rust/kernel/sync/locked_by.rs

Source file repositories/reference/linux-study-clean/rust/kernel/sync/locked_by.rs

File Facts

System
Linux kernel
Corpus path
rust/kernel/sync/locked_by.rs
Extension
.rs
Size
6576 bytes
Lines
170
Domain
Rust Kernel Layer
Bucket
Rust API Membrane
Inferred role
Rust Kernel Layer: implementation source
Status
source implementation candidate

Why This File Exists

Rust-side wrappers and abstractions around kernel C APIs, ownership contracts, allocation, synchronization, and module integration.

Dependency Surface

Detected Declarations

Annotated Snippet

// 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::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: KVec<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!("{} {}\n", 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

Annotation

Implementation Notes