// SPDX-License-Identifier: GPL-2.0
//! A wrapper for data protected by a lock that does not wrap it.
use super::{lock::Backend, lock::Lock};
use crate::build_assert;
use core::{cell::UnsafeCell, mem::size_of, ptr};
/// Allows access to some data to be serialised by a lock that does not wrap it.
///
/// In most cases, data protected by a lock is wrapped by the appropriate lock type, e.g.,
/// [`Mutex`] or [`SpinLock`]. [`LockedBy`] is meant for cases when this is not possible.
/// For example, if a container has a lock and some data in the contained elements needs
/// to be protected by the same lock.
///
/// [`LockedBy`] wraps the data in lieu of another locking primitive, and only allows access to it
/// when the caller shows evidence that the 'external' lock is locked. It panics if the evidence
/// refers to the wrong instance of the lock.
///
/// [`Mutex`]: super::Mutex
/// [`SpinLock`]: super::SpinLock
///
/// # Examples
///
/// The following is an example for illustrative purposes: `InnerDirectory::bytes_used` is an
/// aggregate of all `InnerFile::bytes_used` and must be kept consistent; so we wrap `InnerFile` in
/// a `LockedBy` so that it shares a lock with `InnerDirectory`. This allows us to enforce at
/// compile-time that access to `InnerFile` is only granted when an `InnerDirectory` is also
/// locked; we enforce at run time that the right `InnerDirectory` is locked.
///
/// ```
/// use kernel::sync::{LockedBy, Mutex};
///
/// struct InnerFile {
/// bytes_used: u64,
/// }
///
/// struct File {
/// _ino: u32,
/// inner: LockedBy<InnerFile, InnerDirectory>,
/// }
///
/// struct InnerDirectory {
/// /// The sum of the bytes used by all files.
/// bytes_used: u64,
/// _files: Vec<File>,
/// }
///
/// struct Directory {
/// _ino: u32,
/// inner: Mutex<InnerDirectory>,
/// }
///
/// /// Prints `bytes_used` from both the directory and file.
/// fn print_bytes_used(dir: &Directory, file: &File) {
/// let guard = dir.inner.lock();
/// let inner_file = file.inner.access(&guard);
/// pr_info!("{} {}", guard.bytes_used, inner_file.bytes_used);
/// }
///
/// /// Increments `bytes_used` for both the directory and file.
/// fn inc_bytes_used(dir: &Directory, file: &File) {
/// let mut guard = dir.inner.lock();
/// guard.bytes_used += 10;
///
/// let file_inner = file.inner.access_mut(&mut guard);
/// file_inner.bytes_used += 10;
/// }
///
/// /// Creates a new file.
/// fn new_file(ino: u32, dir: &Directory) -> File {
/// File {
/// _ino: ino,
/// inner: LockedBy::new(&dir.inner, InnerFile { bytes_used: 0 }),
/// }
/// }
/// ```
pub struct LockedBy<T: ?Sized, U: ?Sized> {
owner: *const U,
data: UnsafeCell<T>,
}
// SAFETY: `LockedBy` can be transferred across thread boundaries iff the data it protects can.
unsafe impl<T: ?Sized + Send, U: ?Sized> Send for LockedBy<T, U> {}
// SAFETY: If `T` is not `Sync`, then parallel shared access to this `LockedBy` allows you to use
// `access_mut` to hand out `&mut T` on one thread at the time. The requirement that `T: Send` is
// sufficient to allow that.
//
// If `T` is `Sync`, then the `access` method also becomes available, which allows you to obtain
// several `&T` from several threads at once. However, this is okay as `T` is `Sync`.
unsafe impl<T: ?Sized + Send, U: ?Sized> Sync for LockedBy<T, U> {}
impl<T, U> LockedBy<T, U> {
/// Constructs a new instance of [`LockedBy`].
///
/// It stores a raw pointer to the owner that is never dereferenced. It is only used to ensure
/// that the right owner is being used to access the protected data. If the owner is freed, the
/// data becomes inaccessible; if another instance of the owner is allocated *on the same
/// memory location*, the data becomes accessible again: none of this affects memory safety
/// because in any case at most one thread (or CPU) can access the protected data at a time.
pub fn new<B: Backend>(owner: &Lock<U, B>, data: T) -> Self {
build_assert!(
size_of::<Lock<U, B>>() > 0,
"The lock type cannot be a ZST because it may be impossible to distinguish instances"
);
Self {
owner: owner.data.get(),
data: UnsafeCell::new(data),
}
}
}
impl<T: ?Sized, U> LockedBy<T, U> {
/// Returns a reference to the protected data when the caller provides evidence (via a
/// reference) that the owner is locked.
///
/// `U` cannot be a zero-sized type (ZST) because there are ways to get an `&U` that matches
/// the data protected by the lock without actually holding it.
///
/// # Panics
///
/// Panics if `owner` is different from the data protected by the lock used in
/// [`new`](LockedBy::new).
pub fn access<'a>(&'a self, owner: &'a U) -> &'a T
where
T: Sync,
{
build_assert!(
size_of::<U>() > 0,
"`U` cannot be a ZST because `owner` wouldn't be unique"
);
if !ptr::eq(owner, self.owner) {
panic!("mismatched owners");
}
// SAFETY: `owner` is evidence that there are only shared references to the owner for the
// duration of 'a, so it's not possible to use `Self::access_mut` to obtain a mutable
// reference to the inner value that aliases with this shared reference. The type is `Sync`
// so there are no other requirements.
unsafe { &*self.data.get() }
}
/// Returns a mutable reference to the protected data when the caller provides evidence (via a
/// mutable owner) that the owner is locked mutably.
///
/// `U` cannot be a zero-sized type (ZST) because there are ways to get an `&mut U` that
/// matches the data protected by the lock without actually holding it.
///
/// Showing a mutable reference to the owner is sufficient because we know no other references
/// can exist to it.
///
/// # Panics
///
/// Panics if `owner` is different from the data protected by the lock used in
/// [`new`](LockedBy::new).
pub fn access_mut<'a>(&'a self, owner: &'a mut U) -> &'a mut T {
build_assert!(
size_of::<U>() > 0,
"`U` cannot be a ZST because `owner` wouldn't be unique"
);
if !ptr::eq(owner, self.owner) {
panic!("mismatched owners");
}
// SAFETY: `owner` is evidence that there is only one reference to the owner.
unsafe { &mut *self.data.get() }
}
}
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