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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??



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;
}