Expose release method to file implementations.

This is useful for cases when the implementation needs to defer the
actual freeing of the file object, for example, because it needs to
perform expensive work that is better suited for worker threads than the
current one.

Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>

Author: wedsonaf

rust/kernel/src/file_operations.rs

@@ -113,7 +113,7 @@ unsafe extern "C" fn release_callback<T: FileOperations>(
     file: *mut bindings::file,
 ) -> c_types::c_int {
     let ptr = mem::replace(&mut (*file).private_data, ptr::null_mut());
-    drop(T::Wrapper::from_pointer(ptr as _));
+    T::release(T::Wrapper::from_pointer(ptr as _), &File::from_ptr(file));
     0
 }
 
@@ -223,6 +223,11 @@ pub trait FileOperations: Sync + Sized {
     /// pointer in `struct file_operations`.
     fn open() -> KernelResult<Self::Wrapper>;
 
+    /// Cleans up after the last reference to the file goes away. Note that the object is moved, so
+    /// it will be freed automatically unless the implemention moves it elsewhere. Corresponds to
+    /// the `release` function pointer in `struct file_operations`.
+    fn release(_obj: Self::Wrapper, _file: &File) {}
+
     /// Reads data from this file to userspace. Corresponds to the `read`
     /// function pointer in `struct file_operations`.
     const READ: ReadFn<Self> = None;