Qn: kernel_thread() and init

From: Alpha Beta (abbashake007@lycos.com)
Date: Sun Feb 03 2002 - 03:22:08 EST


In the code of
int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
in arch/i386/kernel/process.c

as can be seen in the code here, a system call is made by trigerring the 0x80 interrupt.
this function kernel_thread() is used to launch the init process during booting by
start_kernel() //in init/main.c
But at that time, the process 0 which calls kernel_thread is executing in Kernel mode, so why should some process in kernel mode make a system call??

ANOTHER BIG DOUBT IS THAT process 0 executes in Kernel mode, it then creates the init process ( process 1)- this process according to BACH ends up running in User mode while process 0 runs in kernel mode.
so why should then we have a kernel thread invoked for init when it is to run in User mode ??

int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
{
        long retval, d0;

        __asm__ __volatile__(
                "movl %%esp,%%esi\n\t"
                "int $0x80\n\t" /* Linux/i386 system call */
                "cmpl %%esp,%%esi\n\t" /* child or parent? */
                "je 1f\n\t" /* parent - jump */
                /* Load the argument into eax, and push it. That way, it does
                 * not matter whether the called function is compiled with
                 * -mregparm or not. */
                "movl %4,%%eax\n\t"
                "pushl %%eax\n\t"
                "call *%5\n\t" /* call fn */
                "movl %3,%0\n\t" /* exit */
                "int $0x80\n"
                "1:\t"
                :"=&a" (retval), "=&S" (d0)
                :"0" (__NR_clone), "i" (__NR_exit),
                 "r" (arg), "r" (fn),
                 "b" (flags | CLONE_VM)
                : "memory");
        return retval;
}

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