Add comments in ch6

os/src/sync/up.rs

@@ -1,3 +1,4 @@
+//! Uniprocessor interior mutability primitives
 use core::cell::{RefCell, RefMut};
 
 /// Wrap a static data structure inside it so that we are
@@ -22,7 +23,7 @@ impl<T> UPSafeCell<T> {
             inner: RefCell::new(value),
         }
     }
-    /// Panic if the data has been borrowed.
+    /// Exclusive access inner data in UPSafeCell. Panic if the data has been borrowed.
     pub fn exclusive_access(&self) -> RefMut<'_, T> {
         self.inner.borrow_mut()
     }