Linux驱动中completion接口浅析wait_for_complete

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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
发表于 @ 2009年12月08日　17:11:00 | | 举报|

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