linux内核数据结构之kfifo（实现）

转载地址：http://www.cnblogs.com/Anker/p/3481373.html


1、前言

　　最近项目中用到一个环形缓冲区（ring buffer），代码是由linux内核的kfifo改过来的。缓冲区在文件系统中经常用到，通过缓冲区缓解cpu读写内存和读写磁盘的速度。例如一个进程A产生数据发给另外一个进程B，进程B需要对进程A传的数据进行处理并写入文件，如果B没有处理完，则A要延迟发送。为了保证进程A减少等待时间，可以在A和B之间采用一个缓冲区，A每次将数据存放在缓冲区中，B每次冲缓冲区中取。这是典型的生产者和消费者模型，缓冲区中数据满足FIFO特性，因此可以采用队列进行实现。Linux内核的kfifo正好是一个环形队列，可以用来当作环形缓冲区。生产者与消费者使用缓冲区如下图所示：



　　环形缓冲区的详细介绍及实现方法可以参考http://en.wikipedia.org/wiki/Circular_buffer，介绍的非常详细，列举了实现环形队列的几种方法。环形队列的不便之处在于如何判断队列是空还是满。维基百科上给三种实现方法。

2、linux 内核kfifo

　　kfifo设计的非常巧妙，代码很精简，对于入队和出对处理的出人意料。首先看一下kfifo的数据结构：

struct kfifo {
unsigned char *buffer;     /* the buffer holding the data */
unsigned int size;         /* the size of the allocated buffer */
unsigned int in;           /* data is added at offset (in % size) */
unsigned int out;          /* data is extracted from off. (out % size) */
spinlock_t *lock;          /* protects concurrent modifications */
};

kfifo提供的方法有：

1 //根据给定buffer创建一个kfifo
2 struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,
3                 gfp_t gfp_mask, spinlock_t *lock);
4 //给定size分配buffer和kfifo
5 struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask,
6                  spinlock_t *lock);
7 //释放kfifo空间
8 void kfifo_free(struct kfifo *fifo)
9 //向kfifo中添加数据
10 unsigned int kfifo_put(struct kfifo *fifo,
11                 const unsigned char *buffer, unsigned int len)
12 //从kfifo中取数据
13 unsigned int kfifo_put(struct kfifo *fifo,
14                 const unsigned char *buffer, unsigned int len)
15 //获取kfifo中有数据的buffer大小
16 unsigned int kfifo_len(struct kfifo *fifo)

       定义自旋锁的目的为了防止多进程/线程并发使用kfifo。因为in和out在每次get和out时，发生改变。初始化和创建kfifo的源代码如下：

1 struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,
2              gfp_t gfp_mask, spinlock_t *lock)
3 {
4     struct kfifo *fifo;
6     /* size must be a power of 2 */
7     BUG_ON(!is_power_of_2(size));
9     fifo = kmalloc(sizeof(struct kfifo), gfp_mask);
10     if (!fifo)
11         return ERR_PTR(-ENOMEM);
13     fifo->buffer = buffer;
14     fifo->size = size;
15     fifo->in = fifo->out = 0;
16     fifo->lock = lock;
17
18     return fifo;
19 }
20 struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock)
21 {
22     unsigned char *buffer;
23     struct kfifo *ret;
29     if (!is_power_of_2(size)) {
30         BUG_ON(size > 0x80000000);
31         size = roundup_pow_of_two(size);
32     }
34     buffer = kmalloc(size, gfp_mask);
35     if (!buffer)
36         return ERR_PTR(-ENOMEM);
38     ret = kfifo_init(buffer, size, gfp_mask, lock);
39
40     if (IS_ERR(ret))
41         kfree(buffer);
43     return ret;
44 }

　　在kfifo_init和kfifo_calloc中，kfifo->size的值总是在调用者传进来的size参数的基础上向2的幂扩展，这是内核一贯的做法。这样的好处不言而喻--对kfifo->size取模运算可以转化为与运算，如：kfifo->in % kfifo->size 可以转化为 kfifo->in & (kfifo->size – 1)

      kfifo的巧妙之处在于in和out定义为无符号类型，在put和get时，in和out都是增加，当达到最大值时，产生溢出，使得从0开始，进行循环使用。put和get代码如下所示：

1 static inline unsigned int kfifo_put(struct kfifo *fifo,
2                 const unsigned char *buffer, unsigned int len)
3 {
4     unsigned long flags;
5     unsigned int ret;
6     spin_lock_irqsave(fifo->lock, flags);
7     ret = __kfifo_put(fifo, buffer, len);
8     spin_unlock_irqrestore(fifo->lock, flags);
9     return ret;
10 }
11
12 static inline unsigned int kfifo_get(struct kfifo *fifo,
13                      unsigned char *buffer, unsigned int len)
14 {
15     unsigned long flags;
16     unsigned int ret;
17     spin_lock_irqsave(fifo->lock, flags);
18     ret = __kfifo_get(fifo, buffer, len);
19         //当fifo->in == fifo->out时，buufer为空
20     if (fifo->in == fifo->out)
21         fifo->in = fifo->out = 0;
22     spin_unlock_irqrestore(fifo->lock, flags);
23     return ret;
24 }
25
26
27 unsigned int __kfifo_put(struct kfifo *fifo,
28             const unsigned char *buffer, unsigned int len)
29 {
30     unsigned int l;
31        //buffer中空的长度
32     len = min(len, fifo->size - fifo->in + fifo->out);
34     /*
35      * Ensure that we sample the fifo->out index -before- we
36      * start putting bytes into the kfifo.
37      */
39     smp_mb();
41     /* first put the data starting from fifo->in to buffer end */
42     l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));
43     memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l);
45     /* then put the rest (if any) at the beginning of the buffer */
46     memcpy(fifo->buffer, buffer + l, len - l);
47
48     /*
49      * Ensure that we add the bytes to the kfifo -before-
50      * we update the fifo->in index.
51      */
53     smp_wmb();
55     fifo->in += len;  //每次累加，到达最大值后溢出，自动转为0
57     return len;
58 }
59
60 unsigned int __kfifo_get(struct kfifo *fifo,
61              unsigned char *buffer, unsigned int len)
62 {
63     unsigned int l;
64         //有数据的缓冲区的长度
65     len = min(len, fifo->in - fifo->out);
67     /*
68      * Ensure that we sample the fifo->in index -before- we
69      * start removing bytes from the kfifo.
70      */
72     smp_rmb();
74     /* first get the data from fifo->out until the end of the buffer */
75     l = min(len, fifo->size - (fifo->out & (fifo->size - 1)));
76     memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l);
78     /* then get the rest (if any) from the beginning of the buffer */
79     memcpy(buffer + l, fifo->buffer, len - l);
81     /*
82      * Ensure that we remove the bytes from the kfifo -before-
83      * we update the fifo->out index.
84      */
86     smp_mb();
88     fifo->out += len; //每次累加，到达最大值后溢出，自动转为0
90     return len;
91 }

　　put和get在调用__put和__get过程都进行加锁，防止并发。从代码中可以看出put和get都调用两次memcpy，这针对的是边界条件。例如下图：蓝色表示空闲，红色表示占用。

（1）空的kfifo，



（2）put一个buffer后



（3）get一个buffer后



（4）当此时put的buffer长度超出in到末尾长度时，则将剩下的移到头部去



3、测试程序

 仿照kfifo编写一个ring_buffer，现有线程互斥量进行并发控制。设计的ring_buffer如下所示：

1 /**@brief 仿照linux kfifo写的ring buffer
2  *@atuher Anker  date:2013-12-18
3 * ring_buffer.h
4  * */
5
6 #ifndef KFIFO_HEADER_H
7 #define KFIFO_HEADER_H
8
9 #include <inttypes.h>
10 #include <string.h>
11 #include <stdlib.h>
12 #include <stdio.h>
13 #include <errno.h>
14 #include <assert.h>
15
16 //判断x是否是2的次方
17 #define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))
18 //取a和b中最小值
19 #define min(a, b) (((a) < (b)) ? (a) : (b))
20
21 struct ring_buffer
22 {
23     void         *buffer;     //缓冲区
24     uint32_t     size;       //大小
25     uint32_t     in;         //入口位置
26     uint32_t       out;        //出口位置
27     pthread_mutex_t *f_lock;    //互斥锁
28 };
29 //初始化缓冲区
30 struct ring_buffer* ring_buffer_init(void *buffer, uint32_t size, pthread_mutex_t *f_lock)
31 {
32     assert(buffer);
33     struct ring_buffer *ring_buf = NULL;
34     if (!is_power_of_2(size))
35     {
36     fprintf(stderr,"size must be power of 2.\n");
37         return ring_buf;
38     }
39     ring_buf = (struct ring_buffer *)malloc(sizeof(struct ring_buffer));
40     if (!ring_buf)
41     {
42         fprintf(stderr,"Failed to malloc memory,errno:%u,reason:%s",
43             errno, strerror(errno));
44         return ring_buf;
45     }
46     memset(ring_buf, 0, sizeof(struct ring_buffer));
47     ring_buf->buffer = buffer;
48     ring_buf->size = size;
49     ring_buf->in = 0;
50     ring_buf->out = 0;
51         ring_buf->f_lock = f_lock;
52     return ring_buf;
53 }
54 //释放缓冲区
55 void ring_buffer_free(struct ring_buffer *ring_buf)
56 {
57     if (ring_buf)
58     {
59     if (ring_buf->buffer)
60     {
61         free(ring_buf->buffer);
62         ring_buf->buffer = NULL;
63     }
64     free(ring_buf);
65     ring_buf = NULL;
66     }
67 }
68
69 //缓冲区的长度
70 uint32_t __ring_buffer_len(const struct ring_buffer *ring_buf)
71 {
72     return (ring_buf->in - ring_buf->out);
73 }
74
75 //从缓冲区中取数据
76 uint32_t __ring_buffer_get(struct ring_buffer *ring_buf, void * buffer, uint32_t size)
77 {
78     assert(ring_buf || buffer);
79     uint32_t len = 0;
80     size  = min(size, ring_buf->in - ring_buf->out);
81     /* first get the data from fifo->out until the end of the buffer */
82     len = min(size, ring_buf->size - (ring_buf->out & (ring_buf->size - 1)));
83     memcpy(buffer, ring_buf->buffer + (ring_buf->out & (ring_buf->size - 1)), len);
84     /* then get the rest (if any) from the beginning of the buffer */
85     memcpy(buffer + len, ring_buf->buffer, size - len);
86     ring_buf->out += size;
87     return size;
88 }
89 //向缓冲区中存放数据
90 uint32_t __ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size)
91 {
92     assert(ring_buf || buffer);
93     uint32_t len = 0;
94     size = min(size, ring_buf->size - ring_buf->in + ring_buf->out);
95     /* first put the data starting from fifo->in to buffer end */
96     len  = min(size, ring_buf->size - (ring_buf->in & (ring_buf->size - 1)));
97     memcpy(ring_buf->buffer + (ring_buf->in & (ring_buf->size - 1)), buffer, len);
98     /* then put the rest (if any) at the beginning of the buffer */
99     memcpy(ring_buf->buffer, buffer + len, size - len);
100     ring_buf->in += size;
101     return size;
102 }
103
104 uint32_t ring_buffer_len(const struct ring_buffer *ring_buf)
105 {
106     uint32_t len = 0;
107     pthread_mutex_lock(ring_buf->f_lock);
108     len = __ring_buffer_len(ring_buf);
109     pthread_mutex_unlock(ring_buf->f_lock);
110     return len;
111 }
112
113 uint32_t ring_buffer_get(struct ring_buffer *ring_buf, void *buffer, uint32_t size)
114 {
115     uint32_t ret;
116     pthread_mutex_lock(ring_buf->f_lock);
117     ret = __ring_buffer_get(ring_buf, buffer, size);
118     //buffer中没有数据
119     if (ring_buf->in == ring_buf->out)
120     ring_buf->in = ring_buf->out = 0;
121     pthread_mutex_unlock(ring_buf->f_lock);
122     return ret;
123 }
124
125 uint32_t ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size)
126 {
127     uint32_t ret;
128     pthread_mutex_lock(ring_buf->f_lock);
129     ret = __ring_buffer_put(ring_buf, buffer, size);
130     pthread_mutex_unlock(ring_buf->f_lock);
131     return ret;
132 }
133 #endif

采用多线程模拟生产者和消费者编写测试程序，如下所示：

1 /**@brief ring buffer测试程序，创建两个线程，一个生产者，一个消费者。
2  * 生产者每隔1秒向buffer中投入数据，消费者每隔2秒去取数据。
3  *@atuher Anker  date:2013-12-18
4  * */
5 #include "ring_buffer.h"
6 #include <pthread.h>
7 #include <time.h>
8
9 #define BUFFER_SIZE  1024 * 1024
10
11 typedef struct student_info
12 {
13     uint64_t stu_id;
14     uint32_t age;
15     uint32_t score;
16 }student_info;
17
18
19 void print_student_info(const student_info *stu_info)
20 {
21     assert(stu_info);
22     printf("id:%lu\t",stu_info->stu_id);
23     printf("age:%u\t",stu_info->age);
24     printf("score:%u\n",stu_info->score);
25 }
26
27 student_info * get_student_info(time_t timer)
28 {
29     student_info *stu_info = (student_info *)malloc(sizeof(student_info));
30     if (!stu_info)
31     {
32     fprintf(stderr, "Failed to malloc memory.\n");
33     return NULL;
34     }
35     srand(timer);
36     stu_info->stu_id = 10000 + rand() % 9999;
37     stu_info->age = rand() % 30;
38     stu_info->score = rand() % 101;
39     print_student_info(stu_info);
40     return stu_info;
41 }
42
43 void * consumer_proc(void *arg)
44 {
45     struct ring_buffer *ring_buf = (struct ring_buffer *)arg;
46     student_info stu_info;
47     while(1)
48     {
49     sleep(2);
50     printf("------------------------------------------\n");
51     printf("get a student info from ring buffer.\n");
52     ring_buffer_get(ring_buf, (void *)&stu_info, sizeof(student_info));
53     printf("ring buffer length: %u\n", ring_buffer_len(ring_buf));
54     print_student_info(&stu_info);
55     printf("------------------------------------------\n");
56     }
57     return (void *)ring_buf;
58 }
59
60 void * producer_proc(void *arg)
61 {
62     time_t cur_time;
63     struct ring_buffer *ring_buf = (struct ring_buffer *)arg;
64     while(1)
65     {
66     time(&cur_time);
67     srand(cur_time);
68     int seed = rand() % 11111;
69     printf("******************************************\n");
70     student_info *stu_info = get_student_info(cur_time + seed);
71     printf("put a student info to ring buffer.\n");
72     ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
73     printf("ring buffer length: %u\n", ring_buffer_len(ring_buf));
74     printf("******************************************\n");
75     sleep(1);
76     }
77     return (void *)ring_buf;
78 }
79
80 int consumer_thread(void *arg)
81 {
82     int err;
83     pthread_t tid;
84     err = pthread_create(&tid, NULL, consumer_proc, arg);
85     if (err != 0)
86     {
87     fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",
88         errno, strerror(errno));
89     return -1;
90     }
91     return tid;
92 }
93 int producer_thread(void *arg)
94 {
95     int err;
96     pthread_t tid;
97     err = pthread_create(&tid, NULL, producer_proc, arg);
98     if (err != 0)
99     {
100     fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",
101         errno, strerror(errno));
102     return -1;
103     }
104     return tid;
105 }
106
107
108 int main()
109 {
110     void * buffer = NULL;
111     uint32_t size = 0;
112     struct ring_buffer *ring_buf = NULL;
113     pthread_t consume_pid, produce_pid;
114
115     pthread_mutex_t *f_lock = (pthread_mutex_t *)malloc(sizeof(pthread_mutex_t));
116     if (pthread_mutex_init(f_lock, NULL) != 0)
117     {
118     fprintf(stderr, "Failed init mutex,errno:%u,reason:%s\n",
119         errno, strerror(errno));
120     return -1;
121     }
122     buffer = (void *)malloc(BUFFER_SIZE);
123     if (!buffer)
124     {
125     fprintf(stderr, "Failed to malloc memory.\n");
126     return -1;
127     }
128     size = BUFFER_SIZE;
129     ring_buf = ring_buffer_init(buffer, size, f_lock);
130     if (!ring_buf)
131     {
132     fprintf(stderr, "Failed to init ring buffer.\n");
133     return -1;
134     }
135 #if 0
136     student_info *stu_info = get_student_info(638946124);
137     ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
138     stu_info = get_student_info(976686464);
139     ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
140     ring_buffer_get(ring_buf, (void *)stu_info, sizeof(student_info));
141     print_student_info(stu_info);
142 #endif
143     printf("multi thread test.......\n");
144     produce_pid  = producer_thread((void*)ring_buf);
145     consume_pid  = consumer_thread((void*)ring_buf);
146     pthread_join(produce_pid, NULL);
147     pthread_join(consume_pid, NULL);
148     ring_buffer_free(ring_buf);
149     free(f_lock);
150     return 0;
151 }

测试结果如下所示：



4、参考资料

http://blog.csdn.net/linyt/article/details/5764312

http://en.wikipedia.org/wiki/Circular_buffer

http://yiphon.diandian.com/post/2011-09-10/4918347