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// SPDX-License-Identifier: GPL-2.0
//! Generic kernel lock and guard.
//!
//! It contains a generic Rust lock and guard that allow for different backends (e.g., mutexes,
//! spinlocks, raw spinlocks) to be provided with minimal effort.
use super::LockClassKey;
use crate::{
init::PinInit,
pin_init,
str::CStr,
types::{NotThreadSafe, Opaque, ScopeGuard},
};
use core::{cell::UnsafeCell, marker::PhantomPinned};
use macros::pin_data;
pub mod mutex;
pub mod spinlock;
/// The "backend" of a lock.
///
/// It is the actual implementation of the lock, without the need to repeat patterns used in all
/// locks.
///
/// # Safety
///
/// - Implementers must ensure that only one thread/CPU may access the protected data once the lock
/// is owned, that is, between calls to [`lock`] and [`unlock`].
/// - Implementers must also ensure that [`relock`] uses the same locking method as the original
/// lock operation.
///
/// [`lock`]: Backend::lock
/// [`unlock`]: Backend::unlock
/// [`relock`]: Backend::relock
pub unsafe trait Backend {
/// The state required by the lock.
type State;
/// The state required to be kept between [`lock`] and [`unlock`].
///
/// [`lock`]: Backend::lock
/// [`unlock`]: Backend::unlock
type GuardState;
/// Initialises the lock.
///
/// # Safety
///
/// `ptr` must be valid for write for the duration of the call, while `name` and `key` must
/// remain valid for read indefinitely.
unsafe fn init(
ptr: *mut Self::State,
name: *const core::ffi::c_char,
key: *mut bindings::lock_class_key,
);
/// Acquires the lock, making the caller its owner.
///
/// # Safety
///
/// Callers must ensure that [`Backend::init`] has been previously called.
#[must_use]
unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState;
/// Releases the lock, giving up its ownership.
///
/// # Safety
///
/// It must only be called by the current owner of the lock.
unsafe fn unlock(ptr: *mut Self::State, guard_state: &Self::GuardState);
/// Reacquires the lock, making the caller its owner.
///
/// # Safety
///
/// Callers must ensure that `guard_state` comes from a previous call to [`Backend::lock`] (or
/// variant) that has been unlocked with [`Backend::unlock`] and will be relocked now.
unsafe fn relock(ptr: *mut Self::State, guard_state: &mut Self::GuardState) {
// SAFETY: The safety requirements ensure that the lock is initialised.
*guard_state = unsafe { Self::lock(ptr) };
}
}
/// A mutual exclusion primitive.
///
/// Exposes one of the kernel locking primitives. Which one is exposed depends on the lock
/// [`Backend`] specified as the generic parameter `B`.
#[pin_data]
pub struct Lock<T: ?Sized, B: Backend> {
/// The kernel lock object.
#[pin]
state: Opaque<B::State>,
/// Some locks are known to be self-referential (e.g., mutexes), while others are architecture
/// or config defined (e.g., spinlocks). So we conservatively require them to be pinned in case
/// some architecture uses self-references now or in the future.
#[pin]
_pin: PhantomPinned,
/// The data protected by the lock.
pub(crate) data: UnsafeCell<T>,
}
// SAFETY: `Lock` can be transferred across thread boundaries iff the data it protects can.
unsafe impl<T: ?Sized + Send, B: Backend> Send for Lock<T, B> {}
// SAFETY: `Lock` serialises the interior mutability it provides, so it is `Sync` as long as the
// data it protects is `Send`.
unsafe impl<T: ?Sized + Send, B: Backend> Sync for Lock<T, B> {}
impl<T, B: Backend> Lock<T, B> {
/// Constructs a new lock initialiser.
pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
pin_init!(Self {
data: UnsafeCell::new(t),
_pin: PhantomPinned,
// SAFETY: `slot` is valid while the closure is called and both `name` and `key` have
// static lifetimes so they live indefinitely.
state <- Opaque::ffi_init(|slot| unsafe {
B::init(slot, name.as_char_ptr(), key.as_ptr())
}),
})
}
}
impl<T: ?Sized, B: Backend> Lock<T, B> {
/// Acquires the lock and gives the caller access to the data protected by it.
pub fn lock(&self) -> Guard<'_, T, B> {
// SAFETY: The constructor of the type calls `init`, so the existence of the object proves
// that `init` was called.
let state = unsafe { B::lock(self.state.get()) };
// SAFETY: The lock was just acquired.
unsafe { Guard::new(self, state) }
}
}
/// A lock guard.
///
/// Allows mutual exclusion primitives that implement the [`Backend`] 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, T: ?Sized, B: Backend> {
pub(crate) lock: &'a Lock<T, B>,
pub(crate) state: B::GuardState,
_not_send: NotThreadSafe,
}
// SAFETY: `Guard` is sync when the data protected by the lock is also sync.
unsafe impl<T: Sync + ?Sized, B: Backend> Sync for Guard<'_, T, B> {}
impl<T: ?Sized, B: Backend> Guard<'_, T, B> {
pub(crate) fn do_unlocked<U>(&mut self, cb: impl FnOnce() -> U) -> U {
// SAFETY: The caller owns the lock, so it is safe to unlock it.
unsafe { B::unlock(self.lock.state.get(), &self.state) };
// SAFETY: The lock was just unlocked above and is being relocked now.
let _relock =
ScopeGuard::new(|| unsafe { B::relock(self.lock.state.get(), &mut self.state) });
cb()
}
}
impl<T: ?Sized, B: Backend> core::ops::Deref for Guard<'_, T, B> {
type Target = T;
fn deref(&self) -> &Self::Target {
// SAFETY: The caller owns the lock, so it is safe to deref the protected data.
unsafe { &*self.lock.data.get() }
}
}
impl<T: ?Sized, B: Backend> core::ops::DerefMut for Guard<'_, T, B> {
fn deref_mut(&mut self) -> &mut Self::Target {
// SAFETY: The caller owns the lock, so it is safe to deref the protected data.
unsafe { &mut *self.lock.data.get() }
}
}
impl<T: ?Sized, B: Backend> Drop for Guard<'_, T, B> {
fn drop(&mut self) {
// SAFETY: The caller owns the lock, so it is safe to unlock it.
unsafe { B::unlock(self.lock.state.get(), &self.state) };
}
}
impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
/// Constructs a new immutable lock guard.
///
/// # Safety
///
/// The caller must ensure that it owns the lock.
pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
Self {
lock,
state,
_not_send: NotThreadSafe,
}
}
}
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