rust: introduce InPlaceModule

This allows modules to be initialised in-place in pinned memory, which enables the usage of pinned types (e.g., mutexes, spinlocks, driver registrations, etc.) in modules without any extra allocations. Drivers that don't need this may continue to implement `Module` without any changes. Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Rust-for-Linux · wedsonaf · Sep 29, 2023 · Sep 29, 2023 · Sep 29, 2023 · Sep 29, 2023
commit 484ec70025ff9887d9ca228ec631264039cee355
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
@@ -17,6 +17,7 @@
 #![feature(dispatch_from_dyn)]
 #![feature(new_uninit)]
 #![feature(receiver_trait)]
+#![feature(return_position_impl_trait_in_trait)]
 #![feature(unsize)]
 
 // Ensure conditional compilation based on the kernel configuration works;
@@ -60,7 +61,7 @@ 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
@@ -70,6 +71,29 @@ 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`

diff --git a/rust/macros/module.rs b/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();
                 }}
             }}
 

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
@@ -262,7 +262,7 @@ $(obj)/%.lst: $(src)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := new_uninit
+rust_allowed_features := new_uninit,impl_trait_in_assoc_type
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree