ch4 代码似乎存在 unsound bug：log on page_table.root_ppn 让 qemu 死循环

[zjp-CN]

1. 给 PageTable 添加一个 root_ppn 方法（或者直接公开 root_ppn 字段）

// impl PageTable 
pub fn root_ppn(&self) -> PhysPageNum {
    self.root_ppn
}

2. 在 MemorySet::from_elf 方法里添加 log

// map user stack with U flags
let max_end_va: VirtAddr = max_end_vpn.into();
+info!(
+    "root_ppn: {:?}, max_end_vpn: {max_end_vpn:?}, max_end_va: {max_end_va:?}, ph_count: {ph_count}",
+    memory_set.page_table.root_ppn()
+);

make run LOG=INFO 会造成死循环。此外，如果把这个 log 变成 info!("root_ppn: {:?}", memory_set.page_table.root_ppn()); 并不能造成死循环。

[zjp-CN]

估计这由于错误的 impl Sync 导致的 data race，应该很容易复现（在 release 中开启 LOG 的话），因为我遇到了很多次。

GDB 定位到 drop core::cell::RefMut 的时候， Cell::get 返回时，程序跳转到 0x0。以下是 mm::remap_test(); 在运行完 println!("remap_test passed!"); 之后，remap_test 返回前的调试情况：

─── Assembly ───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
~
~
~
~
~
 0x0000000080204f3a  core::cell::Cell<isize>::get<isize>+0 ld   a0,16(a6)
~
~
~
~
─── Breakpoints ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
[1] break at 0x0000000080207dfa in src/main.rs:104 for src/main.rs:104 hit 1 time
[2] break for /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25 hit 19 times
[2.1] at 0x00000000802045ae in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.2] at 0x00000000802047f8 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.3] at 0x0000000080204a0a in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.4] at 0x0000000080204b5a in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.5] at 0x0000000080204ecc in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.6] at 0x00000000802052f2 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.7] at 0x0000000080205340 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.8] at 0x00000000802053d0 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.9] at 0x00000000802053fe in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.10] at 0x0000000080205d46 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.11] at 0x0000000080206f2a in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.12] at 0x0000000080207278 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.13] at 0x00000000802077ea in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.14] at 0x000000008020788a in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.15] at 0x0000000080207aa4 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.16] at 0x0000000080208000 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.17] at 0x000000008020844c in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.18] at 0x0000000080208abe in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.19] at 0x0000000080208d24 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.20] at 0x00000000802093a0 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
[2.21] at 0x00000000802094b6 in /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/slice/iter/macros.rs:25
─── Expressions ────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
─── Memory ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
─── Registers ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
  zero 0x0000000000000000   ra 0x0000000080204d7a   sp 0x000000008025bef0   gp 0x0000000000000000   tp 0x0000000000000000    t0 0x00000000000000f0   t1 0x0000000000110000
    t2 0x00000000000000df   fp 0x000000008025bf00   s1 0x0000000000000018   a0 0x000000000000000a   a1 0x0000000000000013    a2 0x0000000000000000   a3 0x0000000000000013
    a4 0x000000008020e9b0   a5 0x0000000000002008   a6 0x0000000000000000   a7 0x0000000000000001   s2 0x000000008225c1c8    s3 0x000000008225c090   s4 0x000000008025c000
    s5 0x0000000000000000   s6 0x0000000000000000   s7 0x0000000000000000   s8 0x0000000000000000   s9 0x0000000000000000   s10 0x0000000000000000  s11 0x0000000000000000
    t3 0x0000000000000013   t4 0x000000008025c000   t5 0x000000008225c000   t6 0x000000008020e768   pc 0x0000000080204f3a
─── Source ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
 516      #[inline]
 517      #[stable(feature = "rust1", since = "1.0.0")]
 518      pub fn get(&self) -> T {
 519          // SAFETY: This can cause data races if called from a separate thread,
 520          // but `Cell` is `!Sync` so this won't happen.
 521          unsafe { *self.value.get() }
 522      }
 523
 524      /// Updates the contained value using a function and returns the new value.
 525      ///
─── Stack ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
[0] from 0x0000000080204f3a in core::cell::Cell<isize>::get<isize>+0 at /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/cell.rs:521
[1] from 0x0000000080204f3a in core::cell::{impl#43}::drop+0 at /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/cell.rs:1777
[2] from 0x0000000080204f3a in core::ptr::drop_in_place<core::cell::BorrowRefMut>+0 at /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/ptr/mod.rs:514
[3] from 0x0000000080204f3a in core::ptr::drop_in_place<core::cell::RefMut<os::mm::memory_set::MemorySet>>+0 at /rustc/faefc618cf48bd794cbc808448df1bf3f59f36af/library/core/src/ptr/mod.rs:514
[4] from 0x0000000080204f3a in os::mm::memory_set::remap_test+472 at src/mm/memory_set.rs:460
[5] from 0x0000000080207e02 in os::rust_main+1576 at src/main.rs:104
[6] from 0x0000000080200010 in stext
─── Variables ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
arg self = <optimized out>
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
>>>