mutex.rs

  1// SPDX-License-Identifier: GPL-2.0
  2
  3//! A kernel mutex.
  4//!
  5//! This module allows Rust code to use the kernel's `struct mutex`.
  6
  7/// Creates a [`Mutex`] initialiser with the given name and a newly-created lock class.
  8///
  9/// It uses the name if one is given, otherwise it generates one based on the file name and line
 10/// number.
 11#[macro_export]
 12macro_rules! new_mutex {
 13    ($inner:expr $(, $name:literal)? $(,)?) => {
 14        $crate::sync::Mutex::new(
 15            $inner, $crate::optional_name!($($name)?), $crate::static_lock_class!())
 16    };
 17}
 18pub use new_mutex;
 19
 20/// A mutual exclusion primitive.
 21///
 22/// Exposes the kernel's [`struct mutex`]. When multiple threads attempt to lock the same mutex,
 23/// only one at a time is allowed to progress, the others will block (sleep) until the mutex is
 24/// unlocked, at which point another thread will be allowed to wake up and make progress.
 25///
 26/// Since it may block, [`Mutex`] needs to be used with care in atomic contexts.
 27///
 28/// Instances of [`Mutex`] need a lock class and to be pinned. The recommended way to create such
 29/// instances is with the [`pin_init`](pin_init::pin_init) and [`new_mutex`] macros.
 30///
 31/// # Examples
 32///
 33/// The following example shows how to declare, allocate and initialise a struct (`Example`) that
 34/// contains an inner struct (`Inner`) that is protected by a mutex.
 35///
 36/// ```
 37/// use kernel::sync::{new_mutex, Mutex};
 38///
 39/// struct Inner {
 40///     a: u32,
 41///     b: u32,
 42/// }
 43///
 44/// #[pin_data]
 45/// struct Example {
 46///     c: u32,
 47///     #[pin]
 48///     d: Mutex<Inner>,
 49/// }
 50///
 51/// impl Example {
 52///     fn new() -> impl PinInit<Self> {
 53///         pin_init!(Self {
 54///             c: 10,
 55///             d <- new_mutex!(Inner { a: 20, b: 30 }),
 56///         })
 57///     }
 58/// }
 59///
 60/// // Allocate a boxed `Example`.
 61/// let e = KBox::pin_init(Example::new(), GFP_KERNEL)?;
 62/// assert_eq!(e.c, 10);
 63/// assert_eq!(e.d.lock().a, 20);
 64/// assert_eq!(e.d.lock().b, 30);
 65/// # Ok::<(), Error>(())
 66/// ```
 67///
 68/// The following example shows how to use interior mutability to modify the contents of a struct
 69/// protected by a mutex despite only having a shared reference:
 70///
 71/// ```
 72/// use kernel::sync::Mutex;
 73///
 74/// struct Example {
 75///     a: u32,
 76///     b: u32,
 77/// }
 78///
 79/// fn example(m: &Mutex<Example>) {
 80///     let mut guard = m.lock();
 81///     guard.a += 10;
 82///     guard.b += 20;
 83/// }
 84/// ```
 85///
 86/// [`struct mutex`]: srctree/include/linux/mutex.h
 87pub type Mutex<T> = super::Lock<T, MutexBackend>;
 88
 89/// A [`Guard`] acquired from locking a [`Mutex`].
 90///
 91/// This is simply a type alias for a [`Guard`] returned from locking a [`Mutex`]. It will unlock
 92/// the [`Mutex`] upon being dropped.
 93///
 94/// [`Guard`]: super::Guard
 95pub type MutexGuard<'a, T> = super::Guard<'a, T, MutexBackend>;
 96
 97/// A kernel `struct mutex` lock backend.
 98pub struct MutexBackend;
 99
100// SAFETY: The underlying kernel `struct mutex` object ensures mutual exclusion.
101unsafe impl super::Backend for MutexBackend {
102    type State = bindings::mutex;
103    type GuardState = ();
104
105    unsafe fn init(
106        ptr: *mut Self::State,
107        name: *const crate::ffi::c_char,
108        key: *mut bindings::lock_class_key,
109    ) {
110        // SAFETY: The safety requirements ensure that `ptr` is valid for writes, and `name` and
111        // `key` are valid for read indefinitely.
112        unsafe { bindings::__mutex_init(ptr, name, key) }
113    }
114
115    unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState {
116        // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
117        // memory, and that it has been initialised before.
118        unsafe { bindings::mutex_lock(ptr) };
119    }
120
121    unsafe fn unlock(ptr: *mut Self::State, _guard_state: &Self::GuardState) {
122        // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that the
123        // caller is the owner of the mutex.
124        unsafe { bindings::mutex_unlock(ptr) };
125    }
126
127    unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState> {
128        // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
129        let result = unsafe { bindings::mutex_trylock(ptr) };
130
131        if result != 0 {
132            Some(())
133        } else {
134            None
135        }
136    }
137
138    unsafe fn assert_is_held(ptr: *mut Self::State) {
139        // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
140        unsafe { bindings::mutex_assert_is_held(ptr) }
141    }
142}