rust: add in-place module

IMPORTANT: This work is by Wedson A.F.! This patch will not be send to
the lists. If patches are send to the list, they will be based on a
tree that contains these patches.

Author: Valentin Obst

rust/kernel/lib.rs

@@ -65,7 +65,7 @@ const __LOG_PREFIX: &[u8] = b"rust_kernel\0";
 /// The top level entrypoint to implementing a kernel module.
 ///
 /// For any teardown or cleanup operations, your type may implement [`Drop`].
-pub trait Module: Sized + Sync {
+pub trait Module: Sized + Sync + Send {
     /// Called at module initialization time.
     ///
     /// Use this method to perform whatever setup or registration your module
@@ -75,6 +75,29 @@ pub trait Module: Sized + Sync {
     fn init(module: &'static ThisModule) -> error::Result<Self>;
 }
 
+/// A module that is pinned and initialised in-place.
+pub trait InPlaceModule: Sync + Send {
+    /// Creates an initialiser for the module.
+    ///
+    /// It is called when the module is loaded.
+    fn init(module: &'static ThisModule) -> impl init::PinInit<Self, error::Error>;
+}
+
+impl<T: Module> InPlaceModule for T {
+    fn init(module: &'static ThisModule) -> impl init::PinInit<Self, error::Error> {
+        let initer = move |slot: *mut Self| {
+            let m = <Self as Module>::init(module)?;
+
+            // SAFETY: `slot` is valid for write per the contract with `pin_init_from_closure`.
+            unsafe { slot.write(m) };
+            Ok(())
+        };
+
+        // SAFETY: On success, `initer` always fully initialises an instance of `Self`.
+        unsafe { init::pin_init_from_closure(initer) }
+    }
+}
+
 /// Equivalent to `THIS_MODULE` in the C API.
 ///
 /// C header: [`include/linux/export.h`](srctree/include/linux/export.h)

rust/macros/module.rs

@@ -208,7 +208,7 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
             #[used]
             static __IS_RUST_MODULE: () = ();
 
-            static mut __MOD: Option<{type_}> = None;
+            static mut __MOD: core::mem::MaybeUninit<{type_}> = core::mem::MaybeUninit::uninit();
 
             // SAFETY: `__this_module` is constructed by the kernel at load time and will not be
             // freed until the module is unloaded.
@@ -270,23 +270,17 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
             }}
 
             fn __init() -> core::ffi::c_int {{
-                match <{type_} as kernel::Module>::init(&THIS_MODULE) {{
-                    Ok(m) => {{
-                        unsafe {{
-                            __MOD = Some(m);
-                        }}
-                        return 0;
-                    }}
-                    Err(e) => {{
-                        return e.to_errno();
-                    }}
+                let initer = <{type_} as kernel::InPlaceModule>::init(&THIS_MODULE);
+                match unsafe {{ initer.__pinned_init(__MOD.as_mut_ptr()) }} {{
+                    Ok(m) => 0,
+                    Err(e) => e.to_errno(),
                 }}
             }}
 
             fn __exit() {{
                 unsafe {{
                     // Invokes `drop()` on `__MOD`, which should be used for cleanup.
-                    __MOD = None;
+                    __MOD.assume_init_drop();
                 }}
             }}