p->pid = pid_nr(pid);
    p->tgid = p->pid;//普通进程
    if (clone_flags & CLONE_THREAD)
        p->tgid = current->tgid；//线程选择叫起它的进程的tgid作为自己的tgid 
    .... 
    p->group_leader = p;//普通进程
    INIT_LIST_HEAD(&p->thread_group); 
    ... 
    if (clone_flags & CLONE_THREAD) {
       current->signal->nr_threads++;
       atomic_inc(&current->signal->live);
       atomic_inc(&current->signal->sigcnt);
       p->group_leader = current->group_leader;//线程选择叫起它的进程作为它的group_leader
       list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group);
}