等待完成(completion)

想等待某件處理中的事件完成的話，可以使用這個方法。用法很簡單，先準備好completion變數，然後呼叫要等待的函式即可。處理結束，會自動呼叫通報完成的函式，並返回等待的函式。由於過程中會sleep因此不能用在interrupt context。定義在<linux/completion.h>

初始化

void init_completion(struct completion *x);

進入睡眠

void fastcall_sched wait_for_completion(struct completion *x);

呼叫此函式後，就會讓user context進入睡眠，在等待對象完成之前都不會醒來。其變化還有用signal喚醒的wait_for_completion_interruptible()以及可以指定等待時間的wait_for_completion_timeout()。

int fastcall_sched wait_for_completion_interruptible(struct completion *x);
unsigned long fastcall_sched wait_for_completion_timeout(struct completion *x,unsigned long timeout);

喚醒

void fastcall completion(struct completion *x);

也有通報完成、同時中止kernel thread的函式

void completion_and_exit(struct completion *x);

範例：

#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 = 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\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);

這個範例很簡單，當user context進入read區域時候便會啟動睡眠，此時可以在write進行工作，當write的工作處離完成之後會發出完成通知，read裡面收到完工通知便會跳出睡眠。