long _do_fork(struct kernel_clone_args *args)
{
    u64 clone_flags = args->flags;
    struct completion vfork;
    struct pid *pid;
    struct task_struct *p;
    int trace = 0;
    long nr;
 
    /*
     * Determine whether and which event to report to ptracer.  When
     * called from kernel_thread or CLONE_UNTRACED is explicitly
     * requested, no event is reported; otherwise, report if the event
     * for the type of forking is enabled.
     */
    if (!(clone_flags & CLONE_UNTRACED)) {
        if (clone_flags & CLONE_VFORK)
            trace = PTRACE_EVENT_VFORK;
        else if (args->exit_signal != SIGCHLD)
            trace = PTRACE_EVENT_CLONE;
        else
            trace = PTRACE_EVENT_FORK;
 
        if (likely(!ptrace_event_enabled(current, trace)))
            trace = 0;
    }
　　//copy_process主要通过dup_task_struct()函数进行进程描述符的复制，将父进程的结构体task_struct变量的值复制给子进程
　　//通过copy_thread()函数进行子进程内核栈的初始化
    p = copy_process(NULL, trace, NUMA_NO_NODE, args);//复制进程描述符
    add_latent_entropy();
 
    if (IS_ERR(p))
        return PTR_ERR(p);
 
    /*
     * Do this prior waking up the new thread - the thread pointer
     * might get invalid after that point, if the thread exits quickly.
     */
    trace_sched_process_fork(current, p);
    pid = get_task_pid(p, PIDTYPE_PID);
    nr = pid_vnr(pid);
 
    if (clone_flags & CLONE_PARENT_SETTID)
        put_user(nr, args->parent_tid);
    if (clone_flags & CLONE_VFORK) {
        p->vfork_done = &vfork;
        init_completion(&vfork);
        get_task_struct(p);
    }
    wake_up_new_task(p);//将已经准备好进程上下文的子进程加入就绪队列
　　//该函数对task_struct结构体中的thread_struct结构体进行了初始化。
　　//将子进程的ax值设置为0，即fork()系统调用最终给子进程的返回值
　　//设置子进程的sp值，即子进程的内核堆栈
　　//将子进程的ip值设置为ret_from_fork，即子进程返回后开始执行的地方

 
    /* forking complete and child started to run, tell ptracer */
    if (unlikely(trace))
        ptrace_event_pid(trace, pid);
    if (clone_flags & CLONE_VFORK) {
        if (!wait_for_vfork_done(p, &vfork))
            ptrace_event_pid(PTRACE_EVENT_VFORK_DONE, pid);
    }
 
    put_pid(pid);
    return nr;
}