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::{
 10    init::PinInit,
 11    pin_init,
 12    str::CStr,
 13    types::{NotThreadSafe, Opaque, ScopeGuard},
 14};
 15use core::{cell::UnsafeCell, marker::PhantomPinned};
 16use macros::pin_data;
 17
 18pub mod mutex;
 19pub mod spinlock;
 20
 21pub(super) mod global;
 22pub use global::{GlobalGuard, GlobalLock, GlobalLockBackend, GlobalLockedBy};
 23
 24/// The "backend" of a lock.
 25///
 26/// It is the actual implementation of the lock, without the need to repeat patterns used in all
 27/// locks.
 28///
 29/// # Safety
 30///
 31/// - Implementers must ensure that only one thread/CPU may access the protected data once the lock
 32///   is owned, that is, between calls to [`lock`] and [`unlock`].
 33/// - Implementers must also ensure that [`relock`] uses the same locking method as the original
 34///   lock operation.
 35///
 36/// [`lock`]: Backend::lock
 37/// [`unlock`]: Backend::unlock
 38/// [`relock`]: Backend::relock
 39pub unsafe trait Backend {
 40    /// The state required by the lock.
 41    type State;
 42
 43    /// The state required to be kept between [`lock`] and [`unlock`].
 44    ///
 45    /// [`lock`]: Backend::lock
 46    /// [`unlock`]: Backend::unlock
 47    type GuardState;
 48
 49    /// Initialises the lock.
 50    ///
 51    /// # Safety
 52    ///
 53    /// `ptr` must be valid for write for the duration of the call, while `name` and `key` must
 54    /// remain valid for read indefinitely.
 55    unsafe fn init(
 56        ptr: *mut Self::State,
 57        name: *const crate::ffi::c_char,
 58        key: *mut bindings::lock_class_key,
 59    );
 60
 61    /// Acquires the lock, making the caller its owner.
 62    ///
 63    /// # Safety
 64    ///
 65    /// Callers must ensure that [`Backend::init`] has been previously called.
 66    #[must_use]
 67    unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState;
 68
 69    /// Tries to acquire the lock.
 70    ///
 71    /// # Safety
 72    ///
 73    /// Callers must ensure that [`Backend::init`] has been previously called.
 74    unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState>;
 75
 76    /// Releases the lock, giving up its ownership.
 77    ///
 78    /// # Safety
 79    ///
 80    /// It must only be called by the current owner of the lock.
 81    unsafe fn unlock(ptr: *mut Self::State, guard_state: &Self::GuardState);
 82
 83    /// Reacquires the lock, making the caller its owner.
 84    ///
 85    /// # Safety
 86    ///
 87    /// Callers must ensure that `guard_state` comes from a previous call to [`Backend::lock`] (or
 88    /// variant) that has been unlocked with [`Backend::unlock`] and will be relocked now.
 89    unsafe fn relock(ptr: *mut Self::State, guard_state: &mut Self::GuardState) {
 90        // SAFETY: The safety requirements ensure that the lock is initialised.
 91        *guard_state = unsafe { Self::lock(ptr) };
 92    }
 93}
 94
 95/// A mutual exclusion primitive.
 96///
 97/// Exposes one of the kernel locking primitives. Which one is exposed depends on the lock
 98/// [`Backend`] specified as the generic parameter `B`.
 99#[pin_data]
100pub struct Lock<T: ?Sized, B: Backend> {
101    /// The kernel lock object.
102    #[pin]
103    state: Opaque<B::State>,
104
105    /// Some locks are known to be self-referential (e.g., mutexes), while others are architecture
106    /// or config defined (e.g., spinlocks). So we conservatively require them to be pinned in case
107    /// some architecture uses self-references now or in the future.
108    #[pin]
109    _pin: PhantomPinned,
110
111    /// The data protected by the lock.
112    pub(crate) data: UnsafeCell<T>,
113}
114
115// SAFETY: `Lock` can be transferred across thread boundaries iff the data it protects can.
116unsafe impl<T: ?Sized + Send, B: Backend> Send for Lock<T, B> {}
117
118// SAFETY: `Lock` serialises the interior mutability it provides, so it is `Sync` as long as the
119// data it protects is `Send`.
120unsafe impl<T: ?Sized + Send, B: Backend> Sync for Lock<T, B> {}
121
122impl<T, B: Backend> Lock<T, B> {
123    /// Constructs a new lock initialiser.
124    pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
125        pin_init!(Self {
126            data: UnsafeCell::new(t),
127            _pin: PhantomPinned,
128            // SAFETY: `slot` is valid while the closure is called and both `name` and `key` have
129            // static lifetimes so they live indefinitely.
130            state <- Opaque::ffi_init(|slot| unsafe {
131                B::init(slot, name.as_char_ptr(), key.as_ptr())
132            }),
133        })
134    }
135}
136
137impl<T: ?Sized, B: Backend> Lock<T, B> {
138    /// Acquires the lock and gives the caller access to the data protected by it.
139    pub fn lock(&self) -> Guard<'_, T, B> {
140        // SAFETY: The constructor of the type calls `init`, so the existence of the object proves
141        // that `init` was called.
142        let state = unsafe { B::lock(self.state.get()) };
143        // SAFETY: The lock was just acquired.
144        unsafe { Guard::new(self, state) }
145    }
146
147    /// Tries to acquire the lock.
148    ///
149    /// Returns a guard that can be used to access the data protected by the lock if successful.
150    pub fn try_lock(&self) -> Option<Guard<'_, T, B>> {
151        // SAFETY: The constructor of the type calls `init`, so the existence of the object proves
152        // that `init` was called.
153        unsafe { B::try_lock(self.state.get()).map(|state| Guard::new(self, state)) }
154    }
155}
156
157/// A lock guard.
158///
159/// Allows mutual exclusion primitives that implement the [`Backend`] trait to automatically unlock
160/// when a guard goes out of scope. It also provides a safe and convenient way to access the data
161/// protected by the lock.
162#[must_use = "the lock unlocks immediately when the guard is unused"]
163pub struct Guard<'a, T: ?Sized, B: Backend> {
164    pub(crate) lock: &'a Lock<T, B>,
165    pub(crate) state: B::GuardState,
166    _not_send: NotThreadSafe,
167}
168
169// SAFETY: `Guard` is sync when the data protected by the lock is also sync.
170unsafe impl<T: Sync + ?Sized, B: Backend> Sync for Guard<'_, T, B> {}
171
172impl<T: ?Sized, B: Backend> Guard<'_, T, B> {
173    pub(crate) fn do_unlocked<U>(&mut self, cb: impl FnOnce() -> U) -> U {
174        // SAFETY: The caller owns the lock, so it is safe to unlock it.
175        unsafe { B::unlock(self.lock.state.get(), &self.state) };
176
177        let _relock = ScopeGuard::new(||
178                // SAFETY: The lock was just unlocked above and is being relocked now.
179                unsafe { B::relock(self.lock.state.get(), &mut self.state) });
180
181        cb()
182    }
183}
184
185impl<T: ?Sized, B: Backend> core::ops::Deref for Guard<'_, T, B> {
186    type Target = T;
187
188    fn deref(&self) -> &Self::Target {
189        // SAFETY: The caller owns the lock, so it is safe to deref the protected data.
190        unsafe { &*self.lock.data.get() }
191    }
192}
193
194impl<T: ?Sized, B: Backend> core::ops::DerefMut for Guard<'_, T, B> {
195    fn deref_mut(&mut self) -> &mut Self::Target {
196        // SAFETY: The caller owns the lock, so it is safe to deref the protected data.
197        unsafe { &mut *self.lock.data.get() }
198    }
199}
200
201impl<T: ?Sized, B: Backend> Drop for Guard<'_, T, B> {
202    fn drop(&mut self) {
203        // SAFETY: The caller owns the lock, so it is safe to unlock it.
204        unsafe { B::unlock(self.lock.state.get(), &self.state) };
205    }
206}
207
208impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
209    /// Constructs a new immutable lock guard.
210    ///
211    /// # Safety
212    ///
213    /// The caller must ensure that it owns the lock.
214    pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
215        Self {
216            lock,
217            state,
218            _not_send: NotThreadSafe,
219        }
220    }
221}