【linux】Linux内核结构体--kfifo 环状缓冲区

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：

1 struct kfifo {
2     unsigned char *buffer;     /* the buffer holding the data */
3     unsigned int size;         /* the size of the allocated buffer */
4     unsigned int in;           /* data is added at offset (in % size) */
5     unsigned int out;          /* data is extracted from off. (out % size) */
6     spinlock_t *lock;          /* protects concurrent modifications */
7 };

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, gfp_t gfp_mask, spinlock_t *lock);
 3 //给定size分配buffer和kfifo
 4 struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock);
 5 //释放kfifo空间
 6 void kfifo_free(struct kfifo *fifo)
 7 //向kfifo中添加数据
 8 unsigned int kfifo_put(struct kfifo *fifo, const unsigned char *buffer, unsigned int len)
 9 //从kfifo中取数据
10 unsigned int kfifo_get(struct kfifo *fifo, const unsigned char *buffer, unsigned int len)
11 //获取kfifo中有数据的buffer大小
12 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, gfp_t gfp_mask, spinlock_t *lock)
 2 {
 3     struct kfifo *fifo;
 4     /* size must be a power of 2 */
 5     BUG_ON(!is_power_of_2(size));
 6     fifo = kmalloc(sizeof(struct kfifo), gfp_mask);
 7     if (!fifo)
 8         return ERR_PTR(-ENOMEM);
 9     fifo->buffer = buffer;
10     fifo->size = size;
11     fifo->in = fifo->out = 0;
12     fifo->lock = lock;
13
14     return fifo;
15 }
16 struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock)
17 {
18     unsigned char *buffer;
19     struct kfifo *ret;
20     if (!is_power_of_2(size)) {
21         BUG_ON(size > 0x80000000);
22         size = roundup_pow_of_two(size);
23     }
24     buffer = kmalloc(size, gfp_mask);
25     if (!buffer)
26         return ERR_PTR(-ENOMEM);
27     ret = kfifo_init(buffer, size, gfp_mask, lock);
28
29     if (IS_ERR(ret))
30         kfree(buffer);
31     return ret;
32 }

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, const unsigned char *buffer, unsigned int len)
 2 {
 3     unsigned long flags;
 4     unsigned int ret;
 5     spin_lock_irqsave(fifo->lock, flags);
 6     ret = __kfifo_put(fifo, buffer, len);
 7     spin_unlock_irqrestore(fifo->lock, flags);
 8     return ret;
 9 }
10
11 static inline unsigned int kfifo_get(struct kfifo *fifo, unsigned char *buffer, unsigned int len)
12 {
13     unsigned long flags;
14     unsigned int ret;
15     spin_lock_irqsave(fifo->lock, flags);
16     ret = __kfifo_get(fifo, buffer, len);
17         //当fifo->in == fifo->out时，buufer为空
18     if (fifo->in == fifo->out)
19         fifo->in = fifo->out = 0;
20     spin_unlock_irqrestore(fifo->lock, flags);
21     return ret;
22 }
23
24
25 unsigned int __kfifo_put(struct kfifo *fifo, const unsigned char *buffer, unsigned int len)
26 {
27     unsigned int l;
28        //buffer中空的长度
29     len = min(len, fifo->size - fifo->in + fifo->out);
30     /*
31      * Ensure that we sample the fifo->out index -before- we
32      * start putting bytes into the kfifo.
33      */
34     smp_mb();
35     /* first put the data starting from fifo->in to buffer end */
36     l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));
37     memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l);
38     /* then put the rest (if any) at the beginning of the buffer */
39     memcpy(fifo->buffer, buffer + l, len - l);
40
41     /*
42      * Ensure that we add the bytes to the kfifo -before-
43      * we update the fifo->in index.
44      */
45     smp_wmb();
46     fifo->in += len;  //每次累加，到达最大值后溢出，自动转为0
47     return len;
48 }
49
50 unsigned int __kfifo_get(struct kfifo *fifo, unsigned char *buffer, unsigned int len)
51 {
52     unsigned int l;
53         //有数据的缓冲区的长度
54     len = min(len, fifo->in - fifo->out);
55     /*
56      * Ensure that we sample the fifo->in index -before- we
57      * start removing bytes from the kfifo.
58      */
59     smp_rmb();
60     /* first get the data from fifo->out until the end of the buffer */
61     l = min(len, fifo->size - (fifo->out & (fifo->size - 1)));
62     memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l);
63     /* then get the rest (if any) from the beginning of the buffer */
64     memcpy(buffer + l, fifo->buffer, len - l);
65     /*
66      * Ensure that we remove the bytes from the kfifo -before-
67      * we update the fifo->out index.
68      */
69     smp_mb();
70     fifo->out += len; //每次累加，到达最大值后溢出，自动转为0
71     return len;
72 }

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数据到buffer后

（3）从buffer中get数据后

（4）当此时put到buffer中的数据长度超出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 }

总结：
len = min(len, fifo->size - fifo->in + fifo->out)
      在 len 和 (fifo->size - fifo->in + fifo->out) 之间取一个较小的值赋给len。注意，当 (fifo->in == fifo->out+fifo->size) 时，表示缓冲区已满，此时得到的较小值一定是0，后面实际写入的字节数也全为0。
      另一种边界情况是当 len 很大时（因为len是无符号的，负数对它来说也是一个很大的正数），这一句也能保证len取到一个较小的值，因为    fifo->in 总是大于等于 fifo->out ，所以后面的那个表达式     l = min(len, fifo->size - (fifo->in & (fifo->size - 1))); 的值不会超过fifo->size的大小。
      smp_mb();  smp_wmb(); 是加内存屏障，这里不是我们讨论的范围，你可以忽略它。
      l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));  是把上一步决定的要写入的字节数len “切开”，这里又使用了一个技巧。注意：实际分配给 fifo->buffer 的字节数 fifo->size，必须是2的幂，否则这里就会出错。既然 fifo->size 是2的幂，那么 (fifo->size-1) 也就是一个后面几位全为1的数，也就能保证(fifo->in & (fifo->size - 1)) 总为不超过 (fifo->size - 1) 的那一部分，和 (fifo->in)% (fifo->size - 1) 的效果一样。
      这样后面的代码就不难理解了，它先向 fifo->in 到缓冲区末端这一块写数据，如果还没写完，在从缓冲区头开始写入剩下的，从而实现了循环缓冲。最后，把写指针后移 len 个字节，并返回len。
      从上面可以看出，fifo->in的值可以从0变化到超过fifo->size的数值，fifo->out也如此，但它们的差不会超过fifo->size。

【来源】 https://blog.csdn.net/yusiguyuan/article/details/41985907