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_mutex::RawMutex;
9
10/// A mutual exclusion primitive 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/// # Fairness
20///
21/// A typical unfair lock can often end up in a situation where a single thread
22/// quickly acquires and releases the same mutex in succession, which can starve
23/// other threads waiting to acquire the mutex. While this improves throughput
24/// because it doesn't force a context switch when a thread tries to re-acquire
25/// a mutex it has just released, this can starve other threads.
26///
27/// This mutex uses [eventual fairness](https://trac.webkit.org/changeset/203350)
28/// to ensure that the lock will be fair on average without sacrificing
29/// throughput. This is done by forcing a fair unlock on average every 0.5ms,
30/// which will force the lock to go to the next thread waiting for the mutex.
31///
32/// Additionally, any critical section longer than 1ms will always use a fair
33/// unlock, which has a negligible impact on throughput considering the length
34/// of the critical section.
35///
36/// You can also force a fair unlock by calling `MutexGuard::unlock_fair` when
37/// unlocking a mutex instead of simply dropping the `MutexGuard`.
38///
39/// # Differences from the standard library `Mutex`
40///
41/// - No poisoning, the lock is released normally on panic.
42/// - Only requires 1 byte of space, whereas the standard library boxes the
43/// `Mutex` due to platform limitations.
44/// - Can be statically constructed.
45/// - Does not require any drop glue when dropped.
46/// - Inline fast path for the uncontended case.
47/// - Efficient handling of micro-contention using adaptive spinning.
48/// - Allows raw locking & unlocking without a guard.
49/// - Supports eventual fairness so that the mutex is fair on average.
50/// - Optionally allows making the mutex fair by calling `MutexGuard::unlock_fair`.
51///
52/// # Examples
53///
54/// ```
55/// use parking_lot::Mutex;
56/// use std::sync::{Arc, mpsc::channel};
57/// use std::thread;
58///
59/// const N: usize = 10;
60///
61/// // Spawn a few threads to increment a shared variable (non-atomically), and
62/// // let the main thread know once all increments are done.
63/// //
64/// // Here we're using an Arc to share memory among threads, and the data inside
65/// // the Arc is protected with a mutex.
66/// let data = Arc::new(Mutex::new(0));
67///
68/// let (tx, rx) = channel();
69/// for _ in 0..10 {
70/// let (data, tx) = (Arc::clone(&data), tx.clone());
71/// thread::spawn(move || {
72/// // The shared state can only be accessed once the lock is held.
73/// // Our non-atomic increment is safe because we're the only thread
74/// // which can access the shared state when the lock is held.
75/// let mut data = data.lock();
76/// *data += 1;
77/// if *data == N {
78/// tx.send(()).unwrap();
79/// }
80/// // the lock is unlocked here when `data` goes out of scope.
81/// });
82/// }
83///
84/// rx.recv().unwrap();
85/// ```
86pub type Mutex<T> = lock_api::Mutex<RawMutex, T>;
87
88/// Creates a new mutex in an unlocked state ready for use.
89///
90/// This allows creating a mutex in a constant context on stable Rust.
91pub const fn const_mutex<T>(val: T) -> Mutex<T> {
92 Mutex::const_new(<RawMutex as lock_api::RawMutex>::INIT, val)
93}
94
95/// An RAII implementation of a "scoped lock" of a mutex. When this structure is
96/// dropped (falls out of scope), the lock will be unlocked.
97///
98/// The data protected by the mutex can be accessed through this guard via its
99/// `Deref` and `DerefMut` implementations.
100pub type MutexGuard<'a, T> = lock_api::MutexGuard<'a, RawMutex, T>;
101
102/// An RAII mutex guard returned by `MutexGuard::map`, which can point to a
103/// subfield of the protected data.
104///
105/// The main difference between `MappedMutexGuard` and `MutexGuard` is that the
106/// former doesn't support temporarily unlocking and re-locking, since that
107/// could introduce soundness issues if the locked object is modified by another
108/// thread.
109pub type MappedMutexGuard<'a, T> = lock_api::MappedMutexGuard<'a, RawMutex, T>;
110
111#[cfg(test)]
112mod tests {
113 use crate::{Condvar, Mutex};
114 use std::sync::atomic::{AtomicUsize, Ordering};
115 use std::sync::mpsc::channel;
116 use std::sync::Arc;
117 use std::thread;
118
119 #[cfg(feature = "serde")]
120 use bincode::{deserialize, serialize};
121
122 struct Packet<T>(Arc<(Mutex<T>, Condvar)>);
123
124 #[derive(Eq, PartialEq, Debug)]
125 struct NonCopy(i32);
126
127 unsafe impl<T: Send> Send for Packet<T> {}
128 unsafe impl<T> Sync for Packet<T> {}
129
130 #[test]
131 fn smoke() {
132 let m = Mutex::new(());
133 drop(m.lock());
134 drop(m.lock());
135 }
136
137 #[test]
138 fn lots_and_lots() {
139 const J: u32 = 1000;
140 const K: u32 = 3;
141
142 let m = Arc::new(Mutex::new(0));
143
144 fn inc(m: &Mutex<u32>) {
145 for _ in 0..J {
146 *m.lock() += 1;
147 }
148 }
149
150 let (tx, rx) = channel();
151 for _ in 0..K {
152 let tx2 = tx.clone();
153 let m2 = m.clone();
154 thread::spawn(move || {
155 inc(&m2);
156 tx2.send(()).unwrap();
157 });
158 let tx2 = tx.clone();
159 let m2 = m.clone();
160 thread::spawn(move || {
161 inc(&m2);
162 tx2.send(()).unwrap();
163 });
164 }
165
166 drop(tx);
167 for _ in 0..2 * K {
168 rx.recv().unwrap();
169 }
170 assert_eq!(*m.lock(), J * K * 2);
171 }
172
173 #[test]
174 fn try_lock() {
175 let m = Mutex::new(());
176 *m.try_lock().unwrap() = ();
177 }
178
179 #[test]
180 fn test_into_inner() {
181 let m = Mutex::new(NonCopy(10));
182 assert_eq!(m.into_inner(), NonCopy(10));
183 }
184
185 #[test]
186 fn test_into_inner_drop() {
187 struct Foo(Arc<AtomicUsize>);
188 impl Drop for Foo {
189 fn drop(&mut self) {
190 self.0.fetch_add(1, Ordering::SeqCst);
191 }
192 }
193 let num_drops = Arc::new(AtomicUsize::new(0));
194 let m = Mutex::new(Foo(num_drops.clone()));
195 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
196 {
197 let _inner = m.into_inner();
198 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
199 }
200 assert_eq!(num_drops.load(Ordering::SeqCst), 1);
201 }
202
203 #[test]
204 fn test_get_mut() {
205 let mut m = Mutex::new(NonCopy(10));
206 *m.get_mut() = NonCopy(20);
207 assert_eq!(m.into_inner(), NonCopy(20));
208 }
209
210 #[test]
211 fn test_mutex_arc_condvar() {
212 let packet = Packet(Arc::new((Mutex::new(false), Condvar::new())));
213 let packet2 = Packet(packet.0.clone());
214 let (tx, rx) = channel();
215 let _t = thread::spawn(move || {
216 // wait until parent gets in
217 rx.recv().unwrap();
218 let (lock, cvar) = &*packet2.0;
219 let mut lock = lock.lock();
220 *lock = true;
221 cvar.notify_one();
222 });
223
224 let (lock, cvar) = &*packet.0;
225 let mut lock = lock.lock();
226 tx.send(()).unwrap();
227 assert!(!*lock);
228 while !*lock {
229 cvar.wait(&mut lock);
230 }
231 }
232
233 #[test]
234 fn test_mutex_arc_nested() {
235 // Tests nested mutexes and access
236 // to underlying data.
237 let arc = Arc::new(Mutex::new(1));
238 let arc2 = Arc::new(Mutex::new(arc));
239 let (tx, rx) = channel();
240 let _t = thread::spawn(move || {
241 let lock = arc2.lock();
242 let lock2 = lock.lock();
243 assert_eq!(*lock2, 1);
244 tx.send(()).unwrap();
245 });
246 rx.recv().unwrap();
247 }
248
249 #[test]
250 fn test_mutex_arc_access_in_unwind() {
251 let arc = Arc::new(Mutex::new(1));
252 let arc2 = arc.clone();
253 let _ = thread::spawn(move || {
254 struct Unwinder {
255 i: Arc<Mutex<i32>>,
256 }
257 impl Drop for Unwinder {
258 fn drop(&mut self) {
259 *self.i.lock() += 1;
260 }
261 }
262 let _u = Unwinder { i: arc2 };
263 panic!();
264 })
265 .join();
266 let lock = arc.lock();
267 assert_eq!(*lock, 2);
268 }
269
270 #[test]
271 fn test_mutex_unsized() {
272 let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
273 {
274 let b = &mut *mutex.lock();
275 b[0] = 4;
276 b[2] = 5;
277 }
278 let comp: &[i32] = &[4, 2, 5];
279 assert_eq!(&*mutex.lock(), comp);
280 }
281
282 #[test]
283 fn test_mutexguard_sync() {
284 fn sync<T: Sync>(_: T) {}
285
286 let mutex = Mutex::new(());
287 sync(mutex.lock());
288 }
289
290 #[test]
291 fn test_mutex_debug() {
292 let mutex = Mutex::new(vec![0u8, 10]);
293
294 assert_eq!(format!("{:?}", mutex), "Mutex { data: [0, 10] }");
295 let _lock = mutex.lock();
296 assert_eq!(format!("{:?}", mutex), "Mutex { data: <locked> }");
297 }
298
299 #[cfg(feature = "serde")]
300 #[test]
301 fn test_serde() {
302 let contents: Vec<u8> = vec![0, 1, 2];
303 let mutex = Mutex::new(contents.clone());
304
305 let serialized = serialize(&mutex).unwrap();
306 let deserialized: Mutex<Vec<u8>> = deserialize(&serialized).unwrap();
307
308 assert_eq!(*(mutex.lock()), *(deserialized.lock()));
309 assert_eq!(contents, *(deserialized.lock()));
310 }
311}
312