linux内核数据结构之kfifo

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 = ;
     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 = ;
     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 - )));
     memcpy(fifo->buffer + (fifo->in & (fifo->size - )), 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 - )));
     memcpy(buffer, fifo->buffer + (fifo->out & (fifo->size - )), 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, , sizeof(struct ring_buffer));
     ring_buf->buffer = buffer;
     ring_buf->size = size;
     ring_buf->in = ;
     ring_buf->out = ;
         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 = ;
     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 - )));
     memcpy(buffer, ring_buf->buffer + (ring_buf->out & (ring_buf->size - )), 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 = ;
     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 - )));
     memcpy(ring_buf->buffer + (ring_buf->in & (ring_buf->size - )), 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 = ;
     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 = ;
     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 =  + rand() % ;
     stu_info->age = rand() % ;
     stu_info->score = rand() % ;
     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()
     {
     sleep();
     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()
     {
     time(&cur_time);
     srand(cur_time);
     int seed = rand() % ;
     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();
     }
     return (void *)ring_buf;
 }

 int consumer_thread(void *arg)
 {
     int err;
     pthread_t tid;
     err = pthread_create(&tid, NULL, consumer_proc, arg);
     if (err != )
     {
     fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",
         errno, strerror(errno));
     return -;
     }
     return tid;
 }
 int producer_thread(void *arg)
 {
     int err;
     pthread_t tid;
     err = pthread_create(&tid, NULL, producer_proc, arg);
     if (err != )
     {
     fprintf(stderr, "Failed to create consumer thread.errno:%u, reason:%s\n",
         errno, strerror(errno));
     return -;
     }
     return tid;
 }

 int main()
 {
     void * buffer = NULL;
     uint32_t size = ;
     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) != )
     {
     fprintf(stderr, "Failed init mutex,errno:%u,reason:%s\n",
         errno, strerror(errno));
     return -;
     }
     buffer = (void *)malloc(BUFFER_SIZE);
     if (!buffer)
     {
     fprintf(stderr, "Failed to malloc memory.\n");
     return -;
     }
     size = BUFFER_SIZE;
     ring_buf = ring_buffer_init(buffer, size, f_lock);
     if (!ring_buf)
     {
     fprintf(stderr, "Failed to init ring buffer.\n");
     return -;
     }
 #if 0
     student_info *stu_info = get_student_info();
     ring_buffer_put(ring_buf, (void *)stu_info, sizeof(student_info));
     stu_info = get_student_info();
     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 ;
 }

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