Implement many process syscalls.

os/src/config.rs

@@ -7,12 +7,6 @@ pub const PAGE_SIZE_BITS: usize = 0xc;
 
 pub const TRAMPOLINE: usize = usize::MAX - PAGE_SIZE + 1;
 pub const TRAP_CONTEXT: usize = TRAMPOLINE - PAGE_SIZE;
-/// Return (bottom, top) of a kernel stack in kernel space.
-pub fn kernel_stack_position(app_id: usize) -> (usize, usize) {
-    let top = TRAMPOLINE - app_id * (KERNEL_STACK_SIZE + PAGE_SIZE);
-    let bottom = top - KERNEL_STACK_SIZE;
-    (bottom, top)
-}
 
 #[cfg(feature = "board_k210")]
 pub const CPU_FREQ: usize = 10000000;

os/src/loader.rs

@@ -45,7 +45,8 @@ fn app_names() -> Vec<&'static str> {
             let slice = core::slice::from_raw_parts(start, end as usize - start as usize);
             let str = core::str::from_utf8(slice).unwrap();
             v.push(str);
-            start = end.add(1);
+            // Mention that there is a extra char between names
+            start = end.add(2);
         }
     }
     v

os/src/main.rs

@@ -41,20 +41,11 @@ pub fn rust_main() -> ! {
     clear_bss();
     println!("[kernel] Hello, world!");
     mm::init();
-    println!("[kernel] back to world!");
     mm::remap_test();
-
+    task::add_initproc();
-    // add apps
-    let num_app = loader::get_num_app();
-    println!("num_app={}", num_app);
-    for i in 0..num_app {
-        println!("i={}", i);
-        task::add_application(loader::get_app_data(i), i);
-    }
     trap::init();
     trap::enable_timer_interrupt();
     timer::set_next_trigger();
-    println!("before task::run_tasks!");
     task::run_tasks();
     panic!("Unreachable in rust_main!");
 }

os/src/mm/address.rs

@@ -110,6 +110,13 @@ impl VirtPageNum {
     }
 }
 
+impl PhysAddr {
+    pub fn get_mut<T>(&self) -> &'static mut T {
+        unsafe {
+            (self.0 as *mut T).as_mut().unwrap()
+        }
+    }
+}
 impl PhysPageNum {
     pub fn get_pte_array(&self) -> &'static mut [PageTableEntry] {
         let pa: PhysAddr = self.clone().into();
@@ -125,9 +132,7 @@ impl PhysPageNum {
     }
     pub fn get_mut<T>(&self) -> &'static mut T {
         let pa: PhysAddr = self.clone().into();
-        unsafe {
+        pa.get_mut()
-            (pa.0 as *mut T).as_mut().unwrap()
-        }
     }
 }
 

os/src/mm/memory_set.rs

@@ -47,6 +47,7 @@ impl MemorySet {
             areas: Vec::new(),
         }
     }
+    #[allow(unused)]
     pub fn dealloc_all_frames(&mut self) {
         *self = Self::new_bare();
     }
@@ -68,7 +69,6 @@ impl MemorySet {
             area.unmap(&mut self.page_table);
             self.areas.remove(idx);
         }
-        panic!("Area not found!");
     }
     fn push(&mut self, mut map_area: MapArea, data: Option<&[u8]>) {
         map_area.map(&mut self.page_table);
@@ -189,6 +189,23 @@ impl MemorySet {
         ), None);
         (memory_set, user_stack_top, elf.header.pt2.entry_point() as usize)
     }
+    pub fn from_existed_user(user_space: &MemorySet) -> MemorySet {
+        let mut memory_set = Self::new_bare();
+        // map trampoline
+        memory_set.map_trampoline();
+        // copy data sections/trap_context/user_stack
+        for area in user_space.areas.iter() {
+            let new_area = MapArea::from_another(area);
+            memory_set.push(new_area, None);
+            // copy data from another space
+            for vpn in area.vpn_range {
+                let src_ppn = user_space.translate(vpn).unwrap().ppn();
+                let dst_ppn = memory_set.translate(vpn).unwrap().ppn();
+                dst_ppn.get_bytes_array().copy_from_slice(src_ppn.get_bytes_array());
+            }
+        }
+        memory_set
+    }
     pub fn activate(&self) {
         let satp = self.page_table.token();
         unsafe {
@@ -199,6 +216,9 @@ impl MemorySet {
     pub fn translate(&self, vpn: VirtPageNum) -> Option<PageTableEntry> {
         self.page_table.translate(vpn)
     }
+    pub fn clear(&mut self) {
+        *self = Self::new_bare();
+    }
 }
 
 pub struct MapArea {
@@ -224,6 +244,14 @@ impl MapArea {
             map_perm,
         }
     }
+    pub fn from_another(another: &MapArea) -> Self {
+        Self {
+            vpn_range: VPNRange::new(another.vpn_range.get_start(), another.vpn_range.get_end()),
+            data_frames: BTreeMap::new(),
+            map_type: another.map_type,
+            map_perm: another.map_perm,
+        }
+    }
     pub fn map_one(&mut self, page_table: &mut PageTable, vpn: VirtPageNum) {
         let ppn: PhysPageNum;
         match self.map_type {

os/src/mm/mod.rs

@@ -8,7 +8,12 @@ use page_table::{PageTable, PTEFlags};
 use address::{VPNRange, StepByOne};
 pub use address::{PhysAddr, VirtAddr, PhysPageNum, VirtPageNum};
 pub use frame_allocator::{FrameTracker, frame_alloc};
-pub use page_table::{PageTableEntry, translated_byte_buffer};
+pub use page_table::{
+    PageTableEntry,
+    translated_byte_buffer,
+    translated_str,
+    translated_refmut,
+};
 pub use memory_set::{MemorySet, KERNEL_SPACE, MapPermission};
 pub use memory_set::remap_test;
 

os/src/mm/page_table.rs

@@ -1,6 +1,15 @@
-use super::{frame_alloc, PhysPageNum, FrameTracker, VirtPageNum, VirtAddr, StepByOne};
+use super::{
+    frame_alloc,
+    PhysPageNum,
+    FrameTracker,
+    VirtPageNum,
+    VirtAddr,
+    PhysAddr,
+    StepByOne
+};
 use alloc::vec::Vec;
 use alloc::vec;
+use alloc::string::String;
 use bitflags::*;
 
 bitflags! {
@@ -126,6 +135,17 @@ impl PageTable {
         self.find_pte(vpn)
             .map(|pte| {pte.clone()})
     }
+    pub fn translate_va(&self, va: VirtAddr) -> Option<PhysAddr> {
+        self.find_pte(va.clone().floor())
+            .map(|pte| {
+                //println!("translate_va:va = {:?}", va);
+                let aligned_pa: PhysAddr = pte.ppn().into();
+                //println!("translate_va:pa_align = {:?}", aligned_pa);
+                let offset = va.page_offset();
+                let aligned_pa_usize: usize = aligned_pa.into();
+                (aligned_pa_usize + offset).into()
+            })
+    }
     pub fn token(&self) -> usize {
         8usize << 60 | self.root_ppn.0
     }
@@ -151,3 +171,27 @@ pub fn translated_byte_buffer(token: usize, ptr: *const u8, len: usize) -> Vec<&
     }
     v
 }
+
+pub fn translated_str(token: usize, ptr: *const u8) -> String {
+    let page_table = PageTable::from_token(token);
+    let mut string = String::new();
+    let mut va = ptr as usize;
+    loop {
+        let ch: u8 = *(page_table.translate_va(VirtAddr::from(va)).unwrap().get_mut());
+        if ch == 0 {
+            break;
+        } else {
+            string.push(ch as char);
+            va += 1;
+        }
+    }
+    string
+}
+
+pub fn translated_refmut<T>(token: usize, ptr: *mut T) -> &'static mut T {
+    //println!("into translated_refmut!");
+    let page_table = PageTable::from_token(token);
+    let va = ptr as usize;
+    //println!("translated_refmut: before translate_va");
+    page_table.translate_va(VirtAddr::from(va)).unwrap().get_mut()
+}

os/src/syscall/fs.rs

@@ -1,5 +1,5 @@
 use crate::mm::translated_byte_buffer;
-use crate::task::{current_user_token, suspend_current_and_run_next};
+use crate::task::{current_user_token};
 use crate::sbi::console_getchar;
 
 const FD_STDIN: usize = 0;
@@ -28,7 +28,7 @@ pub fn sys_read(fd: usize, buf: *const u8, len: usize) -> isize {
             loop {
                 c = console_getchar();
                 if c == 0 {
-                    suspend_current_and_run_next();
+                    //suspend_current_and_run_next();
                     continue;
                 } else {
                     break;

os/src/syscall/mod.rs

@@ -3,6 +3,10 @@ const SYSCALL_WRITE: usize = 64;
 const SYSCALL_EXIT: usize = 93;
 const SYSCALL_YIELD: usize = 124;
 const SYSCALL_GET_TIME: usize = 169;
+const SYSCALL_GETPID: usize = 172;
+const SYSCALL_FORK: usize = 220;
+const SYSCALL_EXEC: usize = 221;
+const SYSCALL_WAITPID: usize = 260;
 
 mod fs;
 mod process;
@@ -17,6 +21,10 @@ pub fn syscall(syscall_id: usize, args: [usize; 3]) -> isize {
         SYSCALL_EXIT => sys_exit(args[0] as i32),
         SYSCALL_YIELD => sys_yield(),
         SYSCALL_GET_TIME => sys_get_time(),
+        SYSCALL_GETPID => sys_getpid(),
+        SYSCALL_FORK => sys_fork(),
+        SYSCALL_EXEC => sys_exec(args[0] as *const u8),
+        SYSCALL_WAITPID => sys_waitpid(args[0] as isize, args[1] as *mut i32),
         _ => panic!("Unsupported syscall_id: {}", syscall_id),
     }
 }

os/src/syscall/process.rs

@@ -1,11 +1,30 @@
 use crate::task::{
     suspend_current_and_run_next,
     exit_current_and_run_next,
+    current_task,
+    current_user_token,
+    add_task,
 };
 use crate::timer::get_time;
+use crate::mm::{
+    translated_str,
+    translated_refmut,
+};
+use crate::loader::get_app_data_by_name;
+use alloc::sync::Arc;
+
+pub fn sys_exit(exit_code: i32) -> ! {
+    // save exit code
+    let task = current_task().unwrap();
+
+    // ---- hold current PCB lock
+    task.acquire_inner_lock().exit_code = exit_code;
+    // ---- release current PCB lock
+
+    // this function will not return
+    // drop task manually to maintain rc correctly
+    drop(task);
 
-pub fn sys_exit(xstate: i32) -> ! {
-    println!("[kernel] Application exited with code {}", xstate);
     exit_current_and_run_next();
     panic!("Unreachable in sys_exit!");
 }
@@ -18,3 +37,59 @@ pub fn sys_yield() -> isize {
 pub fn sys_get_time() -> isize {
     get_time() as isize
 }
+
+pub fn sys_getpid() -> isize {
+    current_task().unwrap().pid.0 as isize
+}
+
+pub fn sys_fork() -> isize {
+    let current_task = current_task().unwrap();
+    let new_task = current_task.fork();
+    let new_pid = new_task.pid.0;
+    // modify trap context of new_task, because it returns immediately after switching
+    let trap_cx = new_task.acquire_inner_lock().get_trap_cx();
+    // we do not have to move to next instruction since we have done it before
+    // for child process, fork returns 0
+    trap_cx.x[10] = 0;
+    // add new task to scheduler
+    add_task(new_task);
+    new_pid as isize
+}
+
+pub fn sys_exec(path: *const u8) -> isize {
+    let token = current_user_token();
+    let path = translated_str(token, path);
+    let data = get_app_data_by_name(path.as_str()).unwrap();
+    let task = current_task().unwrap();
+    task.exec(data);
+    0
+}
+
+pub fn sys_waitpid(pid: isize, exit_code_ptr: *mut i32) -> isize {
+    let task = current_task().unwrap();
+    // find a child process
+    // ---- hold current PCB lock
+    let mut inner = task.acquire_inner_lock();
+    let pair = inner.children
+        .iter()
+        .enumerate()
+        .find(|(_, p)| {
+            // ++++ temporarily hold child PCB lock
+            p.acquire_inner_lock().is_zombie() && (pid == -1 || pid as usize == p.getpid())
+            // ++++ release child PCB lock
+        });
+    if let Some((idx, _)) = pair {
+        let child = inner.children.remove(idx);
+        // confirm that child will be deallocated after removing from children list
+        assert_eq!(Arc::strong_count(&child), 1);
+        let found_pid = child.getpid();
+        // ++++ temporarily hold child lock
+        let exit_code = child.acquire_inner_lock().exit_code;
+        // ++++ release child PCB lock
+        *translated_refmut(inner.memory_set.token(), exit_code_ptr) = exit_code;
+        found_pid as isize
+    } else {
+        -1
+    }
+    // ---- release current PCB lock automatically
+}

os/src/task/manager.rs

@@ -5,7 +5,7 @@ use spin::Mutex;
 use lazy_static::*;
 
 pub struct TaskManager {
-    ready_queue: VecDeque<Arc<Mutex<TaskControlBlock>>>,
+    ready_queue: VecDeque<Arc<TaskControlBlock>>,
 }
 
 /// A simple FIFO scheduler.
@@ -13,10 +13,10 @@ impl TaskManager {
     pub fn new() -> Self {
         Self { ready_queue: VecDeque::new(), }
     }
-    pub fn add(&mut self, task: Arc<Mutex<TaskControlBlock>>) {
+    pub fn add(&mut self, task: Arc<TaskControlBlock>) {
         self.ready_queue.push_back(task);
     }
-    pub fn fetch(&mut self) -> Option<Arc<Mutex<TaskControlBlock>>> {
+    pub fn fetch(&mut self) -> Option<Arc<TaskControlBlock>> {
         self.ready_queue.pop_front()
     }
 }
@@ -25,10 +25,10 @@ lazy_static! {
     pub static ref TASK_MANAGER: Mutex<TaskManager> = Mutex::new(TaskManager::new());
 }
 
-pub fn add_task(task: Arc<Mutex<TaskControlBlock>>) {
+pub fn add_task(task: Arc<TaskControlBlock>) {
     TASK_MANAGER.lock().add(task);
 }
 
-pub fn fetch_task() -> Option<Arc<Mutex<TaskControlBlock>>> {
+pub fn fetch_task() -> Option<Arc<TaskControlBlock>> {
     TASK_MANAGER.lock().fetch()
 }

os/src/task/mod.rs

@@ -5,17 +5,11 @@ mod manager;
 mod processor;
 mod pid;
 
-use crate::loader::{get_num_app, get_app_data};
+use crate::loader::{get_app_data_by_name};
-use crate::trap::TrapContext;
-use core::cell::RefCell;
-use lazy_static::*;
 use switch::__switch;
 use task::{TaskControlBlock, TaskStatus};
-use alloc::vec::Vec;
 use alloc::sync::Arc;
-use spin::Mutex;
 use manager::fetch_task;
-use pid::{PidHandle, pid_alloc, KernelStack};
 
 pub use context::TaskContext;
 pub use processor::{
@@ -27,13 +21,21 @@ pub use processor::{
     schedule,
 };
 pub use manager::add_task;
+pub use pid::{PidHandle, pid_alloc, KernelStack};
 
 pub fn suspend_current_and_run_next() {
     // There must be an application running.
-    let task = current_task().unwrap();
+    let task = take_current_task().unwrap();
-    let task_cx_ptr = task.lock().get_task_cx_ptr2();
+
-    // Change status to Ready.
+    // ---- temporarily hold current PCB lock
-    task.lock().task_status = TaskStatus::Ready;
+    let task_cx_ptr = task.acquire_inner_lock().get_task_cx_ptr2();
+    // ---- release current PCB lock
+
+    // ++++ temporarily hold current PCB lock
+    // Change status to Ready
+    task.acquire_inner_lock().task_status = TaskStatus::Ready;
+    // ++++ release current PCB lock
+
     // push back to ready queue.
     add_task(task);
     // jump to scheduling cycle
@@ -41,10 +43,37 @@ pub fn suspend_current_and_run_next() {
 }
 
 pub fn exit_current_and_run_next() {
-    // The resource recycle mechanism needs child processes. Now we just panic!
+    // take from Processor
-    panic!("An application exited!");
+    let task = take_current_task().unwrap();
+    // **** hold current PCB lock
+    let mut inner = task.acquire_inner_lock();
+    // Change status to Zombie
+    inner.task_status = TaskStatus::Zombie;
+    // move any child to its parent
+
+    // ++++++ hold parent PCB lock here
+    {
+        let parent = inner.parent.as_ref().unwrap().upgrade().unwrap();
+        let mut parent_inner = parent.acquire_inner_lock();
+        for child in inner.children.iter() {
+            parent_inner.children.push(child.clone());
+        }
+    }
+    // ++++++ release parent PCB lock here
+
+    inner.children.clear();
+    // deallocate user space
+    inner.memory_set.clear();
+    drop(inner);
+    // **** release current PCB lock
+    // drop task manually to maintain rc correctly
+    drop(task);
+    // we do not have to save task context
+    let _unused: usize = 0;
+    schedule(&_unused as *const _);
 }
 
-pub fn add_application(elf_data: &[u8], app_id: usize) {
+pub fn add_initproc() {
-    add_task(Arc::new(Mutex::new(TaskControlBlock::new(elf_data, app_id))));
+    let data = get_app_data_by_name("initproc").unwrap();
+    add_task(Arc::new(TaskControlBlock::new(data)));
 }

os/src/task/pid.rs

@@ -46,6 +46,7 @@ pub struct PidHandle(pub usize);
 
 impl Drop for PidHandle {
     fn drop(&mut self) {
+        //println!("drop pid {}", self.0);
         PID_ALLOCATOR.lock().dealloc(self.0);
     }
 }
@@ -82,7 +83,7 @@ impl KernelStack {
     }
     pub fn push_on_top<T>(&self, value: T) -> *mut T where
         T: Sized, {
-        let (_, kernel_stack_top) = kernel_stack_position(self.pid);
+        let kernel_stack_top = self.get_top();
         let ptr_mut = (kernel_stack_top - core::mem::size_of::<T>()) as *mut T;
         unsafe { *ptr_mut = value; }
         ptr_mut

os/src/task/processor.rs

@@ -2,7 +2,7 @@ use super::TaskControlBlock;
 use alloc::sync::Arc;
 use spin::Mutex;
 use lazy_static::*;
-use super::{add_task, fetch_task};
+use super::{fetch_task, TaskStatus};
 use super::__switch;
 use crate::trap::TrapContext;
 
@@ -13,7 +13,7 @@ pub struct Processor {
 unsafe impl Sync for Processor {}
 
 struct ProcessorInner {
-    current: Option<Arc<Mutex<TaskControlBlock>>>,
+    current: Option<Arc<TaskControlBlock>>,
     idle_task_cx_ptr: usize,
 }
 
@@ -31,13 +31,13 @@ impl Processor {
         &inner.idle_task_cx_ptr as *const usize
     }
     pub fn run(&self) {
-        //println!("into Processor::run");
         loop {
             if let Some(task) = fetch_task() {
-                //println!("found task!");
                 let idle_task_cx_ptr = self.get_idle_task_cx_ptr2();
-                let next_task_cx_ptr = task.lock().get_task_cx_ptr2();
+                // acquire
-                //println!("next_task_cx_ptr={:p}", next_task_cx_ptr);
+                let next_task_cx_ptr = task.acquire_inner_lock().get_task_cx_ptr2();
+                task.acquire_inner_lock().task_status = TaskStatus::Running;
+                // release
                 self.inner.lock().current = Some(task);
                 unsafe {
                     __switch(
@@ -48,10 +48,10 @@ impl Processor {
             }
         }
     }
-    pub fn take_current(&self) -> Option<Arc<Mutex<TaskControlBlock>>> {
+    pub fn take_current(&self) -> Option<Arc<TaskControlBlock>> {
         self.inner.lock().current.take()
     }
-    pub fn current(&self) -> Option<Arc<Mutex<TaskControlBlock>>> {
+    pub fn current(&self) -> Option<Arc<TaskControlBlock>> {
         self.inner.lock().current.as_ref().map(|task| task.clone())
     }
 }
@@ -64,25 +64,22 @@ pub fn run_tasks() {
     PROCESSOR.run();
 }
 
-pub fn take_current_task() -> Option<Arc<Mutex<TaskControlBlock>>> {
+pub fn take_current_task() -> Option<Arc<TaskControlBlock>> {
     PROCESSOR.take_current()
 }
 
-pub fn current_task() -> Option<Arc<Mutex<TaskControlBlock>>> {
+pub fn current_task() -> Option<Arc<TaskControlBlock>> {
-    //println!("into current_task!");
     PROCESSOR.current()
 }
 
 pub fn current_user_token() -> usize {
-    //println!("into current_user_token!");
     let task = current_task().unwrap();
-    //println!("Got task in current_user_token!");
+    let token = task.acquire_inner_lock().get_user_token();
-    let token = task.lock().get_user_token();
     token
 }
 
 pub fn current_trap_cx() -> &'static mut TrapContext {
-    current_task().unwrap().as_ref().lock().get_trap_cx()
+    current_task().unwrap().acquire_inner_lock().get_trap_cx()
 }
 
 pub fn schedule(switched_task_cx_ptr2: *const usize) {

os/src/task/task.rs

@@ -1,22 +1,32 @@
-use crate::mm::{MemorySet, MapPermission, PhysPageNum, KERNEL_SPACE, VirtAddr};
+use crate::mm::{MemorySet, PhysPageNum, KERNEL_SPACE, VirtAddr};
 use crate::trap::{TrapContext, trap_handler};
-use crate::config::{TRAP_CONTEXT, kernel_stack_position};
+use crate::config::{TRAP_CONTEXT};
 use super::TaskContext;
 use super::{PidHandle, pid_alloc, KernelStack};
+use alloc::sync::{Weak, Arc};
+use alloc::vec::Vec;
+use spin::{Mutex, MutexGuard};
 
 pub struct TaskControlBlock {
     // immutable
-    pub trap_cx_ppn: PhysPageNum,
-    pub base_size: usize,
     pub pid: PidHandle,
     pub kernel_stack: KernelStack,
     // mutable
+    inner: Mutex<TaskControlBlockInner>,
+}
+
+pub struct TaskControlBlockInner {
+    pub trap_cx_ppn: PhysPageNum,
+    pub base_size: usize,
     pub task_cx_ptr: usize,
     pub task_status: TaskStatus,
     pub memory_set: MemorySet,
+    pub parent: Option<Weak<TaskControlBlock>>,
+    pub children: Vec<Arc<TaskControlBlock>>,
+    pub exit_code: i32,
 }
 
-impl TaskControlBlock {
+impl TaskControlBlockInner {
     pub fn get_task_cx_ptr2(&self) -> *const usize {
         &self.task_cx_ptr as *const usize
     }
@@ -26,7 +36,19 @@ impl TaskControlBlock {
     pub fn get_user_token(&self) -> usize {
         self.memory_set.token()
     }
-    pub fn new(elf_data: &[u8], app_id: usize) -> Self {
+    fn get_status(&self) -> TaskStatus {
+        self.task_status
+    }
+    pub fn is_zombie(&self) -> bool {
+        self.get_status() == TaskStatus::Zombie
+    }
+}
+
+impl TaskControlBlock {
+    pub fn acquire_inner_lock(&self) -> MutexGuard<TaskControlBlockInner> {
+        self.inner.lock()
+    }
+    pub fn new(elf_data: &[u8]) -> Self {
         // memory_set with elf program headers/trampoline/trap context/user stack
         let (memory_set, user_sp, entry_point) = MemorySet::from_elf(elf_data);
         let trap_cx_ppn = memory_set
@@ -41,16 +63,23 @@ impl TaskControlBlock {
         // push a task context which goes to trap_return to the top of kernel stack
         let task_cx_ptr = kernel_stack.push_on_top(TaskContext::goto_trap_return());
         let task_control_block = Self {
-            trap_cx_ppn,
-            base_size: user_sp,
             pid: pid_handle,
             kernel_stack,
+            inner: Mutex::new(TaskControlBlockInner {
+                trap_cx_ppn,
+                base_size: user_sp,
                 task_cx_ptr: task_cx_ptr as usize,
                 task_status,
                 memory_set,
+                parent: None,
+                children: Vec::new(),
+                exit_code: 0,
+            }),
         };
         // prepare TrapContext in user space
-        let trap_cx = task_control_block.get_trap_cx();
+        // ---- acquire child PCB lock
+        let trap_cx = task_control_block.acquire_inner_lock().get_trap_cx();
+        // ---- release child PCB lock
         *trap_cx = TrapContext::app_init_context(
             entry_point,
             user_sp,
@@ -60,12 +89,86 @@ impl TaskControlBlock {
         );
         task_control_block
     }
+    pub fn exec(&self, elf_data: &[u8]) {
+        // memory_set with elf program headers/trampoline/trap context/user stack
+        let (memory_set, user_sp, entry_point) = MemorySet::from_elf(elf_data);
+        let trap_cx_ppn = memory_set
+            .translate(VirtAddr::from(TRAP_CONTEXT).into())
+            .unwrap()
+            .ppn();
+
+        // **** hold current PCB lock
+        let mut inner = self.inner.lock();
+        // substitute memory_set
+        inner.memory_set = memory_set;
+        // update trap_cx ppn
+        inner.trap_cx_ppn = trap_cx_ppn;
+        drop(inner);
+        // **** release current PCB lock manually
+
+        // initialize trap_cx
+        // **** acquire current PCB lock
+        let trap_cx = self.acquire_inner_lock().get_trap_cx();
+        // **** release current PCB lock
+        *trap_cx = TrapContext::app_init_context(
+            entry_point,
+            user_sp,
+            KERNEL_SPACE.lock().token(),
+            self.kernel_stack.get_top(),
+            trap_handler as usize,
+        );
+    }
+    pub fn fork(self: &Arc<TaskControlBlock>) -> Arc<TaskControlBlock> {
+        // ---- hold parent PCB lock
+        let mut parent_inner = self.inner.lock();
+        // copy user space(include trap context)
+        let memory_set = MemorySet::from_existed_user(
+            &parent_inner.memory_set
+        );
+        let trap_cx_ppn = memory_set
+            .translate(VirtAddr::from(TRAP_CONTEXT).into())
+            .unwrap()
+            .ppn();
+        let task_status = TaskStatus::Ready;
+        // alloc a pid and a kernel stack in kernel space
+        let pid_handle = pid_alloc();
+        let kernel_stack = KernelStack::new(&pid_handle);
+        let kernel_stack_top = kernel_stack.get_top();
+        // push a goto_trap_return task_cx on the top of kernel stack
+        let task_cx_ptr = kernel_stack.push_on_top(TaskContext::goto_trap_return());
+        let task_control_block = Arc::new(TaskControlBlock {
+            pid: pid_handle,
+            kernel_stack,
+            inner: Mutex::new(TaskControlBlockInner {
+                trap_cx_ppn,
+                base_size: parent_inner.base_size,
+                task_cx_ptr: task_cx_ptr as usize,
+                task_status,
+                memory_set,
+                parent: Some(Arc::downgrade(self)),
+                children: Vec::new(),
+                exit_code: 0,
+            }),
+        });
+        // add child
+        parent_inner.children.push(task_control_block.clone());
+        // modify kernel_sp in trap_cx
+        // **** acquire child PCB lock
+        let trap_cx = task_control_block.acquire_inner_lock().get_trap_cx();
+        // **** release child PCB lock
+        trap_cx.kernel_sp = kernel_stack_top;
+        // return
+        task_control_block
+        // ---- release parent PCB lock
+    }
+    pub fn getpid(&self) -> usize {
+        self.pid.0
+    }
 }
 
 #[derive(Copy, Clone, PartialEq)]
 pub enum TaskStatus {
     Ready,
     Running,
-    Exited,
     Zombie,
 }

os/src/trap/context.rs

@@ -20,7 +20,9 @@ impl TrapContext {
         trap_handler: usize,
     ) -> Self {
         let mut sstatus = sstatus::read();
+        // set CPU privilege to User after trapping back
         sstatus.set_spp(SPP::User);
+        // enable Supervisor mode interrupt after trapping back
         sstatus.set_spie(true);
         let mut cx = Self {
             x: [0; 32],

os/src/trap/mod.rs

@@ -53,17 +53,22 @@ pub fn enable_timer_interrupt() {
 #[no_mangle]
 pub fn trap_handler() -> ! {
     set_kernel_trap_entry();
-    let cx = current_trap_cx();
     let scause = scause::read();
     let stval = stval::read();
     match scause.cause() {
         Trap::Exception(Exception::UserEnvCall) => {
+            // jump to next instruction anyway
+            let mut cx = current_trap_cx();
             cx.sepc += 4;
-            cx.x[10] = syscall(cx.x[17], [cx.x[10], cx.x[11], cx.x[12]]) as usize;
+            // get system call return value
+            let result = syscall(cx.x[17], [cx.x[10], cx.x[11], cx.x[12]]);
+            // cx is changed during sys_exec, so we have to call it again
+            cx = current_trap_cx();
+            cx.x[10] = result as usize;
         }
         Trap::Exception(Exception::StoreFault) |
         Trap::Exception(Exception::StorePageFault) => {
-            println!("[kernel] PageFault in application, bad addr = {:#x}, bad instruction = {:#x}, core dumped.", stval, cx.sepc);
+            println!("[kernel] PageFault in application, bad addr = {:#x}, bad instruction = {:#x}, core dumped.", stval, current_trap_cx().sepc);
             exit_current_and_run_next();
         }
         Trap::Exception(Exception::IllegalInstruction) => {
@@ -83,7 +88,6 @@ pub fn trap_handler() -> ! {
 
 #[no_mangle]
 pub fn trap_return() -> ! {
-    //println!("into trap_return!");
     set_user_trap_entry();
     let trap_cx_ptr = TRAP_CONTEXT;
     let user_satp = current_user_token();
@@ -92,7 +96,6 @@ pub fn trap_return() -> ! {
         fn __restore();
     }
     let restore_va = __restore as usize - __alltraps as usize + TRAMPOLINE;
-    //println!("before jr!");
     unsafe {
         llvm_asm!("jr $0" :: "r"(restore_va), "{a0}"(trap_cx_ptr), "{a1}"(user_satp) :: "volatile");
     }
@@ -101,7 +104,7 @@ pub fn trap_return() -> ! {
 
 #[no_mangle]
 pub fn trap_from_kernel() -> ! {
-    panic!("a trap from kernel!");
+    panic!("a trap {:?} from kernel!", scause::read().cause());
 }
 
 pub use context::{TrapContext};

user/src/bin/user_shell.rs

@@ -12,7 +12,7 @@ const DL: u8 = 0x7fu8;
 const BS: u8 = 0x08u8;
 
 use alloc::string::String;
-use user_lib::{fork, exec, wait};
+use user_lib::{fork, exec, waitpid, yield_};
 use user_lib::console::getchar;
 
 #[no_mangle]
@@ -37,8 +37,17 @@ pub fn main() -> i32 {
                         unreachable!();
                     } else {
                         let mut xstate: i32 = 0;
-                        wait(&mut xstate);
+                        let mut exit_pid: isize = 0;
+                        loop {
+                            exit_pid = waitpid(pid as usize, &mut xstate);
+                            if exit_pid == -1 {
+                                yield_();
+                            } else {
+                                assert_eq!(pid, exit_pid);
                                 println!("Shell: Process {} exited with code {}", pid, xstate);
+                                break;
+                            }
+                        }
                     }
                     line.clear();
                 }

user/src/lib.rs

@@ -42,7 +42,7 @@ fn main() -> i32 {
 
 pub fn read(fd: usize, buf: &mut [u8]) -> isize { sys_read(fd, buf) }
 pub fn write(fd: usize, buf: &[u8]) -> isize { sys_write(fd, buf) }
-pub fn exit(xstate: i32) -> ! { sys_exit(xstate); }
+pub fn exit(exit_code: i32) -> ! { sys_exit(exit_code); }
 pub fn yield_() -> isize { sys_yield() }
 pub fn get_time() -> isize { sys_get_time() }
 pub fn getpid() -> isize { sys_getpid() }

user/src/syscall.rs

@@ -29,8 +29,8 @@ pub fn sys_write(fd: usize, buffer: &[u8]) -> isize {
     syscall(SYSCALL_WRITE, [fd, buffer.as_ptr() as usize, buffer.len()])
 }
 
-pub fn sys_exit(xstate: i32) -> ! {
+pub fn sys_exit(exit_code: i32) -> ! {
-    syscall(SYSCALL_EXIT, [xstate as usize, 0, 0]);
+    syscall(SYSCALL_EXIT, [exit_code as usize, 0, 0]);
     panic!("sys_exit never returns!");
 }