linux Hash表简介

一、Hash表简介

1. 哈希表(Hash table)又叫散列表，是根据(Key, Value) 键值对进行访问的数据结构。主要目的是提高查询效率，比如Hash表的order为5，也就是同时使用2^5个链表，理论上查询速度可以比链表快2^5倍，典型的
以空间换时间。

2. 主要实现在 include/linux/hashtable.h 中，基于hlist。

二、相关结构

/* include/linux/types.h */
struct hlist_head {
    struct hlist_node *first;
};
struct hlist_node {

struct hlist_node *next, **pprev;

};

使用二级指针 **pprev 可以使在删除节点上的处理变的简单，无需关心是否是head节点。而单链表或双向链表使用 *prev 成员需要单独判断是否是head节点。

 

三、相关函数

1. 定义Hash表

/* Hash表的表头就是一个数组，数组中每一个原生都是一个链表 */
#define DECLARE_HASHTABLE(name, bits)    \
    struct hlist_head name[1 << (bits)]
/ 定义并初始化 /

#define DEFINE_HASHTABLE(name, bits)                        

struct hlist_head name[1 << (bits)] =                    

{ [0 ... ((1 << (bits)) - 1)] = HLIST_HEAD_INIT }

2. Hash表初始化

/* 初始化Hash表 */
#define hash_init(hashtable) __hash_init(hashtable, HASH_SIZE(hashtable))
static inline void __hash_init(struct hlist_head *ht, unsigned int sz)

{

unsigned int i;
</span><span style="color: rgba(0, 0, 255, 1)">for</span> (i = <span style="color: rgba(128, 0, 128, 1)">0</span>; i &lt; sz; i++<span style="color: rgba(0, 0, 0, 1)">)
    INIT_HLIST_HEAD(</span>&amp;<span style="color: rgba(0, 0, 0, 1)">ht[i]);

}

#define INIT_HLIST_HEAD(ptr) ((ptr)->first = NULL)

3. Hash表添加元素

/**
 * hash_add - add an object to a hashtable
 * @hashtable: hashtable to add to
 * @node: the &struct hlist_node of the object to be added
 * @key: the key of the object to be added
 */
#define hash_add(hashtable, node, key)                        \
    hlist_add_head(node, &hashtable[hash_min(key, HASH_BITS(hashtable))])
/

hash_add_rcu - add an object to a rcu enabled hashtable
@hashtable: hashtable to add to
@node: the &struct hlist_node of the object to be added
@key: the key of the object to be added

*/

#define hash_add_rcu(hashtable, node, key)                    <span style="color: rgba(0, 0, 0, 1)">

hlist_add_head_rcu(node, &hashtable[hash_min(key, HASH_BITS(hashtable))])

4. Hash表遍历

/**
 * hash_for_each - iterate over a hashtable
 * @name: hashtable to iterate
 * @bkt: integer to use as bucket loop cursor
 * @obj: the type * to use as a loop cursor for each entry
 * @member: the name of the hlist_node within the struct
 */
#define hash_for_each(name, bkt, obj, member)                \
    for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);\
            (bkt)++)\
        hlist_for_each_entry(obj, &name[bkt], member)
/

hash_for_each_rcu - iterate over a rcu enabled hashtable
@name: hashtable to iterate
@bkt: integer to use as bucket loop cursor
@obj: the type * to use as a loop cursor for each entry
@member: the name of the hlist_node within the struct

*/

#define hash_for_each_rcu(name, bkt, obj, member)            

for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);

(bkt)++)

hlist_for_each_entry_rcu(obj, &name[bkt], member)

/

hash_for_each_safe - iterate over a hashtable safe against removal of
hash entry
@name: hashtable to iterate
@bkt: integer to use as bucket loop cursor
@tmp: a &struct hlist_node used for temporary storage
@obj: the type * to use as a loop cursor for each entry
@member: the name of the hlist_node within the struct

*/

#define hash_for_each_safe(name, bkt, tmp, obj, member)            

for ((bkt) = 0, obj = NULL; obj == NULL && (bkt) < HASH_SIZE(name);

(bkt)++)

hlist_for_each_entry_safe(obj, tmp, &name[bkt], member)

/

hash_for_each_possible - iterate over all possible objects hashing to the
same bucket
@name: hashtable to iterate
@obj: the type * to use as a loop cursor for each entry
@member: the name of the hlist_node within the struct
@key: the key of the objects to iterate over

*/

#define hash_for_each_possible(name, obj, member, key)            <span style="color: rgba(0, 0, 0, 1)">

hlist_for_each_entry(obj, &name[hash_min(key, HASH_BITS(name))], member)

/

hash_for_each_possible_rcu - iterate over all possible objects hashing to the
same bucket in an rcu enabled hashtable
@name: hashtable to iterate
@obj: the type * to use as a loop cursor for each entry
@member: the name of the hlist_node within the struct
@key: the key of the objects to iterate over

*/

#define hash_for_each_possible_rcu(name, obj, member, key, cond...)    <span style="color: rgba(0, 0, 0, 1)">

hlist_for_each_entry_rcu(obj, &name[hash_min(key, HASH_BITS(name))],

member, ## cond)

/

hash_for_each_possible_rcu_notrace - iterate over all possible objects hashing
to the same bucket in an rcu enabled hashtable in a rcu enabled hashtable
@name: hashtable to iterate
@obj: the type * to use as a loop cursor for each entry
@member: the name of the hlist_node within the struct
@key: the key of the objects to iterate over

This is the same as hash_for_each_possible_rcu() except that it does
not do any RCU debugging or tracing.

*/

#define hash_for_each_possible_rcu_notrace(name, obj, member, key) <span style="color: rgba(0, 0, 0, 1)">

hlist_for_each_entry_rcu_notrace(obj, 

&name[hash_min(key, HASH_BITS(name))], member)

/

hash_for_each_possible_safe - iterate over all possible objects hashing to the
same bucket safe against removals
@name: hashtable to iterate
@obj: the type * to use as a loop cursor for each entry
@tmp: a &struct hlist_node used for temporary storage
@member: the name of the hlist_node within the struct
@key: the key of the objects to iterate over

*/

#define hash_for_each_possible_safe(name, obj, tmp, member, key)    <span style="color: rgba(0, 0, 0, 1)">

hlist_for_each_entry_safe(obj, tmp,

&name[hash_min(key, HASH_BITS(name))], member)

5. Hash表元素删除

/**
 * hash_del - remove an object from a hashtable
 * @node: &struct hlist_node of the object to remove
 */
static inline void hash_del(struct hlist_node *node)
{
    hlist_del_init(node);
}
/

hash_del_rcu - remove an object from a rcu enabled hashtable
@node: &struct hlist_node of the object to remove

*/

static inline void hash_del_rcu(struct hlist_node *node)

{

hlist_del_init_rcu(node);

}

注：更多API见 hashtable.h。

 

四、使用举例

1. 使用hash表缓存内核符号

#define pr_fmt(fmt) "Hash Table Debug: " fmt
include <linux/kernel.h>
include <linux/module.h>
include <linux/errno.h>
include <linux/init.h>
include <linux/kallsyms.h>
include <linux/hashtable.h>
#define HASH_TABLE_BIT 5

#define HASH_TABLE_LEN (1 << HASH_TABLE_BIT)

#define HASH_KEY_MASK ((1 << HASH_TABLE_BIT) - 1)
struct hash_node {

unsigned long addr;

char symbol[KSYM_SYMBOL_LEN];

struct hlist_node node;

};
struct hash_test_desc {

struct hlist_head htbl[HASH_TABLE_LEN];

};
static struct hash_test_desc *htd = NULL;
static const char *find_and_cache_symbol(unsigned long caller_addr)

{

struct hash_node *cur_node = NULL;

struct hash_node *new_node = NULL;

const char *cur_symbol = NULL;

unsigned int cur_key = 0;
</span><span style="color: rgba(0, 0, 255, 1)">if</span> (!<span style="color: rgba(0, 0, 0, 1)">htd)
    </span><span style="color: rgba(0, 0, 255, 1)">return</span> <span style="color: rgba(128, 0, 0, 1)">"</span><span style="color: rgba(128, 0, 0, 1)">NULL</span><span style="color: rgba(128, 0, 0, 1)">"</span><span style="color: rgba(0, 0, 0, 1)">;

cur_key </span>= (unsigned <span style="color: rgba(0, 0, 255, 1)">int</span>) caller_addr &amp;<span style="color: rgba(0, 0, 0, 1)"> HASH_KEY_MASK;

</span><span style="color: rgba(0, 128, 0, 1)">/*</span><span style="color: rgba(0, 128, 0, 1)"> try to find symbols from history records </span><span style="color: rgba(0, 128, 0, 1)">*/</span><span style="color: rgba(0, 0, 0, 1)">
hash_for_each_possible(htd</span>-&gt;<span style="color: rgba(0, 0, 0, 1)">htbl, cur_node, node, cur_key) {
    </span><span style="color: rgba(0, 0, 255, 1)">if</span> (cur_node-&gt;addr ==<span style="color: rgba(0, 0, 0, 1)"> caller_addr) {
        cur_symbol </span>= cur_node-&gt;<span style="color: rgba(0, 0, 0, 1)">symbol;
        </span><span style="color: rgba(0, 0, 255, 1)">break</span><span style="color: rgba(0, 0, 0, 1)">;
    }
}

</span><span style="color: rgba(0, 128, 0, 1)">/*</span><span style="color: rgba(0, 128, 0, 1)"> symbol not found, add new a record </span><span style="color: rgba(0, 128, 0, 1)">*/</span>
<span style="color: rgba(0, 0, 255, 1)">if</span> (!<span style="color: rgba(0, 0, 0, 1)">cur_symbol) {
    new_node </span>= kzalloc(<span style="color: rgba(0, 0, 255, 1)">sizeof</span>(<span style="color: rgba(0, 0, 255, 1)">struct</span><span style="color: rgba(0, 0, 0, 1)"> hash_node), GFP_ATOMIC);
    </span><span style="color: rgba(0, 0, 255, 1)">if</span> (!<span style="color: rgba(0, 0, 0, 1)">new_node)
        </span><span style="color: rgba(0, 0, 255, 1)">return</span> <span style="color: rgba(128, 0, 0, 1)">"</span><span style="color: rgba(128, 0, 0, 1)">NULL-no_mem</span><span style="color: rgba(128, 0, 0, 1)">"</span><span style="color: rgba(0, 0, 0, 1)">;
    new_node</span>-&gt;addr =<span style="color: rgba(0, 0, 0, 1)"> caller_addr;
    sprint_symbol(new_node</span>-&gt;<span style="color: rgba(0, 0, 0, 1)">symbol, caller_addr);
    cur_symbol </span>= new_node-&gt;<span style="color: rgba(0, 0, 0, 1)">symbol;
    hash_add(htd</span>-&gt;htbl, &amp;new_node-&gt;<span style="color: rgba(0, 0, 0, 1)">node, cur_key);
}

</span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)"> cur_symbol;

}
static void remove_hash_table(struct hash_test_desc *ld)

{

int bkt = 0;

struct hash_node *cur = NULL;

struct hlist_node *tmp = NULL;

struct hlist_head *htbl = NULL;
</span><span style="color: rgba(0, 0, 255, 1)">if</span> (!<span style="color: rgba(0, 0, 0, 1)">ld)
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)">;

hash_for_each_safe(ld</span>-&gt;<span style="color: rgba(0, 0, 0, 1)">htbl, bkt, tmp, cur, node) {
    hash_del(</span>&amp;cur-&gt;<span style="color: rgba(0, 0, 0, 1)">node);
    kfree(cur);
}

}
static void hash_table_hook_func(void)

{

void *location;
</span><span style="color: rgba(0, 0, 255, 1)">if</span> (!<span style="color: rgba(0, 0, 0, 1)">htd)
    </span><span style="color: rgba(0, 0, 255, 1)">return</span><span style="color: rgba(0, 0, 0, 1)">;

location  </span>= __builtin_return_address(<span style="color: rgba(128, 0, 128, 1)">1</span><span style="color: rgba(0, 0, 0, 1)">);
pr_info(</span><span style="color: rgba(128, 0, 0, 1)">"</span><span style="color: rgba(128, 0, 0, 1)">caller=%s\n</span><span style="color: rgba(128, 0, 0, 1)">"</span>, find_and_cache_symbol((unsigned <span style="color: rgba(0, 0, 255, 1)">long</span><span style="color: rgba(0, 0, 0, 1)">)location));

}

EXPORT_SYMBOL(hash_table_hook_func);
static int __init hash_table_debug_init(void)

{

struct hash_test_desc *ld = kzalloc(sizeof(struct hash_test_desc), GFP_KERNEL);

if (!ld) {

pr_info("kzalloc failed!\n");

return -ENOMEM;

}

htd = ld;
</span><span style="color: rgba(0, 0, 255, 1)">return</span> <span style="color: rgba(128, 0, 128, 1)">0</span><span style="color: rgba(0, 0, 0, 1)">;

}
static void __exit hash_table_debug_exit(void)

{

struct hash_test_desc *ld = htd;

htd = NULL;

remove_hash_table(ld);

kfree(ld);

}
module_init(hash_table_debug_init);

module_exit(hash_table_debug_exit);
MODULE_DESCRIPTION("Hash Table Debug");

MODULE_LICENSE("GPL v2");

 

 

 

参考：https://zhuanlan.zhihu.com/p/360217911

 

分类: 数据结构

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