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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 #[inline]
106 unsafe fn init(
107 ptr: *mut Self::State,
108 name: *const crate::ffi::c_char,
109 key: *mut bindings::lock_class_key,
110 ) {
111 // SAFETY: The safety requirements ensure that `ptr` is valid for writes, and `name` and
112 // `key` are valid for read indefinitely.
113 unsafe { bindings::__mutex_init(ptr, name, key) }
114 }
115
116 #[inline]
117 unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState {
118 // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
119 // memory, and that it has been initialised before.
120 unsafe { bindings::mutex_lock(ptr) };
121 }
122
123 #[inline]
124 unsafe fn unlock(ptr: *mut Self::State, _guard_state: &Self::GuardState) {
125 // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that the
126 // caller is the owner of the mutex.
127 unsafe { bindings::mutex_unlock(ptr) };
128 }
129
130 #[inline]
131 unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState> {
132 // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
133 let result = unsafe { bindings::mutex_trylock(ptr) };
134
135 if result != 0 {
136 Some(())
137 } else {
138 None
139 }
140 }
141
142 #[inline]
143 unsafe fn assert_is_held(ptr: *mut Self::State) {
144 // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
145 unsafe { bindings::mutex_assert_is_held(ptr) }
146 }
147}