All of the process/thread creation calls discussed so far invoke different system calls (except create_thread) to step into kernel mode. All of those system calls in turn converge into the common kernel function _do_fork(), which is invoked with distinct CLONE_* flags. do_fork() internally falls back on copy_process() to complete the task. The following figure sums up the call sequence for process creation:

/* kernel/fork.c */
/*
 * Create a kernel thread.
 */

pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags)
{
 return _do_fork(flags|CLONE_VM|CLONE_UNTRACED, (unsigned long)fn,
 (unsigned long)arg, NULL, NULL, 0);
}

/* sys_fork: create a child process by duplicating caller */
SYSCALL_DEFINE0(fork)
{
#ifdef CONFIG_MMU
 return _do_fork(SIGCHLD, 0, 0, NULL, NULL, 0);
#else
 /* cannot support in nommu mode */
 return -EINVAL;
#endif
}

/* sys_vfork: create vfork child process */
SYSCALL_DEFINE0(vfork)
{
 return _do_fork(CLONE_VFORK | CLONE_VM | SIGCHLD, 0,
 0, NULL, NULL, 0);
}

/* sys_clone: create child process as per clone flags */

#ifdef __ARCH_WANT_SYS_CLONE
#ifdef CONFIG_CLONE_BACKWARDS
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
 int __user *, parent_tidptr,
 unsigned long, tls,
 int __user *, child_tidptr)
#elif defined(CONFIG_CLONE_BACKWARDS2)
SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
 int __user *, parent_tidptr,
 int __user *, child_tidptr,
 unsigned long, tls)
#elif defined(CONFIG_CLONE_BACKWARDS3)
SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
 int, stack_size,
 int __user *, parent_tidptr,
 int __user *, child_tidptr,
 unsigned long, tls)
#else
SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
 int __user *, parent_tidptr,
 int __user *, child_tidptr,
 unsigned long, tls)
#endif
{
 return _do_fork(clone_flags, newsp, 0, parent_tidptr, child_tidptr, tls);
}
#endif