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Description
https://rcore-os.github.io/rCore-Tutorial-Book-v3/chapter8/1thread-kernel.html 中说
如果进程/主线程先调用了 exit 系统调用来退出，那么整个进程（包括所属的所有线程）都会退出
但示例代码显然不能做到终止其他线程的运行，不用运行任何测试就能直接读出来。
当然这也可以用测试来验证。基于commit 9ad3bb4 ，运行如下测试：
#![no_std]
#![no_main]
#[macro_use]
extern crate user_lib;
extern crate alloc;
use user_lib::{thread_create, exit};
use alloc::vec::Vec;
pub fn thread_a() -> ! {
for i in 0..1000 { print!("{}", i); }
exit(1)
}
#[no_mangle]
pub fn main() -> i32 {
thread_create(thread_a as usize, 0);
println!("main thread exited.");
exit(0)
}可能的输出为：
>> exit_test
main thread exited.
[kernel] Panicked at src/sync/up.rs:27 already borrowed: BorrowMutError
[kernel] Panicked at src/task/processor.rs:94 called `Option::unwrap()` on a `None` value
[kernel] Panicked at src/task/processor.rs:94 called `Option::unwrap()` on a `None` value
[kernel] Panicked at src/task/processor.rs:94 called `Option::unwrap()` on a `None` value
...
虽然这段代码不太符合通常的正确代码的模式，但它也不应该让内核crash。