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// 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.
use super::{LockClassKey, NeedsLockClass};
use crate::{str::CStr, Bool, False, True};
use core::pin::Pin;
/// 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<I> + ?Sized, I: LockInfo = WriteLock> {
pub(crate) lock: &'a L,
pub(crate) context: L::GuardContext,
}
// 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, I> Sync for Guard<'_, L, I>
where
L: Lock<I> + ?Sized,
L::Inner: Sync,
I: LockInfo,
{
}
impl<L: Lock<I> + ?Sized, I: LockInfo> core::ops::Deref for Guard<'_, L, I> {
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<I> + ?Sized, I: LockInfo<Writable = True>> core::ops::DerefMut for Guard<'_, L, I> {
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.locked_data().get() }
}
}
impl<L: Lock<I> + ?Sized, I: LockInfo> Drop for Guard<'_, L, I> {
fn drop(&mut self) {
// SAFETY: The caller owns the lock, so it is safe to unlock it.
unsafe { self.lock.unlock(&mut self.context) };
}
}
impl<'a, L: Lock<I> + ?Sized, I: LockInfo> Guard<'a, L, I> {
/// Constructs a new immutable lock guard.
///
/// # Safety
///
/// The caller must ensure that it owns the lock.
pub(crate) unsafe fn new(lock: &'a L, context: L::GuardContext) -> Self {
Self { lock, context }
}
}
/// Specifies properties of a lock.
pub trait LockInfo {
/// Determines if the data protected by a lock is writable.
type Writable: Bool;
}
/// A marker for locks that only allow reading.
pub struct ReadLock;
impl LockInfo for ReadLock {
type Writable = False;
}
/// A marker for locks that allow reading and writing.
pub struct WriteLock;
impl LockInfo for WriteLock {
type Writable = True;
}
/// 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.
///
/// # Safety
///
/// - Implementers of this trait with the [`WriteLock`] marker must ensure that only one thread/CPU
/// may access the protected data once the lock is held, that is, between calls to `lock_noguard`
/// and `unlock`.
/// - Implementers of all other markers must ensure that a mutable reference to the protected data
/// is not active in any thread/CPU because at least one shared reference is active between calls
/// to `lock_noguard` and `unlock`.
pub unsafe trait Lock<I: LockInfo = WriteLock> {
/// The type of the data protected by the lock.
type Inner: ?Sized;
/// The type of context, if any, that needs to be stored in the guard.
type GuardContext;
/// Acquires the lock, making the caller its owner.
#[must_use]
fn lock_noguard(&self) -> Self::GuardContext;
/// Reacquires the lock, making the caller its owner.
///
/// The guard context before the last unlock is passed in.
///
/// Locks that don't require this state on relock can simply use the default implementation
/// that calls [`Lock::lock_noguard`].
fn relock(&self, ctx: &mut Self::GuardContext) {
*ctx = self.lock_noguard();
}
/// 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, context: &mut Self::GuardContext);
/// Returns the data protected by the lock.
fn locked_data(&self) -> &core::cell::UnsafeCell<Self::Inner>;
}
/// A creator of instances of a mutual exclusion (lock) primitive.
pub trait LockFactory {
/// The parametrised type of the mutual exclusion primitive that can be created by this factory.
type LockedType<T>;
/// Constructs a new instance of the mutual exclusion primitive.
///
/// # Safety
///
/// The caller must call [`LockIniter::init_lock`] before using the lock.
unsafe fn new_lock<T>(data: T) -> Self::LockedType<T>;
}
/// A lock that can be initialised with a single lock class key.
pub trait LockIniter {
/// Initialises the lock instance so that it can be safely used.
fn init_lock(self: Pin<&mut Self>, name: &'static CStr, key: &'static LockClassKey);
}
impl<L: LockIniter> NeedsLockClass for L {
fn init(
self: Pin<&mut Self>,
name: &'static CStr,
key: &'static LockClassKey,
_: &'static LockClassKey,
) {
self.init_lock(name, key);
}
}
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