moto_rt/
mutex.rs

1/// A futex-based mutex, to be used in no-std environments.
2//
3// A slightly modified version of
4// https://github.com/m-ou-se/rust-atomics-and-locks/blob/main/src/ch9_locks/mutex_3.rs
5//
6// which has this LICENCE:
7//
8// You may use all code in this repository for any purpose.
9//
10// Attribution is appreciated, but not required.
11// An attribution usually includes the book title, author,
12// publisher, and ISBN. For example: "Rust Atomics and
13// Locks by Mara Bos (O’Reilly). Copyright 2023 Mara Bos,
14// 978-1-098-11944-7."
15//
16use core::cell::UnsafeCell;
17use core::ops::{Deref, DerefMut};
18use core::sync::atomic::AtomicU32;
19use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
20
21const UNLOCKED: u32 = 0;
22const LOCKED_NO_WAITERS: u32 = 1;
23const LOCKED_YES_WAITERS: u32 = 2;
24
25pub struct Mutex<T> {
26    state: AtomicU32,
27    value: UnsafeCell<T>,
28}
29
30unsafe impl<T> Sync for Mutex<T> where T: Send {}
31
32pub struct MutexGuard<'a, T> {
33    pub(crate) mutex: &'a Mutex<T>,
34}
35
36unsafe impl<T> Sync for MutexGuard<'_, T> where T: Sync {}
37
38impl<T> Deref for MutexGuard<'_, T> {
39    type Target = T;
40    #[inline]
41    fn deref(&self) -> &T {
42        unsafe { &*self.mutex.value.get() }
43    }
44}
45
46impl<T> DerefMut for MutexGuard<'_, T> {
47    #[inline]
48    fn deref_mut(&mut self) -> &mut T {
49        unsafe { &mut *self.mutex.value.get() }
50    }
51}
52
53impl<T> Mutex<T> {
54    pub const fn new(value: T) -> Self {
55        Self {
56            state: AtomicU32::new(UNLOCKED),
57            value: UnsafeCell::new(value),
58        }
59    }
60
61    #[inline]
62    pub fn lock(&self) -> MutexGuard<'_, T> {
63        if self
64            .state
65            .compare_exchange(UNLOCKED, LOCKED_NO_WAITERS, Acquire, Relaxed)
66            .is_err()
67        {
68            // The lock was already locked. :(
69            lock_contended(&self.state);
70        }
71        MutexGuard { mutex: self }
72    }
73}
74
75#[cold]
76fn lock_contended(state: &AtomicU32) {
77    let mut spin_count = 0;
78    const MAX_BUSY_LOOP_ITERS: u32 = 100;
79
80    while state.load(Relaxed) == LOCKED_NO_WAITERS && spin_count < MAX_BUSY_LOOP_ITERS {
81        spin_count += 1;
82        core::hint::spin_loop();
83    }
84
85    if state
86        .compare_exchange(UNLOCKED, LOCKED_NO_WAITERS, Acquire, Relaxed)
87        .is_ok()
88    {
89        return;
90    }
91
92    while state.swap(LOCKED_YES_WAITERS, Acquire) != UNLOCKED {
93        crate::futex_wait(state, LOCKED_YES_WAITERS, None);
94    }
95}
96
97impl<T> Drop for MutexGuard<'_, T> {
98    #[inline]
99    fn drop(&mut self) {
100        if self.mutex.state.swap(UNLOCKED, Release) == LOCKED_YES_WAITERS {
101            crate::futex_wake(&self.mutex.state);
102        }
103    }
104}
moto_rt/mutex.rs

moto_rt/
mutex.rs
