Re: Question about the usage of kernel_thread

[Fawad Lateef <fawadlateef@gmail.com>]

On 11/5/05, Yan Zheng <yanzheng@21cn.com> wrote:
> Hi.
>
> In LKD2, Robert say:
> Linux delegates several tasks to kernel threads, most notably the pdflush task and the ksoftirqd task. These threads are created on system boot by other kernel threads. Indeed, a kernel thread can be created only by another kernel thread.
>
>
> But I found that kernel_thread(...) are used wildly like:
>
> #include <linux/kernel.h>
> #include <linux/module.h>
>
> static int noop(void *dummy)
> {
>         printk("current->mm = %p\n", current->mm);
>         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);
>
>
> In this circumstances, The thread created by kernel_thread has "current->mm != NULL".
>
> My question is:
> The new thread is truely kernel thread ? The usage of kernel_thread(...) like this is correct?
>

AFAIK the thread created like above is a true kernel thread but in
general practice what I saw and used that by creating thread from
init_module, the thread first call daemonize which actually drops the
mm related to thread and then through reparent_to_init it makes init
as a parent of the thread/process newly created. So after daemonize
call current->mm becomes NULL and when the scheduling is going to be
done the previous_process->mm will be used as the current->mm and
creating thread like above is correct.

--
Fawad Lateef