diff --git a/os/src/batch.rs b/os/src/batch.rs
index fbe94cd5..f2a0d649 100644
--- a/os/src/batch.rs
+++ b/os/src/batch.rs
@@ -105,16 +105,17 @@ pub fn init() {
 }
 
 pub fn print_app_info() {
-    APP_MANAGER.upsafe_access().print_app_info();
+    APP_MANAGER.exclusive_access().print_app_info();
 }
 
 pub fn run_next_app() -> ! {
-    let app_manager = APP_MANAGER.upsafe_access();
+    let mut app_manager = APP_MANAGER.exclusive_access();
     let current_app = app_manager.get_current_app();
     unsafe {
         app_manager.load_app(current_app);
     }
     app_manager.move_to_next_app();
+    drop(app_manager);
     extern "C" { fn __restore(cx_addr: usize); }
     unsafe {
         __restore(KERNEL_STACK.push_context(
diff --git a/os/src/sync/up.rs b/os/src/sync/up.rs
index 849ac4bc..642668c1 100644
--- a/os/src/sync/up.rs
+++ b/os/src/sync/up.rs
@@ -1,13 +1,15 @@
-/// Wrap a static data structure inside it so that we are 
+use core::cell::{RefCell, RefMut};
+
+/// Wrap a static data structure inside it so that we are
 /// able to access it without any `unsafe`.
 ///
 /// We should only use it in uniprocessor.
 ///
-/// In order to get mutable reference of inner data, call 
-/// `upsafe_access`. 
+/// In order to get mutable reference of inner data, call
+/// `exclusive_access`.
 pub struct UPSafeCell<T> {
     /// inner data
-    data: T,
+    inner: RefCell<T>,
 }
 
 unsafe impl<T> Sync for UPSafeCell<T> {}
@@ -16,12 +18,10 @@ impl<T> UPSafeCell<T> {
     /// User is responsible to guarantee that inner struct is only used in
     /// uniprocessor.
     pub unsafe fn new(value: T) -> Self {
-        Self { data: value, }
+        Self { inner: RefCell::new(value) }
     }
-    /// Mention that user should hold exactly one &mut T at a time. 
-    pub fn upsafe_access(&self) -> &mut T {
-        unsafe {
-            &mut *(&self.data as *const _ as usize as *mut T)
-        }
+    /// Panic if the data has been borrowed.
+    pub fn exclusive_access(&self) -> RefMut<'_, T> {
+        self.inner.borrow_mut()
     }
 }
\ No newline at end of file