Refactor context switching. NFCI (#61)

* Refactor context switching. NFCI

* Fix compile errors

* Embed the symbol table into unit testing kernel images

* Allow clippy::print_with_newline

Makefile

@@ -49,10 +49,13 @@ CARGOFLAGS += -Z build-std=core,alloc -Z build-std-features=compiler-builtins-me
 CARGOFLAGS += --target $(target_json)
 CARGOFLAGS += $(if $(RELEASE),--release,)
 TESTCARGOFLAGS += --package kerla -Z unstable-options
-TESTCARGOFLAGS += --config "target.$(ARCH).runner = '$(PYTHON3) $(topdir)/tools/run-qemu.py --arch $(ARCH)'"
+TESTCARGOFLAGS += --config "target.$(ARCH).runner = './tools/run-unittests.sh'"
 WATCHFLAGS += --clear
 export CARGO_FROM_MAKE=1
 export INITRAMFS_PATH
+export ARCH
+export PYTHON3
+export NM
 
 #
 #  Build Commands

kernel/arch/x64/interrupt.rs

@@ -8,7 +8,7 @@ use crate::{
     timer::handle_timer_irq,
 };
 
-use x86::{controlregs::cr2, current::rflags::RFlags, irq::*};
+use x86::{controlregs::cr2, irq::*};
 
 /// The interrupt stack frame.
 #[derive(Debug, Copy, Clone)]
@@ -195,11 +195,3 @@ unsafe extern "C" fn x64_handle_interrupt(vec: u8, frame: *const InterruptFrame)
         }
     }
 }
-
-pub unsafe fn enable_interrupt() {
-    asm!("sti");
-}
-
-pub fn is_interrupt_enabled() -> bool {
-    x86::current::rflags::read().contains(RFlags::FLAGS_IF)
-}

kernel/arch/x64/mod.rs

@@ -36,7 +36,6 @@ pub use arch_prctl::arch_prctl;
 pub use backtrace::Backtrace;
 pub use boot::init;
 pub use idle::{halt, idle};
-pub use interrupt::{enable_interrupt, is_interrupt_enabled};
 pub use ioapic::enable_irq;
 pub use lock::{SpinLock, SpinLockGuard};
 pub use page_table::{PageFaultReason, PageTable};

kernel/printk.rs

@@ -2,7 +2,6 @@ use alloc::boxed::Box;
 use arrayvec::ArrayVec;
 use kerla_utils::ring_buffer::RingBuffer;
 
-use crate::arch::SpinLock;
 use crate::arch::{print_str, printchar, Backtrace, VAddr};
 use crate::lang_items::PANICKED;
 use core::mem::size_of;
@@ -12,8 +11,10 @@ use core::{fmt, slice};
 pub struct Printer;
 
 pub const KERNEL_LOG_BUF_SIZE: usize = 8192;
-pub static KERNEL_LOG_BUF: SpinLock<RingBuffer<u8, KERNEL_LOG_BUF_SIZE>> =
-    SpinLock::new(RingBuffer::new());
+// We use spin::Mutex here because SpinLock's debugging features may cause a
+// problem (capturing a backtrace requires memory allocation).
+pub static KERNEL_LOG_BUF: spin::Mutex<RingBuffer<u8, KERNEL_LOG_BUF_SIZE>> =
+    spin::Mutex::new(RingBuffer::new());
 
 impl core::fmt::Write for Printer {
     fn write_char(&mut self, c: char) -> core::fmt::Result {

kernel/process/switch.rs

@@ -1,7 +1,7 @@
 use super::*;
 use crate::process::PId;
 use crate::{
-    arch::{self, enable_interrupt, is_interrupt_enabled},
+    arch::{self},
     process::process::PROCESSES,
 };
 
@@ -11,10 +11,6 @@ use core::mem::{self};
 
 /// Yields execution to another thread.
 pub fn switch() {
-    // Save the current interrupt enable flag to restore it in the next execution
-    // of the currently running thread.
-    let interrupt_enabled = is_interrupt_enabled();
-
     let prev = current_process().clone();
     let prev_pid = prev.pid();
     let prev_state = prev.state();
@@ -46,10 +42,17 @@ pub fn switch() {
         lock.page_table().switch();
     }
 
-    // Drop `next_thread` here because `switch_thread` won't return when the current
-    // process is being destroyed (e.g. by exit(2)) and it leads to a memory leak.
+    // Drop `prev` and `next` here because `switch_thread` won't return when the
+    // current process is being destroyed (e.g. by exit(2)).
+    //
+    // Since processes are referenced from at least the following two places,
+    // we can safely decrement reference counts without immediately drop here:
+    //
+    // - prev or next: they holds Arc<Process> (not &Arc<Process>).
+    // - Their parent processes's list of children.
     //
     // To cheat the borrow checker we do so by `Arc::decrement_strong_count`.
+    debug_assert!(Arc::strong_count(&prev) > 1);
     debug_assert!(Arc::strong_count(&next) > 1);
     unsafe {
         Arc::decrement_strong_count(Arc::as_ptr(&prev));
@@ -64,14 +67,4 @@ pub fn switch() {
     // reference count by `Arc::decrement_strong_count` above.
     mem::forget(prev);
     mem::forget(next);
-
-    // Now we're in the next thread. Release held locks and continue executing.
-
-    // Retstore the interrupt enable flag manually because lock guards
-    // (`prev` and `next`) that holds the flag state are `mem::forget`-ed.
-    if interrupt_enabled {
-        unsafe {
-            enable_interrupt();
-        }
-    }
 }

kernel/test_runner.rs

@@ -1,8 +1,9 @@
 #![cfg(test)]
+#![allow(clippy::print_with_newline)]
 
 use crate::arch::*;
 use core::panic::PanicInfo;
-use core::sync::atomic::{AtomicBool, Ordering};
+use core::sync::atomic::Ordering;
 
 pub trait Testable {
     fn run(&self);
@@ -15,7 +16,7 @@ where
     fn run(&self) {
         print!("{} ... ", core::any::type_name::<T>());
         self();
-        println!("\x1b[92mok\x1b[0m");
+        print!("\x1b[92mok\x1b[0m\n");
     }
 }
 
@@ -24,8 +25,8 @@ pub fn run_tests(tests: &[&dyn Testable]) {
     for test in tests {
         test.run();
     }
-    println!();
-    println!("\x1b[92mPassed all tests :)\x1b[0m\n");
+    print!("\n");
+    print!("\x1b[92mPassed all tests :)\x1b[0m\n");
 }
 
 pub fn end_tests() -> ! {
@@ -35,18 +36,17 @@ pub fn end_tests() -> ! {
     loop {}
 }
 
-static ALREADY_PANICED: AtomicBool = AtomicBool::new(false);
-
 #[cfg(test)]
 #[panic_handler]
 fn panic(info: &PanicInfo) -> ! {
-    if ALREADY_PANICED.load(Ordering::SeqCst) {
+    use crate::lang_items::PANICKED;
+
+    if PANICKED.load(Ordering::SeqCst) {
         loop {}
     }
 
-    ALREADY_PANICED.store(true, Ordering::SeqCst);
-
-    print!("\x1b[1;91mfail\x1b[0m\n{}\n", info);
+    PANICKED.store(true, Ordering::SeqCst);
+    print!("\x1b[1;91mfail\npanic: {}\x1b[0m", info);
     semihosting_halt(ExitStatus::Failure);
     loop {}
 }

tools/run-unittests.sh

@@ -0,0 +1,7 @@
+#!/bin/sh
+set -ue
+
+executable="$1"
+$NM $executable | rustfilt | awk '{ $2=""; print $0 }' > $executable.symbols
+$PYTHON3 ../tools/embed-symbol-table.py $executable.symbols $executable
+$PYTHON3 ../tools/run-qemu.py --arch $ARCH $executable