浅谈Linux内核创建新进程的全过程

检查进程数是否超过限制
初始化自旋锁、挂起信号、CPU 定时器等
调用 sched_fork 初始化进程数据结构，并把进程状态设置为 TASK_RUNNING
复制所有进程信息，包括文件系统、信号处理函数、信号、内存管理等
调用 copy_thread 初始化子进程内核栈
为新进程分配并设置新的 pid

4、dup_task_struct 流程 

static struct task_struct *dup_task_struct(struct task_struct *orig)
{
  struct task_struct *tsk;
  struct thread_info *ti;
  int node = tsk_fork_get_node(orig);
  int err;

  //分配一个 task_struct 节点
  tsk = alloc_task_struct_node(node);
  if (!tsk)
    return NULL;

  //分配一个 thread_info 节点，包含进程的内核栈，ti 为栈底
  ti = alloc_thread_info_node(tsk, node);
  if (!ti)
    goto free_tsk;

  //将栈底的值赋给新节点的栈
  tsk->stack = ti;

  //……

  return tsk;

}

调用alloc_task_struct_node分配一个 task_struct 节点
调用alloc_thread_info_node分配一个 thread_info 节点，其实是分配了一个thread_union联合体,将栈底返回给 ti

union thread_union {
  struct thread_info thread_info;
 unsigned long stack[THREAD_SIZE/sizeof(long)];
};

最后将栈底的值 ti 赋值给新节点的栈
最终执行完dup_task_struct之后，子进程除了tsk->stack指针不同之外，全部都一样！
5、sched_fork 流程

core.c

int sched_fork(unsigned long clone_flags, struct task_struct *p)
{
  unsigned long flags;
  int cpu = get_cpu();

  __sched_fork(clone_flags, p);

  //将子进程状态设置为 TASK_RUNNING
  p->state = TASK_RUNNING;

  //……

  //为子进程分配 CPU
  set_task_cpu(p, cpu);

  put_cpu();
  return 0;
}

我们可以看到sched_fork大致完成了两项重要工作，一是将子进程状态设置为 TASK_RUNNING，二是为其分配 CPU
6、copy_thread 流程

int copy_thread(unsigned long clone_flags, unsigned long sp,
  unsigned long arg, struct task_struct *p)
{
  //获取寄存器信息
  struct pt_regs *childregs = task_pt_regs(p);
  struct task_struct *tsk;
  int err;

  p->thread.sp = (unsigned long) childregs;
  p->thread.sp0 = (unsigned long) (childregs+1);
  memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));

  if (unlikely(p->flags & PF_KTHREAD)) {
    //内核线程
    memset(childregs, 0, sizeof(struct pt_regs));
    p->thread.ip = (unsigned long) ret_from_kernel_thread;
    task_user_gs(p) = __KERNEL_STACK_CANARY;
    childregs->ds = __USER_DS;
    childregs->es = __USER_DS;
    childregs->fs = __KERNEL_PERCPU;
    childregs->bx = sp; /* function */
    childregs->bp = arg;
    childregs->orig_ax = -1;
    childregs->cs = __KERNEL_CS | get_kernel_rpl();
    childregs->flags = X86_EFLAGS_IF | X86_EFLAGS_FIXED;
    p->thread.io_bitmap_ptr = NULL;
    return 0;
  }

  //将当前寄存器信息复制给子进程
  *childregs = *current_pt_regs();

  //子进程 eax 置 0，因此fork 在子进程返回0
  childregs->ax = 0;
  if (sp)
    childregs->sp = sp;

  //子进程ip 设置为ret_from_fork，因此子进程从ret_from_fork开始执行
  p->thread.ip = (unsigned long) ret_from_fork;

  //……

  return err;
}

								 
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