xref: /linux/rust/kernel/sync/lock/mutex.rs (revision 7f71507851fc7764b36a3221839607d3a45c2025)
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]
12 macro_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 }
18 pub 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`](crate::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
87 pub type Mutex<T> = super::Lock<T, MutexBackend>;
88 
89 /// A kernel `struct mutex` lock backend.
90 pub struct MutexBackend;
91 
92 // SAFETY: The underlying kernel `struct mutex` object ensures mutual exclusion.
93 unsafe impl super::Backend for MutexBackend {
94     type State = bindings::mutex;
95     type GuardState = ();
96 
97     unsafe fn init(
98         ptr: *mut Self::State,
99         name: *const crate::ffi::c_char,
100         key: *mut bindings::lock_class_key,
101     ) {
102         // SAFETY: The safety requirements ensure that `ptr` is valid for writes, and `name` and
103         // `key` are valid for read indefinitely.
104         unsafe { bindings::__mutex_init(ptr, name, key) }
105     }
106 
107     unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState {
108         // SAFETY: The safety requirements of this function ensure that `ptr` points to valid
109         // memory, and that it has been initialised before.
110         unsafe { bindings::mutex_lock(ptr) };
111     }
112 
113     unsafe fn unlock(ptr: *mut Self::State, _guard_state: &Self::GuardState) {
114         // SAFETY: The safety requirements of this function ensure that `ptr` is valid and that the
115         // caller is the owner of the mutex.
116         unsafe { bindings::mutex_unlock(ptr) };
117     }
118 
119     unsafe fn try_lock(ptr: *mut Self::State) -> Option<Self::GuardState> {
120         // SAFETY: The `ptr` pointer is guaranteed to be valid and initialized before use.
121         let result = unsafe { bindings::mutex_trylock(ptr) };
122 
123         if result != 0 {
124             Some(())
125         } else {
126             None
127         }
128     }
129 }
130