1#![cfg_attr(all(not(feature = "std"), not(test)), no_std)]
2#![cfg_attr(docsrs, feature(doc_cfg))]
3#![deny(missing_docs)]
4
5//! This crate provides [spin-based](https://en.wikipedia.org/wiki/Spinlock) versions of the
6//! primitives in `std::sync` and `std::lazy`. Because synchronization is done through spinning,
7//! the primitives are suitable for use in `no_std` environments.
8//!
9//! # Features
10//!
11//! - `Mutex`, `RwLock`, `Once`/`SyncOnceCell`, and `SyncLazy` equivalents
12//!
13//! - Support for `no_std` environments
14//!
15//! - [`lock_api`](https://crates.io/crates/lock_api) compatibility
16//!
17//! - Upgradeable `RwLock` guards
18//!
19//! - Guards can be sent and shared between threads
20//!
21//! - Guard leaking
22//!
23//! - Ticket locks
24//!
25//! - Different strategies for dealing with contention
26//!
27//! # Relationship with `std::sync`
28//!
29//! While `spin` is not a drop-in replacement for `std::sync` (and
30//! [should not be considered as such](https://matklad.github.io/2020/01/02/spinlocks-considered-harmful.html))
31//! an effort is made to keep this crate reasonably consistent with `std::sync`.
32//!
33//! Many of the types defined in this crate have 'additional capabilities' when compared to `std::sync`:
34//!
35//! - Because spinning does not depend on the thread-driven model of `std::sync`, guards ([`MutexGuard`],
36//! [`RwLockReadGuard`], [`RwLockWriteGuard`], etc.) may be sent and shared between threads.
37//!
38//! - [`RwLockUpgradableGuard`] supports being upgraded into a [`RwLockWriteGuard`].
39//!
40//! - Guards support [leaking](https://doc.rust-lang.org/nomicon/leaking.html).
41//!
42//! - [`Once`] owns the value returned by its `call_once` initializer.
43//!
44//! - [`RwLock`] supports counting readers and writers.
45//!
46//! Conversely, the types in this crate do not have some of the features `std::sync` has:
47//!
48//! - Locks do not track [panic poisoning](https://doc.rust-lang.org/nomicon/poisoning.html).
49//!
50//! ## Feature flags
51//!
52//! The crate comes with a few feature flags that you may wish to use.
53//!
54//! - `lock_api` enables support for [`lock_api`](https://crates.io/crates/lock_api)
55//!
56//! - `ticket_mutex` uses a ticket lock for the implementation of `Mutex`
57//!
58//! - `fair_mutex` enables a fairer implementation of `Mutex` that uses eventual fairness to avoid
59//! starvation
60//!
61//! - `std` enables support for thread yielding instead of spinning
62
63#[cfg(any(test, feature = "std"))]
64extern crate core;
65
66#[cfg(feature = "portable_atomic")]
67extern crate portable_atomic;
68
69#[cfg(not(feature = "portable_atomic"))]
70use core::sync::atomic;
71#[cfg(feature = "portable_atomic")]
72use portable_atomic as atomic;
73
74#[cfg(feature = "barrier")]
75#[cfg_attr(docsrs, doc(cfg(feature = "barrier")))]
76pub mod barrier;
77#[cfg(feature = "lazy")]
78#[cfg_attr(docsrs, doc(cfg(feature = "lazy")))]
79pub mod lazy;
80#[cfg(feature = "mutex")]
81#[cfg_attr(docsrs, doc(cfg(feature = "mutex")))]
82pub mod mutex;
83#[cfg(feature = "once")]
84#[cfg_attr(docsrs, doc(cfg(feature = "once")))]
85pub mod once;
86pub mod relax;
87#[cfg(feature = "rwlock")]
88#[cfg_attr(docsrs, doc(cfg(feature = "rwlock")))]
89pub mod rwlock;
90
91#[cfg(feature = "mutex")]
92#[cfg_attr(docsrs, doc(cfg(feature = "mutex")))]
93pub use mutex::MutexGuard;
94#[cfg(feature = "std")]
95#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
96pub use relax::Yield;
97pub use relax::{RelaxStrategy, Spin};
98#[cfg(feature = "rwlock")]
99#[cfg_attr(docsrs, doc(cfg(feature = "rwlock")))]
100pub use rwlock::RwLockReadGuard;
101
102// Avoid confusing inference errors by aliasing away the relax strategy parameter. Users that need to use a different
103// relax strategy can do so by accessing the types through their fully-qualified path. This is a little bit horrible
104// but sadly adding a default type parameter is *still* a breaking change in Rust (for understandable reasons).
105
106/// A primitive that synchronizes the execution of multiple threads. See [`barrier::Barrier`] for documentation.
107///
108/// A note for advanced users: this alias exists to avoid subtle type inference errors due to the default relax
109/// strategy type parameter. If you need a non-default relax strategy, use the fully-qualified path.
110#[cfg(feature = "barrier")]
111#[cfg_attr(docsrs, doc(cfg(feature = "barrier")))]
112pub type Barrier = crate::barrier::Barrier;
113
114/// A value which is initialized on the first access. See [`lazy::Lazy`] for documentation.
115///
116/// A note for advanced users: this alias exists to avoid subtle type inference errors due to the default relax
117/// strategy type parameter. If you need a non-default relax strategy, use the fully-qualified path.
118#[cfg(feature = "lazy")]
119#[cfg_attr(docsrs, doc(cfg(feature = "lazy")))]
120pub type Lazy<T, F = fn() -> T> = crate::lazy::Lazy<T, F>;
121
122/// A primitive that synchronizes the execution of multiple threads. See [`mutex::Mutex`] for documentation.
123///
124/// A note for advanced users: this alias exists to avoid subtle type inference errors due to the default relax
125/// strategy type parameter. If you need a non-default relax strategy, use the fully-qualified path.
126#[cfg(feature = "mutex")]
127#[cfg_attr(docsrs, doc(cfg(feature = "mutex")))]
128pub type Mutex<T> = crate::mutex::Mutex<T>;
129
130/// A primitive that provides lazy one-time initialization. See [`once::Once`] for documentation.
131///
132/// A note for advanced users: this alias exists to avoid subtle type inference errors due to the default relax
133/// strategy type parameter. If you need a non-default relax strategy, use the fully-qualified path.
134#[cfg(feature = "once")]
135#[cfg_attr(docsrs, doc(cfg(feature = "once")))]
136pub type Once<T = ()> = crate::once::Once<T>;
137
138/// A lock that provides data access to either one writer or many readers. See [`rwlock::RwLock`] for documentation.
139///
140/// A note for advanced users: this alias exists to avoid subtle type inference errors due to the default relax
141/// strategy type parameter. If you need a non-default relax strategy, use the fully-qualified path.
142#[cfg(feature = "rwlock")]
143#[cfg_attr(docsrs, doc(cfg(feature = "rwlock")))]
144pub type RwLock<T> = crate::rwlock::RwLock<T>;
145
146/// A guard that provides immutable data access but can be upgraded to [`RwLockWriteGuard`]. See
147/// [`rwlock::RwLockUpgradableGuard`] for documentation.
148///
149/// A note for advanced users: this alias exists to avoid subtle type inference errors due to the default relax
150/// strategy type parameter. If you need a non-default relax strategy, use the fully-qualified path.
151#[cfg(feature = "rwlock")]
152#[cfg_attr(docsrs, doc(cfg(feature = "rwlock")))]
153pub type RwLockUpgradableGuard<'a, T> = crate::rwlock::RwLockUpgradableGuard<'a, T>;
154
155/// A guard that provides mutable data access. See [`rwlock::RwLockWriteGuard`] for documentation.
156///
157/// A note for advanced users: this alias exists to avoid subtle type inference errors due to the default relax
158/// strategy type parameter. If you need a non-default relax strategy, use the fully-qualified path.
159#[cfg(feature = "rwlock")]
160#[cfg_attr(docsrs, doc(cfg(feature = "rwlock")))]
161pub type RwLockWriteGuard<'a, T> = crate::rwlock::RwLockWriteGuard<'a, T>;
162
163/// Spin synchronisation primitives, but compatible with [`lock_api`](https://crates.io/crates/lock_api).
164#[cfg(feature = "lock_api")]
165#[cfg_attr(docsrs, doc(cfg(feature = "lock_api")))]
166pub mod lock_api {
167 /// A lock that provides mutually exclusive data access (compatible with [`lock_api`](https://crates.io/crates/lock_api)).
168 #[cfg(feature = "mutex")]
169 #[cfg_attr(docsrs, doc(cfg(feature = "mutex")))]
170 pub type Mutex<T> = lock_api_crate::Mutex<crate::Mutex<()>, T>;
171
172 /// A guard that provides mutable data access (compatible with [`lock_api`](https://crates.io/crates/lock_api)).
173 #[cfg(feature = "mutex")]
174 #[cfg_attr(docsrs, doc(cfg(feature = "mutex")))]
175 pub type MutexGuard<'a, T> = lock_api_crate::MutexGuard<'a, crate::Mutex<()>, T>;
176
177 /// A lock that provides data access to either one writer or many readers (compatible with [`lock_api`](https://crates.io/crates/lock_api)).
178 #[cfg(feature = "rwlock")]
179 #[cfg_attr(docsrs, doc(cfg(feature = "rwlock")))]
180 pub type RwLock<T> = lock_api_crate::RwLock<crate::RwLock<()>, T>;
181
182 /// A guard that provides immutable data access (compatible with [`lock_api`](https://crates.io/crates/lock_api)).
183 #[cfg(feature = "rwlock")]
184 #[cfg_attr(docsrs, doc(cfg(feature = "rwlock")))]
185 pub type RwLockReadGuard<'a, T> = lock_api_crate::RwLockReadGuard<'a, crate::RwLock<()>, T>;
186
187 /// A guard that provides mutable data access (compatible with [`lock_api`](https://crates.io/crates/lock_api)).
188 #[cfg(feature = "rwlock")]
189 #[cfg_attr(docsrs, doc(cfg(feature = "rwlock")))]
190 pub type RwLockWriteGuard<'a, T> = lock_api_crate::RwLockWriteGuard<'a, crate::RwLock<()>, T>;
191
192 /// A guard that provides immutable data access but can be upgraded to [`RwLockWriteGuard`] (compatible with [`lock_api`](https://crates.io/crates/lock_api)).
193 #[cfg(feature = "rwlock")]
194 #[cfg_attr(docsrs, doc(cfg(feature = "rwlock")))]
195 pub type RwLockUpgradableReadGuard<'a, T> =
196 lock_api_crate::RwLockUpgradableReadGuard<'a, crate::RwLock<()>, T>;
197}
198
199/// In the event of an invalid operation, it's best to abort the current process.
200#[cfg(feature = "fair_mutex")]
201fn abort() -> ! {
202 #[cfg(not(feature = "std"))]
203 {
204 // Panicking while panicking is defined by Rust to result in an abort.
205 struct Panic;
206
207 impl Drop for Panic {
208 fn drop(&mut self) {
209 panic!("aborting due to invalid operation");
210 }
211 }
212
213 let _panic = Panic;
214 panic!("aborting due to invalid operation");
215 }
216
217 #[cfg(feature = "std")]
218 {
219 std::process::abort();
220 }
221}
222