rust: lock: introduce SpinLock

This is the `spinlock_t` lock backend and allows Rust code to use the
kernel spinlock idiomatically.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20230419174426.132207-1-wedsonaf@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/helpers.c

@@ -23,6 +23,7 @@
 #include <linux/err.h>
 #include <linux/refcount.h>
 #include <linux/mutex.h>
+#include <linux/spinlock.h>
 
 __noreturn void rust_helper_BUG(void)
 {
@@ -36,6 +37,29 @@ void rust_helper_mutex_lock(struct mutex *lock)
 }
 EXPORT_SYMBOL_GPL(rust_helper_mutex_lock);
 
+void rust_helper___spin_lock_init(spinlock_t *lock, const char *name,
+				  struct lock_class_key *key)
+{
+#ifdef CONFIG_DEBUG_SPINLOCK
+	__raw_spin_lock_init(spinlock_check(lock), name, key, LD_WAIT_CONFIG);
+#else
+	spin_lock_init(lock);
+#endif
+}
+EXPORT_SYMBOL_GPL(rust_helper___spin_lock_init);
+
+void rust_helper_spin_lock(spinlock_t *lock)
+{
+	spin_lock(lock);
+}
+EXPORT_SYMBOL_GPL(rust_helper_spin_lock);
+
+void rust_helper_spin_unlock(spinlock_t *lock)
+{
+	spin_unlock(lock);
+}
+EXPORT_SYMBOL_GPL(rust_helper_spin_unlock);
+
 refcount_t rust_helper_REFCOUNT_INIT(int n)
 {
 	return (refcount_t)REFCOUNT_INIT(n);

rust/kernel/sync.rs

@@ -11,7 +11,7 @@
 pub mod lock;
 
 pub use arc::{Arc, ArcBorrow, UniqueArc};
-pub use lock::mutex::Mutex;
+pub use lock::{mutex::Mutex, spinlock::SpinLock};
 
 /// Represents a lockdep class. It's a wrapper around C's `lock_class_key`.
 #[repr(transparent)]

rust/kernel/sync/lock.rs

@@ -11,6 +11,7 @@
 use macros::pin_data;
 
 pub mod mutex;
+pub mod spinlock;
 
 /// The "backend" of a lock.
 ///

rust/kernel/sync/lock/spinlock.rs

@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! A kernel spinlock.
+//!
+//! This module allows Rust code to use the kernel's `spinlock_t`.
+
+use crate::bindings;
+
+/// Creates a [`SpinLock`] initialiser with the given name and a newly-created lock class.
+///
+/// It uses the name if one is given, otherwise it generates one based on the file name and line
+/// number.
+#[macro_export]
+macro_rules! new_spinlock {
+    ($inner:expr $(, $name:literal)? $(,)?) => {
+        $crate::sync::SpinLock::new(
+            $inner, $crate::optional_name!($($name)?), $crate::static_lock_class!())
+    };
+}
+
+/// A spinlock.
+///
+/// Exposes the kernel's [`spinlock_t`]. When multiple CPUs attempt to lock the same spinlock, only
+/// one at a time is allowed to progress, the others will block (spinning) until the spinlock is
+/// unlocked, at which point another CPU will be allowed to make progress.
+///
+/// Instances of [`SpinLock`] need a lock class and to be pinned. The recommended way to create such
+/// instances is with the [`pin_init`](crate::pin_init) and [`new_spinlock`] macros.
+///
+/// # Examples
+///
+/// The following example shows how to declare, allocate and initialise a struct (`Example`) that
+/// contains an inner struct (`Inner`) that is protected by a spinlock.
+///
+/// ```
+/// use kernel::{init::InPlaceInit, init::PinInit, new_spinlock, pin_init, sync::SpinLock};
+///
+/// struct Inner {
+///     a: u32,
+///     b: u32,
+/// }
+///
+/// #[pin_data]
+/// struct Example {
+///     c: u32,
+///     #[pin]
+///     d: SpinLock<Inner>,
+/// }
+///
+/// impl Example {
+///     fn new() -> impl PinInit<Self> {
+///         pin_init!(Self {
+///             c: 10,
+///             d <- new_spinlock!(Inner { a: 20, b: 30 }),
+///         })
+///     }
+/// }
+///
+/// // Allocate a boxed `Example`.
+/// let e = Box::pin_init(Example::new())?;
+/// assert_eq!(e.c, 10);
+/// assert_eq!(e.d.lock().a, 20);
+/// assert_eq!(e.d.lock().b, 30);
+/// ```
+///
+/// The following example shows how to use interior mutability to modify the contents of a struct
+/// protected by a spinlock despite only having a shared reference:
+///
+/// ```
+/// use kernel::sync::SpinLock;
+///
+/// struct Example {
+///     a: u32,
+///     b: u32,
+/// }
+///
+/// fn example(m: &SpinLock<Example>) {
+///     let mut guard = m.lock();
+///     guard.a += 10;
+///     guard.b += 20;
+/// }
+/// ```
+///
+/// [`spinlock_t`]: ../../../../include/linux/spinlock.h
+pub type SpinLock<T> = super::Lock<T, SpinLockBackend>;
+
+/// A kernel `spinlock_t` lock backend.
+pub struct SpinLockBackend;
+
+// SAFETY: The underlying kernel `spinlock_t` object ensures mutual exclusion.
+unsafe impl super::Backend for SpinLockBackend {
+    type State = bindings::spinlock_t;
+    type GuardState = ();
+
+    unsafe fn init(
+        ptr: *mut Self::State,
+        name: *const core::ffi::c_char,
+        key: *mut bindings::lock_class_key,
+    ) {
+        // SAFETY: The safety requirements ensure that `ptr` is valid for writes, and `name` and
+        // `key` are valid for read indefinitely.
+        unsafe { bindings::__spin_lock_init(ptr, name, key) }
+    }
+
+    unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState {
+        // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
+        // memory, and that it has been initialised before.
+        unsafe { bindings::spin_lock(ptr) }
+    }
+
+    unsafe fn unlock(ptr: *mut Self::State, _guard_state: &Self::GuardState) {
+        // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that the
+        // caller is the owner of the mutex.
+        unsafe { bindings::spin_unlock(ptr) }
+    }
+}