kernel/sync/
set_once.rs

1// SPDX-License-Identifier: GPL-2.0
2
3//! A container that can be initialized at most once.
4
5use super::atomic::{
6    ordering::{Acquire, Relaxed, Release},
7    Atomic,
8};
9use core::{cell::UnsafeCell, mem::MaybeUninit};
10
11/// A container that can be populated at most once. Thread safe.
12///
13/// Once the a [`SetOnce`] is populated, it remains populated by the same object for the
14/// lifetime `Self`.
15///
16/// # Invariants
17///
18/// - `init` may only increase in value.
19/// - `init` may only assume values in the range `0..=2`.
20/// - `init == 0` if and only if `value` is uninitialized.
21/// - `init == 1` if and only if there is exactly one thread with exclusive
22///   access to `self.value`.
23/// - `init == 2` if and only if `value` is initialized and valid for shared
24///   access.
25///
26/// # Example
27///
28/// ```
29/// # use kernel::sync::SetOnce;
30/// let value = SetOnce::new();
31/// assert_eq!(None, value.as_ref());
32///
33/// let status = value.populate(42u8);
34/// assert_eq!(true, status);
35/// assert_eq!(Some(&42u8), value.as_ref());
36/// assert_eq!(Some(42u8), value.copy());
37///
38/// let status = value.populate(101u8);
39/// assert_eq!(false, status);
40/// assert_eq!(Some(&42u8), value.as_ref());
41/// assert_eq!(Some(42u8), value.copy());
42/// ```
43pub struct SetOnce<T> {
44    init: Atomic<u32>,
45    value: UnsafeCell<MaybeUninit<T>>,
46}
47
48impl<T> Default for SetOnce<T> {
49    fn default() -> Self {
50        Self::new()
51    }
52}
53
54impl<T> SetOnce<T> {
55    /// Create a new [`SetOnce`].
56    ///
57    /// The returned instance will be empty.
58    pub const fn new() -> Self {
59        // INVARIANT: The container is empty and we initialize `init` to `0`.
60        Self {
61            value: UnsafeCell::new(MaybeUninit::uninit()),
62            init: Atomic::new(0),
63        }
64    }
65
66    /// Get a reference to the contained object.
67    ///
68    /// Returns [`None`] if this [`SetOnce`] is empty.
69    pub fn as_ref(&self) -> Option<&T> {
70        if self.init.load(Acquire) == 2 {
71            // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
72            // is initialized and valid for shared access.
73            Some(unsafe { &*self.value.get().cast() })
74        } else {
75            None
76        }
77    }
78
79    /// Populate the [`SetOnce`].
80    ///
81    /// Returns `true` if the [`SetOnce`] was successfully populated.
82    pub fn populate(&self, value: T) -> bool {
83        // INVARIANT: If the swap succeeds:
84        //  - We increase `init`.
85        //  - We write the valid value `1` to `init`.
86        //  - Only one thread can succeed in this write, so we have exclusive access after the
87        //    write.
88        if let Ok(0) = self.init.cmpxchg(0, 1, Relaxed) {
89            // SAFETY: By the type invariants of `Self`, the fact that we succeeded in writing `1`
90            // to `self.init` means we obtained exclusive access to `self.value`.
91            unsafe { core::ptr::write(self.value.get().cast(), value) };
92            // INVARIANT:
93            //  - We increase `init`.
94            //  - We write the valid value `2` to `init`.
95            //  - We release our exclusive access to `self.value` and it is now valid for shared
96            //    access.
97            self.init.store(2, Release);
98            true
99        } else {
100            false
101        }
102    }
103
104    /// Get a copy of the contained object.
105    ///
106    /// Returns [`None`] if the [`SetOnce`] is empty.
107    pub fn copy(&self) -> Option<T>
108    where
109        T: Copy,
110    {
111        self.as_ref().copied()
112    }
113}
114
115impl<T> Drop for SetOnce<T> {
116    fn drop(&mut self) {
117        if *self.init.get_mut() == 2 {
118            let value = self.value.get_mut();
119            // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
120            // contains a valid value. We have exclusive access, as we hold a `mut` reference to
121            // `self`.
122            unsafe { value.assume_init_drop() };
123        }
124    }
125}
kernel/sync/set_once.rs

kernel/sync/
set_once.rs
