rust: add support for static synchronisation primitives

This allows drivers to declare global shared variables (`static`) that
can be mutated when the synchronisation primitive is held.

Although we want to discourage the use of globals, some drivers still
need them. For example, a driver that needs to allocate an id to each
instance of a device that is attached (and free it up when it's
detached) may have a global variable to manage ids, access to which
needs to synchronised.

We use constructors to implement this, so we select
`CONFIG_CONSTRUCTORS` automatically when `RUST` is selected.

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

Author: wedsonaf

init/Kconfig

@@ -2062,6 +2062,7 @@ config RUST
 	depends on ARM64 || CPU_32v6 || CPU_32v6K || (PPC64 && CPU_LITTLE_ENDIAN) || X86_64 || RISCV
 	depends on !MODVERSIONS
 	depends on !GCC_PLUGIN_RANDSTRUCT
+	select CONSTRUCTORS
 	default n
 	help
 	  Enables Rust support in the kernel.

rust/kernel/str.rs

@@ -338,7 +338,7 @@ where
 /// ```
 #[macro_export]
 macro_rules! c_str {
-    ($str:literal) => {{
+    ($str:expr) => {{
         const S: &str = concat!($str, "\0");
         const C: &$crate::str::CStr = $crate::str::CStr::from_bytes_with_nul_unwrap(S.as_bytes());
         C

rust/kernel/sync/condvar.rs

@@ -48,7 +48,7 @@ impl CondVar {
     /// # Safety
     ///
     /// The caller must call `CondVar::init` before using the conditional variable.
-    pub unsafe fn new() -> Self {
+    pub const unsafe fn new() -> Self {
         Self {
             wait_list: Opaque::uninit(),
             _pin: PhantomPinned,

rust/kernel/sync/mod.rs

@@ -47,7 +47,7 @@ pub use spinlock::SpinLock;
 #[doc(hidden)]
 #[macro_export]
 macro_rules! init_with_lockdep {
-    ($obj:expr, $name:literal) => {{
+    ($obj:expr, $name:expr) => {{
         static mut CLASS1: core::mem::MaybeUninit<$crate::bindings::lock_class_key> =
             core::mem::MaybeUninit::uninit();
         static mut CLASS2: core::mem::MaybeUninit<$crate::bindings::lock_class_key> =
@@ -85,6 +85,69 @@ pub trait NeedsLockClass {
     );
 }
 
+/// Automatically initialises static instances of synchronisation primitives.
+///
+/// The syntax resembles that of regular static variables, except that the value assigned is that
+/// of the protected type (if one exists). In the examples below, all primitives except for
+/// [`CondVar`] require the inner value to be supplied.
+///
+/// # Examples
+///
+/// ```ignore
+/// # use kernel::{init_static_sync, sync::{CondVar, Mutex, RevocableMutex, SpinLock}};
+/// struct Test {
+///     a: u32,
+///     b: u32,
+/// }
+///
+/// init_static_sync! {
+///     static A: Mutex<Test> = Test { a: 10, b: 20 };
+///
+///     /// Documentation for `B`.
+///     pub static B: Mutex<u32> = 0;
+///
+///     pub(crate) static C: SpinLock<Test> = Test { a: 10, b: 20 };
+///     static D: CondVar;
+///
+///     static E: RevocableMutex<Test> = Test { a: 30, b: 40 };
+/// }
+/// ```
+#[macro_export]
+macro_rules! init_static_sync {
+    ($($(#[$outer:meta])* $v:vis static $id:ident : $t:ty $(= $value:expr)?;)*) => {
+        $(
+            $(#[$outer])*
+            $v static $id: $t = {
+                #[link_section = ".ctors"]
+                #[used]
+                static TMP: extern "C" fn() = {
+                    extern "C" fn constructor() {
+                        // SAFETY: This locally-defined function is only called from a constructor,
+                        // which guarantees that `$id` is not accessible from other threads
+                        // concurrently.
+                        #[allow(clippy::cast_ref_to_mut)]
+                        let mutable = unsafe { &mut *(&$id as *const _ as *mut $t) };
+                        // SAFETY: It's a shared static, so it cannot move.
+                        let pinned = unsafe { core::pin::Pin::new_unchecked(mutable) };
+                        $crate::init_with_lockdep!(pinned, stringify!($id));
+                    }
+                    constructor
+                };
+                $crate::init_static_sync!(@call_new $t, $($value)?)
+            };
+        )*
+    };
+    (@call_new $t:ty, $value:expr) => {{
+        let v = $value;
+        // SAFETY: the initialisation function is called by the constructor above.
+        unsafe { <$t>::new(v) }
+    }};
+    (@call_new $t:ty,) => {
+        // SAFETY: the initialisation function is called by the constructor above.
+        unsafe { <$t>::new() }
+    };
+}
+
 /// Reschedules the caller's task if needed.
 pub fn cond_resched() -> bool {
     // SAFETY: No arguments, reschedules `current` if needed.

rust/kernel/sync/mutex.rs

@@ -53,7 +53,7 @@ impl<T> Mutex<T> {
     /// # Safety
     ///
     /// The caller must call [`Mutex::init_lock`] before using the mutex.
-    pub unsafe fn new(t: T) -> Self {
+    pub const unsafe fn new(t: T) -> Self {
         Self {
             mutex: Opaque::uninit(),
             data: UnsafeCell::new(t),

rust/kernel/sync/revocable_mutex.rs

@@ -80,7 +80,7 @@ impl<T> RevocableMutex<T> {
     /// # Safety
     ///
     /// The caller must call [`RevocableMutex::init`] before using the revocable mutex.
-    pub unsafe fn new(data: T) -> Self {
+    pub const unsafe fn new(data: T) -> Self {
         Self {
             // SAFETY: The safety requirements of this function require that `RevocableMutex::init`
             // be called before the returned object can be used. Mutex initialisation is called

rust/kernel/sync/spinlock.rs

@@ -96,7 +96,7 @@ impl<T> SpinLock<T> {
     /// # Safety
     ///
     /// The caller must call [`SpinLock::init_lock`] before using the spinlock.
-    pub unsafe fn new(t: T) -> Self {
+    pub const unsafe fn new(t: T) -> Self {
         Self {
             spin_lock: Opaque::uninit(),
             data: UnsafeCell::new(t),

rust/kernel/types.rs

@@ -283,7 +283,7 @@ impl<T> Opaque<T> {
     }
 
     /// Creates an uninitialised value.
-    pub fn uninit() -> Self {
+    pub const fn uninit() -> Self {
         Self(MaybeUninit::uninit())
     }