diff --git a/crate/process/src/lib.rs b/crate/process/src/lib.rs
index 07c2e2c..d6e4c7d 100644
--- a/crate/process/src/lib.rs
+++ b/crate/process/src/lib.rs
@@ -4,6 +4,7 @@
 #![feature(linkage)]
 #![feature(nll)]
 #![feature(vec_resize_default)]
+#![feature(exact_size_is_empty)]
 
 extern crate alloc;
 #[macro_use]
diff --git a/crate/process/src/process_manager.rs b/crate/process/src/process_manager.rs
index b8931e7..63878c1 100644
--- a/crate/process/src/process_manager.rs
+++ b/crate/process/src/process_manager.rs
@@ -56,9 +56,17 @@ impl ProcessManager {
 
     fn alloc_pid(&self) -> Pid {
         for (i, proc) in self.procs.iter().enumerate() {
-            if proc.lock().is_none() {
+            let mut proc_lock = proc.lock();
+            if proc_lock.is_none() {
                 return i;
             }
+            match proc_lock.as_mut().unwrap().status {
+                Status::Exited(_) => if self.wait_queue[i].lock().is_empty() {
+                    *proc_lock = None;
+                    return i;
+                },
+                _ => {},
+            }
         }
         panic!("Process number exceeded");
     }
@@ -130,7 +138,7 @@ impl ProcessManager {
     fn set_status(&self, pid: Pid, status: Status) {
         let mut proc_lock = self.procs[pid].lock();
         let mut proc = proc_lock.as_mut().expect("process not exist");
-        trace!("process {} {:?} -> {:?}", pid, proc.status, status);
+        //info!("process {} {:?} -> {:?}", pid, proc.status, status);
         {   // limit the lifetime of scheduler
             let mut scheduler = self.scheduler.lock();
             match (&proc.status, &status) {
@@ -157,14 +165,25 @@ impl ProcessManager {
 
 
     pub fn get_status(&self, pid: Pid) -> Option<Status> {
-        self.procs[pid].lock().as_ref().map(|p| p.status.clone())
+        let mut proc_lock = self.procs[pid].lock();
+        if proc_lock.is_none() {
+            return None;
+        }
+        match proc_lock.as_ref().unwrap().status {
+            Status::Exited(_) => if self.wait_queue[pid].lock().is_empty() {
+                *proc_lock = None;
+                return None;
+            },
+            _ => {},
+        }
+        proc_lock.as_ref().map(|p| p.status.clone())
     }
 
-    pub fn remove(&self, pid: Pid) {
-        let mut proc_lock = self.procs[pid].lock();
+    pub fn wait_done(&self, pid: Pid, target: Pid) {
+        let mut proc_lock = self.procs[target].lock();
         let proc = proc_lock.as_ref().expect("process not exist");
         match proc.status {
-            Status::Exited(_) => *proc_lock = None,
+            Status::Exited(_) => self.wait_queue[target].lock().retain(|&i| i != pid),
             _ => panic!("can not remove non-exited process"),
         }
     }
@@ -185,17 +204,28 @@ impl ProcessManager {
         self.set_status(pid, Status::Waiting(target));
         self.wait_queue[target].lock().push(pid);
     }
+    pub fn wait_child(&self, pid: Pid) {
+        self.set_status(pid, Status::Waiting(0));
+    }
+
+    pub fn set_parent(&self, pid: Pid, target: Pid) {
+        self.wait_queue[target].lock().push(pid);
+    }
+    pub fn del_parent(&self, pid: Pid, target: Pid) {
+        self.wait_queue[target].lock().retain(|&i| i != pid);
+    }
 
     pub fn exit(&self, pid: Pid, code: ExitCode) {
+        (self.exit_handler)(pid);
         self.set_status(pid, Status::Exited(code));
     }
      /// Called when a process exit
     fn exit_handler(&self, pid: Pid, proc: &mut Process) {
-        for waiter in self.wait_queue[pid].lock().drain(..) {
-            self.wakeup(waiter);
+        for waiter in self.wait_queue[pid].lock().iter() {
+            self.wakeup(*waiter);
         }
-        proc.context = None;
-        (self.exit_handler)(pid);
+
+        //proc.context = None;
     }
 }
  fn new_vec_default<T: Default>(size: usize) -> Vec<T> {
diff --git a/crate/process/src/thread.rs b/crate/process/src/thread.rs
index 7ffa0e6..3742a21 100644
--- a/crate/process/src/thread.rs
+++ b/crate/process/src/thread.rs
@@ -151,7 +151,7 @@ impl<T> JoinHandle<T> {
         loop {
             match processor().manager().get_status(self.thread.pid) {
                 Some(Status::Exited(exit_code)) => {
-                    processor().manager().remove(self.thread.pid);
+                    processor().manager().wait_done(current().id(), self.thread.pid);
                     // Find return value on the heap from the exit code.
                     return Ok(unsafe { *Box::from_raw(exit_code as *mut T) });
                 }
diff --git a/kernel/src/fs.rs b/kernel/src/fs.rs
index 938dff2..c90499d 100644
--- a/kernel/src/fs.rs
+++ b/kernel/src/fs.rs
@@ -63,9 +63,11 @@ pub fn shell() {
             //memory_set_map_swappable(processor().get_context_mut(pid).get_memory_set_mut());
             processor().manager().wait(thread::current().id(), pid);
             processor().yield_now();
+            processor().manager().wait_done(thread::current().id(), pid);
         } else {
             println!("Program not exist");
         }
+        //break;
     }
     unsafe { dealloc(buf.as_mut_ptr(), layout) };
 }
diff --git a/kernel/src/process/mod.rs b/kernel/src/process/mod.rs
index 8bdacc5..fdc197a 100644
--- a/kernel/src/process/mod.rs
+++ b/kernel/src/process/mod.rs
@@ -36,21 +36,19 @@ static PROCESSORS: [Processor; MAX_CPU_NUM] = [Processor::new(), Processor::new(
 pub struct Process {
     parent: AtomicUsize,
     children: Mutex<Vec<usize>>,
-    subproc_exit: Condvar,  // Trigger parent's when exit
 }
 
 impl Process {
     pub fn new_fork(pid: usize, parent: usize) {
         PROCESS[pid].parent.store(parent, Ordering::Relaxed);
-        PROCESS[pid].subproc_exit._clear();
         PROCESS[parent].children.lock().push(pid);
     }
     pub fn proc_exit(pid: usize) {
-        let parent = PROCESS[pid].parent.load(Ordering::Relaxed);
-        PROCESS[parent].subproc_exit.notify_all();
-    }
-    pub fn wait_child() {
-        Self::current().subproc_exit._wait();
+        info!("proc_exit");
+        for child in PROCESS[pid].children.lock().clone() {
+            info!("del child {}", child);
+            processor().manager().del_parent(pid, child);
+        }
     }
     pub fn get_children() -> Vec<usize> {
         Self::current().children.lock().clone()
diff --git a/kernel/src/syscall.rs b/kernel/src/syscall.rs
index bcc9bf7..0b755d0 100644
--- a/kernel/src/syscall.rs
+++ b/kernel/src/syscall.rs
@@ -65,6 +65,7 @@ fn sys_fork(tf: &TrapFrame) -> i32 {
     let mut context = process().fork(tf);
     //memory_set_map_swappable(context.get_memory_set_mut());
     let pid = processor().manager().add(context);
+    processor().manager().set_parent(thread::current().id(), pid);
     Process::new_fork(pid, thread::current().id());
     //memory_set_map_swappable(processor.get_context_mut(pid).get_memory_set_mut());
     info!("fork: {} -> {}", thread::current().id(), pid);
@@ -88,7 +89,7 @@ fn sys_wait(pid: usize, code: *mut i32) -> i32 {
                     if !code.is_null() {
                         unsafe { code.write(exit_code as i32); }
                     }
-                    processor().manager().remove(pid);
+                    processor().manager().wait_done(thread::current().id(), pid);
                     Process::do_wait(pid);
                     info!("wait: {} -> {}", thread::current().id(), pid);
                     return 0;
@@ -98,7 +99,8 @@ fn sys_wait(pid: usize, code: *mut i32) -> i32 {
             }
         }
         if pid == 0 {
-            Process::wait_child();
+            processor().manager().wait_child(thread::current().id());
+            processor().yield_now();
         } else {
             processor().manager().wait(thread::current().id(), pid);
             processor().yield_now();