Add sync module with a mutex implementation.

drivers/char/rust_example.rs

@@ -9,7 +9,7 @@
 use alloc::boxed::Box;
 use core::pin::Pin;
 use kernel::prelude::*;
-use kernel::{chrdev, cstr, file_operations::FileOperations, miscdev};
+use kernel::{chrdev, cstr, file_operations::FileOperations, miscdev, mutex_init, sync::Mutex};
 
 module! {
     type: RustExample,
@@ -74,6 +74,15 @@ impl KernelModule for RustExample {
             println!("  my_usize:   {}", my_usize.read(&lock));
         }
 
+        // Test mutexes.
+        {
+            // SAFETY: `init` is called below.
+            let data = Pin::from(Box::try_new(unsafe { Mutex::new(0) })?);
+            mutex_init!(data.as_ref(), "RustExample::init::data");
+            *data.lock() = 10;
+            println!("Value: {}", *data.lock());
+        }
+
         // Including this large variable on the stack will trigger
         // stack probing on the supported archs.
         // This will verify that stack probing does not lead to

rust/kernel/lib.rs

@@ -41,6 +41,7 @@ pub mod prelude;
 pub mod printk;
 pub mod random;
 mod static_assert;
+pub mod sync;
 
 #[cfg(CONFIG_SYSCTL)]
 pub mod sysctl;

rust/kernel/sync/guard.rs

@@ -0,0 +1,86 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! A generic lock guard and trait.
+//!
+//! This module contains a lock guard that can be used with any locking primitive that implements
+//! the ([`Lock`]) trait. It also contains the definition of the trait, which can be leveraged by
+//! other constructs to work on generic locking primitives.
+
+/// Allows mutual exclusion primitives that implement the [`Lock`] trait to automatically unlock
+/// when a guard goes out of scope. It also provides a safe and convenient way to access the data
+/// protected by the lock.
+#[must_use = "the lock unlocks immediately when the guard is unused"]
+pub struct Guard<'a, L: Lock + ?Sized> {
+    pub(crate) lock: &'a L,
+}
+
+// SAFETY: `Guard` is sync when the data protected by the lock is also sync. This is more
+// conservative than the default compiler implementation; more details can be found on
+// https://github.com/rust-lang/rust/issues/41622 -- it refers to `MutexGuard` from the standard
+// library.
+unsafe impl<L> Sync for Guard<'_, L>
+where
+    L: Lock + ?Sized,
+    L::Inner: Sync,
+{
+}
+
+impl<L: Lock + ?Sized> core::ops::Deref for Guard<'_, L> {
+    type Target = L::Inner;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The caller owns the lock, so it is safe to deref the protected data.
+        unsafe { &*self.lock.locked_data().get() }
+    }
+}
+
+impl<L: Lock + ?Sized> core::ops::DerefMut for Guard<'_, L> {
+    fn deref_mut(&mut self) -> &mut L::Inner {
+        // SAFETY: The caller owns the lock, so it is safe to deref the protected data.
+        unsafe { &mut *self.lock.locked_data().get() }
+    }
+}
+
+impl<L: Lock + ?Sized> Drop for Guard<'_, L> {
+    fn drop(&mut self) {
+        // SAFETY: The caller owns the lock, so it is safe to unlock it.
+        unsafe { self.lock.unlock() };
+    }
+}
+
+impl<'a, L: Lock + ?Sized> Guard<'a, L> {
+    /// Constructs a new lock guard.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that it owns the lock.
+    pub(crate) unsafe fn new(lock: &'a L) -> Self {
+        Self { lock }
+    }
+}
+
+/// A generic mutual exclusion primitive.
+///
+/// [`Guard`] is written such that any mutual exclusion primitive that can implement this trait can
+/// also benefit from having an automatic way to unlock itself.
+pub trait Lock {
+    /// The type of the data protected by the lock.
+    type Inner: ?Sized;
+
+    /// Acquires the lock, making the caller its owner.
+    fn lock_noguard(&self);
+
+    /// Releases the lock, giving up ownership of the lock.
+    ///
+    /// # Safety
+    ///
+    /// It must only be called by the current owner of the lock.
+    unsafe fn unlock(&self);
+
+    /// Returns the data protected by the lock.
+    ///
+    /// # Safety
+    ///
+    /// It must only be called by the current owner of the lock.
+    unsafe fn locked_data(&self) -> &core::cell::UnsafeCell<Self::Inner>;
+}

rust/kernel/sync/mod.rs

@@ -0,0 +1,59 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Synchronisation primitives.
+//!
+//! This module contains the kernel APIs related to synchronisation that have been ported or
+//! wrapped for usage by Rust code in the kernel and is shared by all of them.
+//!
+//! # Example
+//!
+//!```
+//! fn test() {
+//!     // SAFETY: `init` is called below.
+//!     let data = alloc::sync::Arc::pin(unsafe{ Mutex::new(0) });
+//!     mutex_init!(data.as_ref(), "test::data");
+//!     *data.lock() = 10;
+//!     kernel::println!("{}", *data.lock());
+//! }
+//! ```
+
+use crate::{bindings, CStr};
+use core::pin::Pin;
+
+mod guard;
+mod mutex;
+
+pub use guard::{Guard, Lock};
+pub use mutex::Mutex;
+
+/// Safely initialises an object that has an `init` function that takes a name and a lock class as
+/// arguments, for example, [`Mutex`].
+#[doc(hidden)]
+#[macro_export]
+macro_rules! init_with_lockdep {
+    ($obj:expr, $name:literal) => {{
+        static mut CLASS: core::mem::MaybeUninit<$crate::bindings::lock_class_key> =
+            core::mem::MaybeUninit::uninit();
+        // SAFETY: `CLASS` is never used by Rust code directly; the kernel may change it though.
+        #[allow(unused_unsafe)]
+        unsafe {
+            $crate::sync::NeedsLockClass::init($obj, $crate::cstr!($name), CLASS.as_mut_ptr())
+        };
+    }};
+}
+
+/// A trait for types that need a lock class during initialisation.
+///
+/// Implementers of this trait benefit from the [`init_with_lockdep`] macro that generates a new
+/// class for each initialisation call site.
+pub trait NeedsLockClass {
+    /// Initialises the type instance so that it can be safely used.
+    ///
+    /// Callers are encouraged to use the [`init_with_lockdep`] macro as it automatically creates a
+    /// new lock class on each usage.
+    ///
+    /// # Safety
+    ///
+    /// `key` must point to a valid memory location as it will be used by the kernel.
+    unsafe fn init(self: Pin<&Self>, name: CStr<'static>, key: *mut bindings::lock_class_key);
+}

rust/kernel/sync/mutex.rs

@@ -0,0 +1,101 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! A kernel mutex.
+//!
+//! This module allows Rust code to use the kernel's [`struct mutex`].
+
+use super::{Guard, Lock, NeedsLockClass};
+use crate::{bindings, CStr};
+use core::{cell::UnsafeCell, marker::PhantomPinned, pin::Pin};
+
+/// Safely initialises a [`Mutex`] with the given name, generating a new lock class.
+#[macro_export]
+macro_rules! mutex_init {
+    ($mutex:expr, $name:literal) => {
+        $crate::init_with_lockdep!($mutex, $name)
+    };
+}
+
+/// Exposes the kernel's [`struct mutex`]. When multiple threads attempt to lock the same mutex,
+/// only one at a time is allowed to progress, the others will block (sleep) until the mutex is
+/// unlocked, at which point another thread will be allowed to wake up and make progress.
+///
+/// A [`Mutex`] must first be initialised with a call to [`Mutex::init`] before it can be used. The
+/// [`mutex_init`] macro is provided to automatically assign a new lock class to a mutex instance.
+///
+/// Since it may block, [`Mutex`] needs to be used with care in atomic contexts.
+///
+/// [`struct mutex`]: ../../../include/linux/mutex.h
+pub struct Mutex<T: ?Sized> {
+    /// The kernel `struct mutex` object.
+    mutex: UnsafeCell<bindings::mutex>,
+
+    /// A mutex needs to be pinned because it contains a [`struct list_head`] that is
+    /// self-referential, so it cannot be safely moved once it is initialised.
+    _pin: PhantomPinned,
+
+    /// The data protected by the mutex.
+    data: UnsafeCell<T>,
+}
+
+// SAFETY: `Mutex` can be transferred across thread boundaries iff the data it protects can.
+unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
+
+// SAFETY: `Mutex` serialises the interior mutability it provides, so it is `Sync` as long as the
+// data it protects is `Send`.
+unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
+
+impl<T> Mutex<T> {
+    /// Constructs a new mutex.
+    ///
+    /// # Safety
+    ///
+    /// The caller must call [`Mutex::init`] before using the mutex.
+    pub unsafe fn new(t: T) -> Self {
+        Self {
+            mutex: UnsafeCell::new(bindings::mutex::default()),
+            data: UnsafeCell::new(t),
+            _pin: PhantomPinned,
+        }
+    }
+}
+
+impl<T: ?Sized> Mutex<T> {
+    /// Locks the mutex and gives the caller access to the data protected by it. Only one thread at
+    /// a time is allowed to access the protected data.
+    pub fn lock(&self) -> Guard<Self> {
+        self.lock_noguard();
+        // SAFETY: The mutex was just acquired.
+        unsafe { Guard::new(self) }
+    }
+}
+
+impl<T: ?Sized> NeedsLockClass for Mutex<T> {
+    unsafe fn init(self: Pin<&Self>, name: CStr<'static>, key: *mut bindings::lock_class_key) {
+        bindings::__mutex_init(self.mutex.get(), name.as_ptr() as _, key);
+    }
+}
+
+impl<T: ?Sized> Lock for Mutex<T> {
+    type Inner = T;
+
+    #[cfg(not(CONFIG_DEBUG_LOCK_ALLOC))]
+    fn lock_noguard(&self) {
+        // SAFETY: `mutex` points to valid memory.
+        unsafe { bindings::mutex_lock(self.mutex.get()) };
+    }
+
+    #[cfg(CONFIG_DEBUG_LOCK_ALLOC)]
+    fn lock_noguard(&self) {
+        // SAFETY: `mutex` points to valid memory.
+        unsafe { bindings::mutex_lock_nested(self.mutex.get(), 0) };
+    }
+
+    unsafe fn unlock(&self) {
+        bindings::mutex_unlock(self.mutex.get());
+    }
+
+    unsafe fn locked_data(&self) -> &UnsafeCell<T> {
+        &self.data
+    }
+}