。

　　获取hash entry代码如下：先计算出一个unsigned int(32bits)的hash，然后稍微操作一下，取高8字节作为 key 值。

static inline struct hlist_head *dev_name_hash(struct net *net, const char *name)
{
    unsigned int hash = full_name_hash(name, strnlen(name, IFNAMSIZ));

    return &net->dev_name_head[hash_32(hash, NETDEV_HASHBITS)];
}

static inline u32 hash_32(u32 val, unsigned int bits)
{
    /* On some cpus multiply is faster, on others gcc will do shifts */
    u32 hash = val * GOLDEN_RATIO_PRIME_32;

    /* High bits are more random, so use them. */
    return hash >> ( - bits);
}

　　访问hash表代码如下：先获取所在的entry，然后遍历该entry。

struct net_device *dev_get_by_name_rcu(struct net *net, const char *name)
{
    struct net_device *dev;
    struct hlist_head *head = dev_name_hash(net, name);

    hlist_for_each_entry_rcu(dev, head, name_hlist)
        if (!strncmp(dev->name, name, IFNAMSIZ))
            return dev;       

    return NULL;
}