linux内核数据结构之kfifo

Posted 2022-02-05 一口Linux

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：