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SubjectQn: kernel_thread()
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"
:"=&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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