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：

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：

//根据给定buffer创建一个kfifo



struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,



                gfp_t gfp_mask, spinlock_t *lock);



//给定size分配buffer和kfifo



struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask,



                 spinlock_t *lock);



//释放kfifo空间



void kfifo_free(struct kfifo *fifo)



//向kfifo中添加数据




unsigned int kfifo_put(struct kfifo *fifo,




                const unsigned char *buffer, unsigned int len)



//从kfifo中取数据




unsigned int kfifo_put(struct kfifo *fifo,




                const unsigned char *buffer, unsigned int len)



//获取kfifo中有数据的buffer大小




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：

struct kfifo *kfifo_init(unsigned char *buffer, unsigned int size,



             gfp_t gfp_mask, spinlock_t *lock)



{



    struct kfifo *fifo;




    /* size must be a power of 2 */




    BUG_ON(!is_power_of_2(size));



    fifo = kmalloc(sizeof(struct kfifo), gfp_mask);



    if (!fifo)



        return ERR_PTR(-ENOMEM);



    fifo->buffer = buffer;



    fifo->size = size;



    fifo->in = fifo->out = 0;



    fifo->lock = lock;



 



    return fifo;



}



struct kfifo *kfifo_alloc(unsigned int size, gfp_t gfp_mask, spinlock_t *lock)



{



    unsigned char *buffer;



    struct kfifo *ret;




    if (!is_power_of_2(size)) {



        BUG_ON(size > 0x80000000);



        size = roundup_pow_of_two(size);



    }



    buffer = kmalloc(size, gfp_mask);



    if (!buffer)



        return ERR_PTR(-ENOMEM);



    ret = kfifo_init(buffer, size, gfp_mask, lock);



 



    if (IS_ERR(ret))



        kfree(buffer);



    return ret;



}

 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：

static inline unsigned int kfifo_put(struct kfifo *fifo,



                const unsigned char *buffer, unsigned int len)



{



    unsigned long flags;



    unsigned int ret;



    spin_lock_irqsave(fifo->lock, flags);



    ret = __kfifo_put(fifo, buffer, len);



    spin_unlock_irqrestore(fifo->lock, flags);



    return ret;



}



 



static inline unsigned int kfifo_get(struct kfifo *fifo,



                     unsigned char *buffer, unsigned int len)



{



    unsigned long flags;



    unsigned int ret;



    spin_lock_irqsave(fifo->lock, flags);



    ret = __kfifo_get(fifo, buffer, len);



        //当fifo->in == fifo->out时，buufer为空




    if (fifo->in == fifo->out)



        fifo->in = fifo->out = 0;



    spin_unlock_irqrestore(fifo->lock, flags);



    return ret;



}



 



 




unsigned int __kfifo_put(struct kfifo *fifo,



            const unsigned char *buffer, unsigned int len)



{



    unsigned int l;



       //buffer中空的长度




    len = min(len, fifo->size - fifo->in + fifo->out);



    /*




     * Ensure that we sample the fifo->out index -before- we



     * start putting bytes into the kfifo.



     */



    smp_mb();



    /* first put the data starting from fifo->in to buffer end */




    l = min(len, fifo->size - (fifo->in & (fifo->size - 1)));



    memcpy(fifo->buffer + (fifo->in & (fifo->size - 1)), buffer, l);



    /* then put the rest (if any) at the beginning of the buffer */




    memcpy(fifo->buffer, buffer + l, len - l);



 



    /*




     * Ensure that we add the bytes to the kfifo -before-



     * we update the fifo->in index.



     */



    smp_wmb();



    fifo->in += len;  //每次累加，到达最大值后溢出，自动转为0




    return len;



}



 




unsigned int __kfifo_get(struct kfifo *fifo,



             unsigned char *buffer, unsigned int len)



{



    unsigned int l;



        //有数据的缓冲区的长度




    len = min(len, fifo->in - fifo->out);



    /*




     * Ensure that we sample the fifo->in index -before- we



     * start removing bytes from the kfifo.



     */



    smp_rmb();



    /* first get the data from fifo->out until the end of the buffer */




    l = min(len, fifo->size - (fifo->out & (fifo->size - 1)));



    memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - 1)), l);



    /* then get the rest (if any) from the beginning of the buffer */




    memcpy(buffer + l, fifo->buffer, len - l);



    /*




     * Ensure that we remove the bytes from the kfifo -before-



     * we update the fifo->out index.



     */



    smp_mb();



    fifo->out += len; //每次累加，到达最大值后溢出，自动转为0




    return len;



}

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：

/**@brief 仿照linux kfifo写的ring buffer



 *@atuher Anker  date:2013-12-18



* ring_buffer.h



 * */



 



#ifndef KFIFO_HEADER_H 



#define KFIFO_HEADER_H



 



#include <inttypes.h>



#include <string.h>



#include <stdlib.h>



#include <stdio.h>



#include <errno.h>



#include <assert.h>



 



//判断x是否是2的次方



#define is_power_of_2(x) ((x) != 0 && (((x) & ((x) - 1)) == 0))



//取a和b中最小值



#define min(a, b) (((a) < (b)) ? (a) : (b))



 



struct ring_buffer



{



    void         *buffer;     //缓冲区




    uint32_t     size;       //大小




    uint32_t     in;         //入口位置




    uint32_t       out;        //出口位置




    pthread_mutex_t *f_lock;    //互斥锁




};



//初始化缓冲区



struct ring_buffer* ring_buffer_init(void *buffer, uint32_t size, pthread_mutex_t *f_lock)



{



    assert(buffer);



    struct ring_buffer *ring_buf = NULL;




    if (!is_power_of_2(size))



    {



    fprintf(stderr,"size must be power of 2.\n");



        return ring_buf;



    }



    ring_buf = (struct ring_buffer *)malloc(sizeof(struct ring_buffer));



    if (!ring_buf)



    {



        fprintf(stderr,"Failed to malloc memory,errno:%u,reason:%s",



            errno, strerror(errno));



        return ring_buf;



    }



    memset(ring_buf, 0, sizeof(struct ring_buffer));



    ring_buf->buffer = buffer;



    ring_buf->size = size;



    ring_buf->in = 0;



    ring_buf->out = 0;



        ring_buf->f_lock = f_lock;



    return ring_buf;



}



//释放缓冲区



void ring_buffer_free(struct ring_buffer *ring_buf)



{



    if (ring_buf)



    {



    if (ring_buf->buffer)



    {



        free(ring_buf->buffer);



        ring_buf->buffer = NULL;



    }



    free(ring_buf);



    ring_buf = NULL;



    }



}



 



//缓冲区的长度




uint32_t __ring_buffer_len(const struct ring_buffer *ring_buf)



{



    return (ring_buf->in - ring_buf->out);



}



 



//从缓冲区中取数据




uint32_t __ring_buffer_get(struct ring_buffer *ring_buf, void * buffer, uint32_t size)



{



    assert(ring_buf || buffer);



    uint32_t len = 0;



    size  = min(size, ring_buf->in - ring_buf->out);        



    /* first get the data from fifo->out until the end of the buffer */




    len = min(size, ring_buf->size - (ring_buf->out & (ring_buf->size - 1)));



    memcpy(buffer, ring_buf->buffer + (ring_buf->out & (ring_buf->size - 1)), len);



    /* then get the rest (if any) from the beginning of the buffer */




    memcpy(buffer + len, ring_buf->buffer, size - len);



    ring_buf->out += size;



    return size;



}



//向缓冲区中存放数据




uint32_t __ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size)



{



    assert(ring_buf || buffer);



    uint32_t len = 0;



    size = min(size, ring_buf->size - ring_buf->in + ring_buf->out);



    /* first put the data starting from fifo->in to buffer end */




    len  = min(size, ring_buf->size - (ring_buf->in & (ring_buf->size - 1)));



    memcpy(ring_buf->buffer + (ring_buf->in & (ring_buf->size - 1)), buffer, len);



    /* then put the rest (if any) at the beginning of the buffer */




    memcpy(ring_buf->buffer, buffer + len, size - len);



    ring_buf->in += size;



    return size;



}



 




uint32_t ring_buffer_len(const struct ring_buffer *ring_buf)



{



    uint32_t len = 0;



    pthread_mutex_lock(ring_buf->f_lock);



    len = __ring_buffer_len(ring_buf);



    pthread_mutex_unlock(ring_buf->f_lock);



    return len;



}



 




uint32_t ring_buffer_get(struct ring_buffer *ring_buf, void *buffer, uint32_t size)



{



    uint32_t ret;



    pthread_mutex_lock(ring_buf->f_lock);



    ret = __ring_buffer_get(ring_buf, buffer, size);



    //buffer中没有数据




    if (ring_buf->in == ring_buf->out)



    ring_buf->in = ring_buf->out = 0;



    pthread_mutex_unlock(ring_buf->f_lock);



    return ret;



}



 




uint32_t ring_buffer_put(struct ring_buffer *ring_buf, void *buffer, uint32_t size)



{



    uint32_t ret;



    pthread_mutex_lock(ring_buf->f_lock);



    ret = __ring_buffer_put(ring_buf, buffer, size);



    pthread_mutex_unlock(ring_buf->f_lock);



    return ret;



}



#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：

/**@brief ring buffer测试程序，创建两个线程，一个生产者，一个消费者。



 * 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。



 *@atuher Anker  date:2013-12-18



 * */



#include "ring_buffer.h"



#include <pthread.h>



#include <time.h>



 



#define BUFFER_SIZE  1024 * 1024



 




typedef struct student_info




{



    uint64_t stu_id;



    uint32_t age;



    uint32_t score;



}student_info;



 



 



void print_student_info(const student_info *stu_info)



{



    assert(stu_info);



    printf("id:%lu\t",stu_info->stu_id);



    printf("age:%u\t",stu_info->age);



    printf("score:%u\n",stu_info->score);



}



 



student_info * get_student_info(time_t timer)



{



    student_info *stu_info = (student_info *)malloc(sizeof(student_info));



    if (!stu_info)



    {



    fprintf(stderr, "Failed to malloc memory.\n");



    return NULL;



    }



    srand(timer);



    stu_info->stu_id = 10000 + rand() % 9999;



    stu_info->age = rand() % 30;



    stu_info->score = rand() % 101;



    print_student_info(stu_info);



    return stu_info;



}



 



void * consumer_proc(void *arg)



{



    struct ring_buffer *ring_buf = (struct ring_buffer *)arg;




    student_info stu_info; 



    while(1)



    {



    sleep(2);



    printf("------------------------------------------\n");



    printf("get a student info from ring buffer.\n");



    ring_buffer_get(ring_buf, (void *)&stu_info, sizeof(student_info));



    printf("ring buffer length: %u\n", ring_buffer_len(ring_buf));



    print_student_info(&stu_info);



    printf("------------------------------------------\n");



    }



    return (void *)ring_buf;



}



 



void * producer_proc(void *arg)



{



    time_t cur_time;



    struct ring_buffer *ring_buf = (struct ring_buffer *)arg;




    while(1)



    {



    time(&cur_time);



    srand(cur_time);



    int seed = rand() % 11111;



    printf("******************************************\n");



    student_info *stu_info = get_student_info(cur_time + seed);



    printf("put a student info to ring buffer.\n");



    ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));



    printf("ring buffer length: %u\n", ring_buffer_len(ring_buf));



    printf("******************************************\n");



    sleep(1);



    }



    return (void *)ring_buf;



}



 



int consumer_thread(void *arg)



{



    int err;



    pthread_t tid;



    err = pthread_create(&tid, NULL, consumer_proc, arg);



    if (err != 0)



    {



    fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",



        errno, strerror(errno));



    return -1;



    }



    return tid;



}



int producer_thread(void *arg)



{



    int err;



    pthread_t tid;



    err = pthread_create(&tid, NULL, producer_proc, arg);



    if (err != 0)



    {



    fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",



        errno, strerror(errno));



    return -1;



    }



    return tid;



}



 



 



int main()



{



    void * buffer = NULL;



    uint32_t size = 0;



    struct ring_buffer *ring_buf = NULL;




    pthread_t consume_pid, produce_pid;



 



    pthread_mutex_t *f_lock = (pthread_mutex_t *)malloc(sizeof(pthread_mutex_t));



    if (pthread_mutex_init(f_lock, NULL) != 0)



    {



    fprintf(stderr, "Failed init mutex,errno:%u,reason:%s\n",



        errno, strerror(errno));



    return -1;



    }



    buffer = (void *)malloc(BUFFER_SIZE);



    if (!buffer)



    {



    fprintf(stderr, "Failed to malloc memory.\n");



    return -1;



    }



    size = BUFFER_SIZE;



    ring_buf = ring_buffer_init(buffer, size, f_lock);



    if (!ring_buf)



    {



    fprintf(stderr, "Failed to init ring buffer.\n");



    return -1;



    }



#if 0



    student_info *stu_info = get_student_info(638946124);



    ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));



    stu_info = get_student_info(976686464);



    ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));



    ring_buffer_get(ring_buf, (void *)stu_info, sizeof(student_info));



    print_student_info(stu_info);



#endif



    printf("multi thread test.......\n");



    produce_pid  = producer_thread((void*)ring_buf);



    consume_pid  = consumer_thread((void*)ring_buf);



    pthread_join(produce_pid, NULL);



    pthread_join(consume_pid, NULL);



    ring_buffer_free(ring_buf);



    free(f_lock);



    return 0;



}

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。

原文链接：https://blog.****.net/yusiguyuan/article/details/41985907