In kernel 4.16 and above init_task is initialized directly

Kernel entry point -> First steps in the start_kernel

> You can see the definition of the init_task and it initialized by the INIT_TASK macro. This macro is from [include/linux/init_task.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/init_task.h) and it just fills the init_task with the values for the first process.

In earlier kernels init_task used to be initialized with the INIT_TASK macro, which is now absent. From 4.16 and on, there's a direct initialization in the init_task.c file.

该 issue 指向内核初始化的一个变更。查看 start_kernel 中的内核入口点代码以及 init_task.c 中的 init_task 定义。将当前的直接初始化与来自 include/linux/init_task.h 的旧 INIT_TASK 宏用法进行比较。更新书籍文本以反映从内核 4.16 开始的变更。

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