转自：
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 $*/#include <linux/module.h>
#include <linux/init.h>#include <linux/sched.h> /* current and everything */
#include <linux/kernel.h> /* printk() */#include <linux/fs.h> /* everything... */#include <linux/types.h> /* size_t */#include <linux/completion.h>MODULE_LICENSE("Dual BSD/GPL");
static int complete_major = 253;//指定主设备号
DECLARE_COMPLETION(comp);
ssize_t complete_read (struct file *filp, char __user *buf, size_t count, loff_t *pos)
{ printk(KERN_DEBUG "process %i (%s) going to sleep/n", current->pid, current->comm); wait_for_completion(&comp); printk(KERN_DEBUG "awoken %i (%s)/n", current->pid, current->comm); return 0; /* EOF */}ssize_t complete_write (struct file *filp, const char __user *buf, size_t count,
loff_t *pos){ printk(KERN_DEBUG "process %i (%s) awakening the readers.../n", current->pid, current->comm); complete(&comp); return count; /* succeed, to avoid retrial */} struct file_operations complete_fops = { .owner = THIS_MODULE, .read = complete_read, .write = complete_write,}; int complete_init(void){ int result;/*
* Register your major, and accept a dynamic number */ result = register_chrdev(complete_major, "complete", &complete_fops); if (result < 0) return result; if (complete_major == 0) complete_major = result; /* dynamic */ return 0;}void complete_cleanup(void)
{ unregister_chrdev(complete_major, "complete");}module_init(complete_init);
module_exit(complete_cleanup); 该模块定义了一个简单的completion设备：任何试图从该设备中读取的进程都将等待，直到其他设备写入该设备为止。编译此模块的Makefile如下：obj-m := complete.oKDIR := /lib/modules/$(shell uname -r)/buildPWD := $(shell pwd)default:$(MAKE) -C $(KDIR) M=$(PWD) modulesclean: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。
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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