1// Copyright 2016 Amanieu d'Antras
2//
3// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
4// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
5// http://opensource.org/licenses/MIT>, at your option. This file may not be
6// copied, modified, or distributed except according to those terms.
7
8use crate::raw_fair_mutex::RawFairMutex;
9
10/// A mutual exclusive primitive that is always fair, useful for protecting shared data
11///
12/// This mutex will block threads waiting for the lock to become available. The
13/// mutex can be statically initialized or created by the `new`
14/// constructor. Each mutex has a type parameter which represents the data that
15/// it is protecting. The data can only be accessed through the RAII guards
16/// returned from `lock` and `try_lock`, which guarantees that the data is only
17/// ever accessed when the mutex is locked.
18///
19/// The regular mutex provided by `parking_lot` uses eventual fairness
20/// (after some time it will default to the fair algorithm), but eventual
21/// fairness does not provide the same guarantees an always fair method would.
22/// Fair mutexes are generally slower, but sometimes needed.
23///
24/// In a fair mutex the waiters form a queue, and the lock is always granted to
25/// the next requester in the queue, in first-in first-out order. This ensures
26/// that one thread cannot starve others by quickly re-acquiring the lock after
27/// releasing it.
28///
29/// A fair mutex may not be interesting if threads have different priorities (this is known as
30/// priority inversion).
31///
32/// # Differences from the standard library `Mutex`
33///
34/// - No poisoning, the lock is released normally on panic.
35/// - Only requires 1 byte of space, whereas the standard library boxes the
36/// `FairMutex` due to platform limitations.
37/// - Can be statically constructed.
38/// - Does not require any drop glue when dropped.
39/// - Inline fast path for the uncontended case.
40/// - Efficient handling of micro-contention using adaptive spinning.
41/// - Allows raw locking & unlocking without a guard.
42///
43/// # Examples
44///
45/// ```
46/// use parking_lot::FairMutex;
47/// use std::sync::{Arc, mpsc::channel};
48/// use std::thread;
49///
50/// const N: usize = 10;
51///
52/// // Spawn a few threads to increment a shared variable (non-atomically), and
53/// // let the main thread know once all increments are done.
54/// //
55/// // Here we're using an Arc to share memory among threads, and the data inside
56/// // the Arc is protected with a mutex.
57/// let data = Arc::new(FairMutex::new(0));
58///
59/// let (tx, rx) = channel();
60/// for _ in 0..10 {
61/// let (data, tx) = (Arc::clone(&data), tx.clone());
62/// thread::spawn(move || {
63/// // The shared state can only be accessed once the lock is held.
64/// // Our non-atomic increment is safe because we're the only thread
65/// // which can access the shared state when the lock is held.
66/// let mut data = data.lock();
67/// *data += 1;
68/// if *data == N {
69/// tx.send(()).unwrap();
70/// }
71/// // the lock is unlocked here when `data` goes out of scope.
72/// });
73/// }
74///
75/// rx.recv().unwrap();
76/// ```
77pub type FairMutex<T> = lock_api::Mutex<RawFairMutex, T>;
78
79/// Creates a new fair mutex in an unlocked state ready for use.
80///
81/// This allows creating a fair mutex in a constant context on stable Rust.
82pub const fn const_fair_mutex<T>(val: T) -> FairMutex<T> {
83 FairMutex::const_new(<RawFairMutex as lock_api::RawMutex>::INIT, val)
84}
85
86/// An RAII implementation of a "scoped lock" of a mutex. When this structure is
87/// dropped (falls out of scope), the lock will be unlocked.
88///
89/// The data protected by the mutex can be accessed through this guard via its
90/// `Deref` and `DerefMut` implementations.
91pub type FairMutexGuard<'a, T> = lock_api::MutexGuard<'a, RawFairMutex, T>;
92
93/// An RAII mutex guard returned by `FairMutexGuard::map`, which can point to a
94/// subfield of the protected data.
95///
96/// The main difference between `MappedFairMutexGuard` and `FairMutexGuard` is that the
97/// former doesn't support temporarily unlocking and re-locking, since that
98/// could introduce soundness issues if the locked object is modified by another
99/// thread.
100pub type MappedFairMutexGuard<'a, T> = lock_api::MappedMutexGuard<'a, RawFairMutex, T>;
101
102#[cfg(test)]
103mod tests {
104 use crate::FairMutex;
105 use std::sync::atomic::{AtomicUsize, Ordering};
106 use std::sync::mpsc::channel;
107 use std::sync::Arc;
108 use std::thread;
109
110 #[cfg(feature = "serde")]
111 use bincode::{deserialize, serialize};
112
113 #[derive(Eq, PartialEq, Debug)]
114 struct NonCopy(i32);
115
116 #[test]
117 fn smoke() {
118 let m = FairMutex::new(());
119 drop(m.lock());
120 drop(m.lock());
121 }
122
123 #[test]
124 fn lots_and_lots() {
125 const J: u32 = 1000;
126 const K: u32 = 3;
127
128 let m = Arc::new(FairMutex::new(0));
129
130 fn inc(m: &FairMutex<u32>) {
131 for _ in 0..J {
132 *m.lock() += 1;
133 }
134 }
135
136 let (tx, rx) = channel();
137 for _ in 0..K {
138 let tx2 = tx.clone();
139 let m2 = m.clone();
140 thread::spawn(move || {
141 inc(&m2);
142 tx2.send(()).unwrap();
143 });
144 let tx2 = tx.clone();
145 let m2 = m.clone();
146 thread::spawn(move || {
147 inc(&m2);
148 tx2.send(()).unwrap();
149 });
150 }
151
152 drop(tx);
153 for _ in 0..2 * K {
154 rx.recv().unwrap();
155 }
156 assert_eq!(*m.lock(), J * K * 2);
157 }
158
159 #[test]
160 fn try_lock() {
161 let m = FairMutex::new(());
162 *m.try_lock().unwrap() = ();
163 }
164
165 #[test]
166 fn test_into_inner() {
167 let m = FairMutex::new(NonCopy(10));
168 assert_eq!(m.into_inner(), NonCopy(10));
169 }
170
171 #[test]
172 fn test_into_inner_drop() {
173 struct Foo(Arc<AtomicUsize>);
174 impl Drop for Foo {
175 fn drop(&mut self) {
176 self.0.fetch_add(1, Ordering::SeqCst);
177 }
178 }
179 let num_drops = Arc::new(AtomicUsize::new(0));
180 let m = FairMutex::new(Foo(num_drops.clone()));
181 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
182 {
183 let _inner = m.into_inner();
184 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
185 }
186 assert_eq!(num_drops.load(Ordering::SeqCst), 1);
187 }
188
189 #[test]
190 fn test_get_mut() {
191 let mut m = FairMutex::new(NonCopy(10));
192 *m.get_mut() = NonCopy(20);
193 assert_eq!(m.into_inner(), NonCopy(20));
194 }
195
196 #[test]
197 fn test_mutex_arc_nested() {
198 // Tests nested mutexes and access
199 // to underlying data.
200 let arc = Arc::new(FairMutex::new(1));
201 let arc2 = Arc::new(FairMutex::new(arc));
202 let (tx, rx) = channel();
203 let _t = thread::spawn(move || {
204 let lock = arc2.lock();
205 let lock2 = lock.lock();
206 assert_eq!(*lock2, 1);
207 tx.send(()).unwrap();
208 });
209 rx.recv().unwrap();
210 }
211
212 #[test]
213 fn test_mutex_arc_access_in_unwind() {
214 let arc = Arc::new(FairMutex::new(1));
215 let arc2 = arc.clone();
216 let _ = thread::spawn(move || {
217 struct Unwinder {
218 i: Arc<FairMutex<i32>>,
219 }
220 impl Drop for Unwinder {
221 fn drop(&mut self) {
222 *self.i.lock() += 1;
223 }
224 }
225 let _u = Unwinder { i: arc2 };
226 panic!();
227 })
228 .join();
229 let lock = arc.lock();
230 assert_eq!(*lock, 2);
231 }
232
233 #[test]
234 fn test_mutex_unsized() {
235 let mutex: &FairMutex<[i32]> = &FairMutex::new([1, 2, 3]);
236 {
237 let b = &mut *mutex.lock();
238 b[0] = 4;
239 b[2] = 5;
240 }
241 let comp: &[i32] = &[4, 2, 5];
242 assert_eq!(&*mutex.lock(), comp);
243 }
244
245 #[test]
246 fn test_mutexguard_sync() {
247 fn sync<T: Sync>(_: T) {}
248
249 let mutex = FairMutex::new(());
250 sync(mutex.lock());
251 }
252
253 #[test]
254 fn test_mutex_debug() {
255 let mutex = FairMutex::new(vec![0u8, 10]);
256
257 assert_eq!(format!("{:?}", mutex), "Mutex { data: [0, 10] }");
258 let _lock = mutex.lock();
259 assert_eq!(format!("{:?}", mutex), "Mutex { data: <locked> }");
260 }
261
262 #[cfg(feature = "serde")]
263 #[test]
264 fn test_serde() {
265 let contents: Vec<u8> = vec![0, 1, 2];
266 let mutex = FairMutex::new(contents.clone());
267
268 let serialized = serialize(&mutex).unwrap();
269 let deserialized: FairMutex<Vec<u8>> = deserialize(&serialized).unwrap();
270
271 assert_eq!(*(mutex.lock()), *(deserialized.lock()));
272 assert_eq!(contents, *(deserialized.lock()));
273 }
274}
275

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