1// SPDX-License-Identifier: GPL-2.0
  2
  3//! Generic kernel lock and guard.
  4//!
  5//! It contains a generic Rust lock and guard that allow for different backends (e.g., mutexes,
  6//! spinlocks, raw spinlocks) to be provided with minimal effort.
  7
  8use super::LockClassKey;
  9use crate::{bindings, init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
 10use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
 11use macros::pin_data;
 12
 13pub mod mutex;
 14pub mod spinlock;
 15
 16/// The "backend" of a lock.
 17///
 18/// It is the actual implementation of the lock, without the need to repeat patterns used in all
 19/// locks.
 20///
 21/// # Safety
 22///
 23/// - Implementers must ensure that only one thread/CPU may access the protected data once the lock
 24///   is owned, that is, between calls to [`lock`] and [`unlock`].
 25/// - Implementers must also ensure that [`relock`] uses the same locking method as the original
 26///   lock operation.
 27///
 28/// [`lock`]: Backend::lock
 29/// [`unlock`]: Backend::unlock
 30/// [`relock`]: Backend::relock
 31pub unsafe trait Backend {
 32    /// The state required by the lock.
 33    type State;
 34
 35    /// The state required to be kept between [`lock`] and [`unlock`].
 36    ///
 37    /// [`lock`]: Backend::lock
 38    /// [`unlock`]: Backend::unlock
 39    type GuardState;
 40
 41    /// Initialises the lock.
 42    ///
 43    /// # Safety
 44    ///
 45    /// `ptr` must be valid for write for the duration of the call, while `name` and `key` must
 46    /// remain valid for read indefinitely.
 47    unsafe fn init(
 48        ptr: *mut Self::State,
 49        name: *const core::ffi::c_char,
 50        key: *mut bindings::lock_class_key,
 51    );
 52
 53    /// Acquires the lock, making the caller its owner.
 54    ///
 55    /// # Safety
 56    ///
 57    /// Callers must ensure that [`Backend::init`] has been previously called.
 58    #[must_use]
 59    unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState;
 60
 61    /// Releases the lock, giving up its ownership.
 62    ///
 63    /// # Safety
 64    ///
 65    /// It must only be called by the current owner of the lock.
 66    unsafe fn unlock(ptr: *mut Self::State, guard_state: &Self::GuardState);
 67
 68    /// Reacquires the lock, making the caller its owner.
 69    ///
 70    /// # Safety
 71    ///
 72    /// Callers must ensure that `guard_state` comes from a previous call to [`Backend::lock`] (or
 73    /// variant) that has been unlocked with [`Backend::unlock`] and will be relocked now.
 74    unsafe fn relock(ptr: *mut Self::State, guard_state: &mut Self::GuardState) {
 75        // SAFETY: The safety requirements ensure that the lock is initialised.
 76        *guard_state = unsafe { Self::lock(ptr) };
 77    }
 78}
 79
 80/// A mutual exclusion primitive.
 81///
 82/// Exposes one of the kernel locking primitives. Which one is exposed depends on the lock
 83/// [`Backend`] specified as the generic parameter `B`.
 84#[pin_data]
 85pub struct Lock<T: ?Sized, B: Backend> {
 86    /// The kernel lock object.
 87    #[pin]
 88    state: Opaque<B::State>,
 89
 90    /// Some locks are known to be self-referential (e.g., mutexes), while others are architecture
 91    /// or config defined (e.g., spinlocks). So we conservatively require them to be pinned in case
 92    /// some architecture uses self-references now or in the future.
 93    #[pin]
 94    _pin: PhantomPinned,
 95
 96    /// The data protected by the lock.
 97    pub(crate) data: UnsafeCell<T>,
 98}
 99
100// SAFETY: `Lock` can be transferred across thread boundaries iff the data it protects can.
101unsafe impl<T: ?Sized + Send, B: Backend> Send for Lock<T, B> {}
102
103// SAFETY: `Lock` serialises the interior mutability it provides, so it is `Sync` as long as the
104// data it protects is `Send`.
105unsafe impl<T: ?Sized + Send, B: Backend> Sync for Lock<T, B> {}
106
107impl<T, B: Backend> Lock<T, B> {
108    /// Constructs a new lock initialiser.
109    pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
110        pin_init!(Self {
111            data: UnsafeCell::new(t),
112            _pin: PhantomPinned,
113            // SAFETY: `slot` is valid while the closure is called and both `name` and `key` have
114            // static lifetimes so they live indefinitely.
115            state <- Opaque::ffi_init(|slot| unsafe {
116                B::init(slot, name.as_char_ptr(), key.as_ptr())
117            }),
118        })
119    }
120}
121
122impl<T: ?Sized, B: Backend> Lock<T, B> {
123    /// Acquires the lock and gives the caller access to the data protected by it.
124    pub fn lock(&self) -> Guard<'_, T, B> {
125        // SAFETY: The constructor of the type calls `init`, so the existence of the object proves
126        // that `init` was called.
127        let state = unsafe { B::lock(self.state.get()) };
128        // SAFETY: The lock was just acquired.
129        unsafe { Guard::new(self, state) }
130    }
131}
132
133/// A lock guard.
134///
135/// Allows mutual exclusion primitives that implement the [`Backend`] trait to automatically unlock
136/// when a guard goes out of scope. It also provides a safe and convenient way to access the data
137/// protected by the lock.
138#[must_use = "the lock unlocks immediately when the guard is unused"]
139pub struct Guard<'a, T: ?Sized, B: Backend> {
140    pub(crate) lock: &'a Lock<T, B>,
141    pub(crate) state: B::GuardState,
142    _not_send: PhantomData<*mut ()>,
143}
144
145// SAFETY: `Guard` is sync when the data protected by the lock is also sync.
146unsafe impl<T: Sync + ?Sized, B: Backend> Sync for Guard<'_, T, B> {}
147
148impl<T: ?Sized, B: Backend> Guard<'_, T, B> {
149    pub(crate) fn do_unlocked<U>(&mut self, cb: impl FnOnce() -> U) -> U {
150        // SAFETY: The caller owns the lock, so it is safe to unlock it.
151        unsafe { B::unlock(self.lock.state.get(), &self.state) };
152
153        // SAFETY: The lock was just unlocked above and is being relocked now.
154        let _relock =
155            ScopeGuard::new(|| unsafe { B::relock(self.lock.state.get(), &mut self.state) });
156
157        cb()
158    }
159}
160
161impl<T: ?Sized, B: Backend> core::ops::Deref for Guard<'_, T, B> {
162    type Target = T;
163
164    fn deref(&self) -> &Self::Target {
165        // SAFETY: The caller owns the lock, so it is safe to deref the protected data.
166        unsafe { &*self.lock.data.get() }
167    }
168}
169
170impl<T: ?Sized, B: Backend> core::ops::DerefMut for Guard<'_, T, B> {
171    fn deref_mut(&mut self) -> &mut Self::Target {
172        // SAFETY: The caller owns the lock, so it is safe to deref the protected data.
173        unsafe { &mut *self.lock.data.get() }
174    }
175}
176
177impl<T: ?Sized, B: Backend> Drop for Guard<'_, T, B> {
178    fn drop(&mut self) {
179        // SAFETY: The caller owns the lock, so it is safe to unlock it.
180        unsafe { B::unlock(self.lock.state.get(), &self.state) };
181    }
182}
183
184impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
185    /// Constructs a new immutable lock guard.
186    ///
187    /// # Safety
188    ///
189    /// The caller must ensure that it owns the lock.
190    pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
191        Self {
192            lock,
193            state,
194            _not_send: PhantomData,
195        }
196    }
197}