Rm spin::Mutex except for easy-fs & add new test huge_write & flush cache to disk after a write transaction

os/src/sync/mod.rs

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

os/src/sync/up.rs

@@ -0,0 +1,27 @@
+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
+/// `exclusive_access`.
+pub struct UPSafeCell<T> {
+    /// inner data
+    inner: RefCell<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 { inner: RefCell::new(value) }
+    }
+    /// Panic if the data has been borrowed.
+    pub fn exclusive_access(&self) -> RefMut<'_, T> {
+        self.inner.borrow_mut()
+    }
+}