RefCell->UPSafeCell

os/src/sync/mod.rs

@@ -0,0 +1,3 @@
+mod up;
+
+pub use up::UPSafeCell;

os/src/sync/up.rs

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+/// 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`. 
+pub struct UPSafeCell<T> {
+    /// inner data
+    data: T,
+}
+
+unsafe impl<T> Sync for UPSafeCell<T> {}
+
+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, }
+    }
+    /// 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)
+        }
+    }
+}