Linux驱动中completion接口浅析(wait_for_complete例子，很好)

 completion是一种轻量级的机制，它允许一个线程告诉另一个线程工作已经完成。可以利用下面的宏静态创建completion：

                         DECLARE_COMPLETION(my_completion); 
       

        如果运行时创建completion，则必须采用以下方法动态创建和初始化：

                         struct compltion my_completion;

                          init_completion(&my_completion);

        completion的相关定义包含在kernel/include/linux/completion.h中：

                        struct completion {

                                     unsigned int done;

                                     wait_queue_head_t wait;

                         };

#define COMPLETION_INITIALIZER(work) /

                                                           { 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }

#define DECLARE_COMPLETION(work) /

                                                      struct completion work = COMPLETION_INITIALIZER(work)

static inline void init_completion(struct completion *x)

{

          x->done = 0;

          init_waitqueue_head(&x->wait);

}

       要等待completion，可进行如下调用：

                    void wait_for_completion(struct completion *c);

       触发completion事件，调用：

                   void complete(struct completion *c);    //唤醒一个等待线程

                   void complete_all(struct completion *c);//唤醒所有的等待线程

        为说明completion的使用方法，将《Linux设备驱动程序》一书中的complete模块的代码摘抄如下：
/*

* complete.c -- the writers awake the readers

*

* Copyright (C) 2003 Alessandro Rubini and Jonathan Corbet

* Copyright (C) 2003 O'Reilly & Associates

*

* The source code in this file can be freely used, adapted,

* and redistributed in source or binary form, so long as an

* acknowledgment appears in derived source files.    The citation

* should list that the code comes from the book "Linux Device

* Drivers" by Alessandro Rubini and Jonathan Corbet, published

* by O'Reilly & Associates.     No warranty is attached;

* we cannot take responsibility for errors or fitness for use.

*

* $Id: complete.c,v 1.2 2004/09/26 07:02:43 gregkh Exp $

*/

#include <linux/module.h>

#include <linux/init.h>

#include <linux/sched.h>   /* current and everything */
#include <linux/kernel.h> /* printk() */
#include <linux/fs.h>      /* everything... */
#include <linux/types.h>   /* size_t */
#include <linux/completion.h>

MODULE_LICENSE("Dual BSD/GPL");

static int complete_major = 253;//指定主设备号

DECLARE_COMPLETION(comp);

ssize_t complete_read (struct file *filp, char __user *buf, size_t count, loff_t *pos)

{

         printk(KERN_DEBUG "process %i (%s) going to sleep/n",

         current->pid, current->comm);

         wait_for_completion(&comp);

         printk(KERN_DEBUG "awoken %i (%s)/n", current->pid, current->comm);

         return 0; /* EOF */

}

ssize_t complete_write (struct file *filp, const char __user *buf, size_t count,

    loff_t *pos)

{

         printk(KERN_DEBUG "process %i (%s) awakening the readers.../n",

         current->pid, current->comm);

         complete(&comp);

         return count; /* succeed, to avoid retrial */
}

struct file_operations complete_fops = {

         .owner = THIS_MODULE,

         .read =    complete_read,

         .write = complete_write,

};

int complete_init(void)

{

         int result;

/*

    * Register your major, and accept a dynamic number

    */
        result = register_chrdev(complete_major, "complete", &complete_fops);

        if (result < 0)

                return result;

        if (complete_major == 0)

                complete_major = result; /* dynamic */
        return 0;

}

void complete_cleanup(void)

{

         unregister_chrdev(complete_major, "complete");

}

module_init(complete_init);

module_exit(complete_cleanup);

        该模块定义了一个简单的completion设备：任何试图从该设备中读取的进程都将等待，直到其他设备写入该设备为止。编译此模块的Makefile如下：
obj-m := complete.o

KDIR := /lib/modules/$(shell uname -r)/build

PWD := $(shell pwd)

default:

$(MAKE) -C $(KDIR) M=$(PWD) modules

clean:

rm -f *.ko *.o *.mod.c

在linux终端中执行以下命令，编译生成模块，并进行动态加载。

#make

#mknod completion c 253 0

#insmod complete.ko

再打开三个终端，一个用于读进程：

#cat completion

一个用于写进程：

#echo >completion

另一个查看系统日志：

#tail -f /var/log/messages

         值得注意的是，当我们使用的complete_all接口时，如果要重复使用一个completion结构，则必须执行 INIT_COMPLETION(struct completion c)来重新初始化它。可以在kernel/include/linux/completion.h中找到这个宏的定义：

          #define INIT_COMPLETION(x) ((x).done = 0)

        以下代码对书中原有的代码进行了一番变动，将唤醒接口由原来的complete换成了complete_all，并且为了重复利用completion结构，所有读进程都结束后就重新初始化completion结构，具体代码如下：

#include <linux/module.h>

#include <linux/init.h>

#include <linux/sched.h>

#include <linux/kernel.h>

#include <linux/fs.h>

#include <linux/types.h>

#include <linux/completion.h>

MODULE_LICENSE("Dual BSD/GPL");

#undef KERN_DEBUG

#define KERN_DEBUG "<1>"

static int complete_major=253;

static int reader_count = 0;

DECLARE_COMPLETION(comp);

ssize_t complete_read (struct file *filp,char __user *buf,size_t count,loff_t *pos)

{

           printk(KERN_DEBUG "process %i (%s) going to sleep,waiting for writer/n",current->pid,current->comm);

           reader_count++;

           printk(KERN_DEBUG "In read ,before comletion: reader count = %d /n",reader_count);

           wait_for_completion(&comp);

           reader_count--;

           printk(KERN_DEBUG "awoken %s (%i) /n",current->comm,current->pid);

           printk(KERN_DEBUG "In read,after completion : reader count = %d /n",reader_count);

/*如果使用complete_all，则completion结构只能用一次，再次使用它时必须调用此宏进行重新初始化*/
           if(reader_count == 0)

                       INIT_COMPLETION(comp);

           return 0;

}

ssize_t complete_write(struct file *filp,const char __user *buf,size_t count,loff_t *pos)

{

           printk(KERN_DEBUG "process %i (%s) awoking the readers.../n",current->pid,current->comm);

           printk(KERN_DEBUG "In write ,before do complete_all : reader count = %d /n",reader_count);

           if(reader_count != 0)  

                   complete_all(&comp);

           printk(KERN_DEBUG "In write ,after do complete_all : reader count = %d /n",reader_count);

           return count;

}

struct file_operations complete_fops={

           .owner = THIS_MODULE,

           .read = complete_read,

           .write = complete_write,

};

int complete_init(void)

{

           int result;

           result=register_chrdev(complete_major,"complete",&complete_fops);

           if(result<0)

                    return result;

           if(complete_major==0)

                   complete_major =result;

           printk(KERN_DEBUG    "complete driver test init! complete_major=%d/n",complete_major);

           printk(KERN_DEBUG "静态初始化completion/n");

           return 0;

}

void complete_exit(void)

{

           unregister_chrdev(complete_major,"complete");

           printk(KERN_DEBUG    "complete driver    is removed/n");

}

module_init(complete_init);

module_exit(complete_exit);

这里测试步骤和上述一样，只不过需要多打开几个终端来执行多个进程同时读操作。

____________

参考资料：

1.Jonathan Corbet等著，魏永明等译.linux设备驱动程序(第三版)

2.Linux Kernel