在kernel态创建一个守护进程

函数kernel_thread

pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
flags参数是什莫？
kernel_thread()这样调用do_fork()：
do_fork(flags|CLONE_VM|CLONE_UNTRACED, 0, ®s, 0, NULL, NULL);
这也是kernel_thread()唯一使用flags的地方，能够看出，无论怎样CLONE_VM|CLONE_UNTRACED两个flag都是要用到的。
察看CLONE_VM的定义能够发现：

/*
 * cloning flags:
 */
#define CSIGNAL        0x000000ff    /* signal mask to be sent at exit */
#define CLONE_VM    0x00000100    /* set if VM shared between processes */
#define CLONE_FS    0x00000200    /* set if fs info shared between processes */
#define CLONE_FILES    0x00000400    
    /* set if open files shared between processes */
#define CLONE_SIGHAND    0x00000800    
    /* set if signal handlers and blocked signals shared */
#define CLONE_IDLETASK    0x00001000   
    /* set if new pid should be 0 (kernel only)*/
#define CLONE_PTRACE    0x00002000    
    /* set if we want to let tracing continue on the child too */
#define CLONE_VFORK    0x00004000    
    /* set if the parent wants the child to wake it up on mm_release */
#define CLONE_PARENT    0x00008000    
    /* set if we want to have the same parent as the cloner */
#define CLONE_THREAD    0x00010000    /* Same thread group? */
#define CLONE_NEWNS    0x00020000    /* New namespace group? */
#define CLONE_SYSVSEM    0x00040000    /* share system V SEM_UNDO semantics */
#define CLONE_SETTLS    0x00080000    /* create a new TLS for the child */
#define CLONE_PARENT_SETTID    0x00100000    /* set the TID in the parent */
#define CLONE_CHILD_CLEARTID    0x00200000    /* clear the TID in the child */
#define CLONE_DETACHED        0x00400000    /* Unused, ignored */
#define CLONE_UNTRACED        0x00800000    
   /* set if the tracing process can’t force CLONE_PTRACE on  this clone */ 
#define CLONE_CHILD_SETTID    0x01000000    /* set the TID in the child */
#define CLONE_STOPPED        0x02000000    /* Start in stopped state */
/*
 * List of flags we want to share for kernel threads,
 * if only because they are not used by them anyway.
 */
#define CLONE_KERNEL    (CLONE_FS | CLONE_FILES | CLONE_SIGHAND)

内核线程常用的flags就是CLONE_KERNEL，为什莫这样？肯定更有很多具体的细节。

用kernel_thread创建的线程复制了父进程的资源（像fork), 而有些资源(如mm)对于内核线程是不需要的, daemonize用来释放这些资源。

schedule_timeout()

schedule_timeout() -- 将当前进程推后执行

fastcall signed long __sched schedule_timeout(signed long timeout)
{
    struct timer_list timer;
    unsigned long expire;
    switch (timeout)
    {
    case MAX_SCHEDULE_TIMEOUT:
        schedule();
        goto out;
    default:
        if (timeout < 0) {
            printk(KERN_ERR "schedule_timeout: wrong timeout "
                "value %lx\n", timeout);
            dump_stack();
            current->state = TASK_RUNNING;
            goto out;
        }
    }
    expire = timeout + jiffies;
    setup_timer(&timer, process_timeout, (unsigned long)current);
    __mod_timer(&timer, expire);
    schedule();
    del_singleshot_timer_sync(&timer);
    timeout = expire - jiffies;
out:
   return timeout < 0 ? 0 : timeout;
}

source code 1

 1 #include 
 2 #include 
 3 #include 
 4 #include "rc2032dx.h"
 5 
 6 int check_button(void)
 7 {
 8     if((GPIO_DATA_INPUT_HIGH & GPIO_BUTTON_POW) == 0)
 9     {
10         return 0;
11     }
12     else
13     {
14         return 1;
15     }
16 }
17 
18 static int noop(void *dummy)
19 {
20     int i = 0;
21     int putton_flash = 0;
22     printk("[dancy log:%d]insmod pushbutton.\n",__LINE__);
23     daemonize("pushbutton");
24     GPIO_DATA_ENABLE_HIGH &= ~GPIO_BUTTON_POW;
25     GPIO_DATA_ENABLE_LOW |= GPIO_FOR_REBOOT;
26     GPIO_DATA_OUTPUT_LOW &= ~GPIO_FOR_REBOOT;
27     while(1)
28     {
29         if(check_button()==0)
30         {
31             putton_flash = 1;
32             i++;
33             if(i >= 20)
34             {
35                 printk("reset board.\n");
36                 GPIO_DATA_OUTPUT_LOW |= GPIO_FOR_REBOOT;
37             }
38         }
39         else
40         {
41             if(putton_flash == 1)
42             {
43                 if(i < 20)
44                 {
45                     printk("turn off led.\n");
46                 }
47                 else
48                 {
49                     printk("reset board.\n");
50                     GPIO_DATA_OUTPUT_LOW |= GPIO_FOR_REBOOT;
51                 }
52             }
53             i = 0;
54             putton_flash = 0;
55         }
56         set_current_state(TASK_INTERRUPTIBLE);
57         schedule_timeout(100);
58     }
59     return 0;
60 }
61 
62 static int pushbutton_init(void)
63 {
64     printk("[dancy log:%d]insmod pushbutton.\n",__LINE__);
65     kernel_thread(noop, NULL, CLONE_KERNEL | SIGCHLD);
66     return 0;
67 }
68 
69 static void pushbutton_exit(void) {}
70 module_init(pushbutton_init);
71 module_exit(pushbutton_exit);

source code 2

#include 
#include 

static int noop(void *dummy)
{
 int i = 0;
 daemonize("mythread");
 while(i++ < 5) 
 {
   printk("current->mm = %p\n", current->mm);
   printk("current->active_mm = %p\n", current->active_mm);
   set_current_state(TASK_INTERRUPTIBLE);
   schedule_timeout(10 * HZ);
 }
 return 0;
}

static int test_init(void)
{
 kernel_thread(noop, NULL, CLONE_KERNEL | SIGCHLD);
 return 0;
}

static void test_exit(void) {}
module_init(test_init);
module_exit(test_exit); 

Makefile

 1 CROSS_COMPILE=/opt/toolchains/uclibc-crosstools-gcc-4.2.3-3/usr/bin
        /mips-linux-uclibc-
 2 KER=../../../../kernel/linux/
 3 obj-m := pushbutton.o
 4 pushbutton.ko:pushbutton.c
 5     $(MAKE) -C $(KER) CROSS_COMPILE=$(CROSS_COMPILE)  SUBDIRS=$(shell pwd) 
    modules
 6 clean:
 7     rm *.ko *.o Module.symvers pushbutton.mod.c .*.cmd .tmp_versions -fr