1 #[cfg(all(test, not(target_os = "emscripten")))]
4 use crate::cell::UnsafeCell;
6 use crate::ops::{Deref, DerefMut};
7 use crate::sync::{poison, LockResult, TryLockError, TryLockResult};
8 use crate::sys_common::mutex as sys;
10 /// A mutual exclusion primitive useful for protecting shared data
12 /// This mutex will block threads waiting for the lock to become available. The
13 /// mutex can be created via a [`new`] constructor. Each mutex has a type parameter
14 /// which represents the data that it is protecting. The data can only be accessed
15 /// through the RAII guards returned from [`lock`] and [`try_lock`], which
16 /// guarantees that the data is only ever accessed when the mutex is locked.
20 /// The mutexes in this module implement a strategy called "poisoning" where a
21 /// mutex is considered poisoned whenever a thread panics while holding the
22 /// mutex. Once a mutex is poisoned, all other threads are unable to access the
23 /// data by default as it is likely tainted (some invariant is not being
26 /// For a mutex, this means that the [`lock`] and [`try_lock`] methods return a
27 /// [`Result`] which indicates whether a mutex has been poisoned or not. Most
28 /// usage of a mutex will simply [`unwrap()`] these results, propagating panics
29 /// among threads to ensure that a possibly invalid invariant is not witnessed.
31 /// A poisoned mutex, however, does not prevent all access to the underlying
32 /// data. The [`PoisonError`] type has an [`into_inner`] method which will return
33 /// the guard that would have otherwise been returned on a successful lock. This
34 /// allows access to the data, despite the lock being poisoned.
36 /// [`new`]: Self::new
37 /// [`lock`]: Self::lock
38 /// [`try_lock`]: Self::try_lock
39 /// [`unwrap()`]: Result::unwrap
40 /// [`PoisonError`]: super::PoisonError
41 /// [`into_inner`]: super::PoisonError::into_inner
46 /// use std::sync::{Arc, Mutex};
48 /// use std::sync::mpsc::channel;
50 /// const N: usize = 10;
52 /// // Spawn a few threads to increment a shared variable (non-atomically), and
53 /// // let the main thread know once all increments are done.
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(Mutex::new(0));
59 /// let (tx, rx) = channel();
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.
67 /// // We unwrap() the return value to assert that we are not expecting
68 /// // threads to ever fail while holding the lock.
69 /// let mut data = data.lock().unwrap();
72 /// tx.send(()).unwrap();
74 /// // the lock is unlocked here when `data` goes out of scope.
78 /// rx.recv().unwrap();
81 /// To recover from a poisoned mutex:
84 /// use std::sync::{Arc, Mutex};
87 /// let lock = Arc::new(Mutex::new(0_u32));
88 /// let lock2 = Arc::clone(&lock);
90 /// let _ = thread::spawn(move || -> () {
91 /// // This thread will acquire the mutex first, unwrapping the result of
92 /// // `lock` because the lock has not been poisoned.
93 /// let _guard = lock2.lock().unwrap();
95 /// // This panic while holding the lock (`_guard` is in scope) will poison
100 /// // The lock is poisoned by this point, but the returned result can be
101 /// // pattern matched on to return the underlying guard on both branches.
102 /// let mut guard = match lock.lock() {
103 /// Ok(guard) => guard,
104 /// Err(poisoned) => poisoned.into_inner(),
110 /// It is sometimes necessary to manually drop the mutex guard to unlock it
111 /// sooner than the end of the enclosing scope.
114 /// use std::sync::{Arc, Mutex};
117 /// const N: usize = 3;
119 /// let data_mutex = Arc::new(Mutex::new(vec![1, 2, 3, 4]));
120 /// let res_mutex = Arc::new(Mutex::new(0));
122 /// let mut threads = Vec::with_capacity(N);
123 /// (0..N).for_each(|_| {
124 /// let data_mutex_clone = Arc::clone(&data_mutex);
125 /// let res_mutex_clone = Arc::clone(&res_mutex);
127 /// threads.push(thread::spawn(move || {
128 /// let mut data = data_mutex_clone.lock().unwrap();
129 /// // This is the result of some important and long-ish work.
130 /// let result = data.iter().fold(0, |acc, x| acc + x * 2);
131 /// data.push(result);
133 /// *res_mutex_clone.lock().unwrap() += result;
137 /// let mut data = data_mutex.lock().unwrap();
138 /// // This is the result of some important and long-ish work.
139 /// let result = data.iter().fold(0, |acc, x| acc + x * 2);
140 /// data.push(result);
141 /// // We drop the `data` explicitly because it's not necessary anymore and the
142 /// // thread still has work to do. This allow other threads to start working on
143 /// // the data immediately, without waiting for the rest of the unrelated work
144 /// // to be done here.
146 /// // It's even more important here than in the threads because we `.join` the
147 /// // threads after that. If we had not dropped the mutex guard, a thread could
148 /// // be waiting forever for it, causing a deadlock.
150 /// // Here the mutex guard is not assigned to a variable and so, even if the
151 /// // scope does not end after this line, the mutex is still released: there is
153 /// *res_mutex.lock().unwrap() += result;
155 /// threads.into_iter().for_each(|thread| {
158 /// .expect("The thread creating or execution failed !")
161 /// assert_eq!(*res_mutex.lock().unwrap(), 800);
163 #[stable(feature = "rust1", since = "1.0.0")]
164 #[cfg_attr(not(test), rustc_diagnostic_item = "Mutex")]
165 pub struct Mutex<T: ?Sized> {
166 inner: sys::MovableMutex,
167 poison: poison::Flag,
171 // these are the only places where `T: Send` matters; all other
172 // functionality works fine on a single thread.
173 #[stable(feature = "rust1", since = "1.0.0")]
174 unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
175 #[stable(feature = "rust1", since = "1.0.0")]
176 unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
178 /// An RAII implementation of a "scoped lock" of a mutex. When this structure is
179 /// dropped (falls out of scope), the lock will be unlocked.
181 /// The data protected by the mutex can be accessed through this guard via its
182 /// [`Deref`] and [`DerefMut`] implementations.
184 /// This structure is created by the [`lock`] and [`try_lock`] methods on
187 /// [`lock`]: Mutex::lock
188 /// [`try_lock`]: Mutex::try_lock
189 #[must_use = "if unused the Mutex will immediately unlock"]
190 #[must_not_suspend = "holding a MutexGuard across suspend \
191 points can cause deadlocks, delays, \
192 and cause Futures to not implement `Send`"]
193 #[stable(feature = "rust1", since = "1.0.0")]
194 #[clippy::has_significant_drop]
195 pub struct MutexGuard<'a, T: ?Sized + 'a> {
197 poison: poison::Guard,
200 #[stable(feature = "rust1", since = "1.0.0")]
201 impl<T: ?Sized> !Send for MutexGuard<'_, T> {}
202 #[stable(feature = "mutexguard", since = "1.19.0")]
203 unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}
206 /// Creates a new mutex in an unlocked state ready for use.
211 /// use std::sync::Mutex;
213 /// let mutex = Mutex::new(0);
215 #[stable(feature = "rust1", since = "1.0.0")]
216 #[rustc_const_stable(feature = "const_locks", since = "1.63.0")]
218 pub const fn new(t: T) -> Mutex<T> {
220 inner: sys::MovableMutex::new(),
221 poison: poison::Flag::new(),
222 data: UnsafeCell::new(t),
227 impl<T: ?Sized> Mutex<T> {
228 /// Acquires a mutex, blocking the current thread until it is able to do so.
230 /// This function will block the local thread until it is available to acquire
231 /// the mutex. Upon returning, the thread is the only thread with the lock
232 /// held. An RAII guard is returned to allow scoped unlock of the lock. When
233 /// the guard goes out of scope, the mutex will be unlocked.
235 /// The exact behavior on locking a mutex in the thread which already holds
236 /// the lock is left unspecified. However, this function will not return on
237 /// the second call (it might panic or deadlock, for example).
241 /// If another user of this mutex panicked while holding the mutex, then
242 /// this call will return an error once the mutex is acquired.
246 /// This function might panic when called if the lock is already held by
247 /// the current thread.
252 /// use std::sync::{Arc, Mutex};
255 /// let mutex = Arc::new(Mutex::new(0));
256 /// let c_mutex = Arc::clone(&mutex);
258 /// thread::spawn(move || {
259 /// *c_mutex.lock().unwrap() = 10;
260 /// }).join().expect("thread::spawn failed");
261 /// assert_eq!(*mutex.lock().unwrap(), 10);
263 #[stable(feature = "rust1", since = "1.0.0")]
264 pub fn lock(&self) -> LockResult<MutexGuard<'_, T>> {
266 self.inner.raw_lock();
267 MutexGuard::new(self)
271 /// Attempts to acquire this lock.
273 /// If the lock could not be acquired at this time, then [`Err`] is returned.
274 /// Otherwise, an RAII guard is returned. The lock will be unlocked when the
275 /// guard is dropped.
277 /// This function does not block.
281 /// If another user of this mutex panicked while holding the mutex, then
282 /// this call will return the [`Poisoned`] error if the mutex would
283 /// otherwise be acquired.
285 /// If the mutex could not be acquired because it is already locked, then
286 /// this call will return the [`WouldBlock`] error.
288 /// [`Poisoned`]: TryLockError::Poisoned
289 /// [`WouldBlock`]: TryLockError::WouldBlock
294 /// use std::sync::{Arc, Mutex};
297 /// let mutex = Arc::new(Mutex::new(0));
298 /// let c_mutex = Arc::clone(&mutex);
300 /// thread::spawn(move || {
301 /// let mut lock = c_mutex.try_lock();
302 /// if let Ok(ref mut mutex) = lock {
305 /// println!("try_lock failed");
307 /// }).join().expect("thread::spawn failed");
308 /// assert_eq!(*mutex.lock().unwrap(), 10);
310 #[stable(feature = "rust1", since = "1.0.0")]
311 pub fn try_lock(&self) -> TryLockResult<MutexGuard<'_, T>> {
313 if self.inner.try_lock() {
314 Ok(MutexGuard::new(self)?)
316 Err(TryLockError::WouldBlock)
321 /// Immediately drops the guard, and consequently unlocks the mutex.
323 /// This function is equivalent to calling [`drop`] on the guard but is more self-documenting.
324 /// Alternately, the guard will be automatically dropped when it goes out of scope.
327 /// #![feature(mutex_unlock)]
329 /// use std::sync::Mutex;
330 /// let mutex = Mutex::new(0);
332 /// let mut guard = mutex.lock().unwrap();
334 /// Mutex::unlock(guard);
336 #[unstable(feature = "mutex_unlock", issue = "81872")]
337 pub fn unlock(guard: MutexGuard<'_, T>) {
341 /// Determines whether the mutex is poisoned.
343 /// If another thread is active, the mutex can still become poisoned at any
344 /// time. You should not trust a `false` value for program correctness
345 /// without additional synchronization.
350 /// use std::sync::{Arc, Mutex};
353 /// let mutex = Arc::new(Mutex::new(0));
354 /// let c_mutex = Arc::clone(&mutex);
356 /// let _ = thread::spawn(move || {
357 /// let _lock = c_mutex.lock().unwrap();
358 /// panic!(); // the mutex gets poisoned
360 /// assert_eq!(mutex.is_poisoned(), true);
363 #[stable(feature = "sync_poison", since = "1.2.0")]
364 pub fn is_poisoned(&self) -> bool {
368 /// Clear the poisoned state from a mutex
370 /// If the mutex is poisoned, it will remain poisoned until this function is called. This
371 /// allows recovering from a poisoned state and marking that it has recovered. For example, if
372 /// the value is overwritten by a known-good value, then the mutex can be marked as
373 /// un-poisoned. Or possibly, the value could be inspected to determine if it is in a
374 /// consistent state, and if so the poison is removed.
379 /// #![feature(mutex_unpoison)]
381 /// use std::sync::{Arc, Mutex};
384 /// let mutex = Arc::new(Mutex::new(0));
385 /// let c_mutex = Arc::clone(&mutex);
387 /// let _ = thread::spawn(move || {
388 /// let _lock = c_mutex.lock().unwrap();
389 /// panic!(); // the mutex gets poisoned
392 /// assert_eq!(mutex.is_poisoned(), true);
393 /// let x = mutex.lock().unwrap_or_else(|mut e| {
394 /// **e.get_mut() = 1;
395 /// mutex.clear_poison();
398 /// assert_eq!(mutex.is_poisoned(), false);
399 /// assert_eq!(*x, 1);
402 #[unstable(feature = "mutex_unpoison", issue = "96469")]
403 pub fn clear_poison(&self) {
407 /// Consumes this mutex, returning the underlying data.
411 /// If another user of this mutex panicked while holding the mutex, then
412 /// this call will return an error instead.
417 /// use std::sync::Mutex;
419 /// let mutex = Mutex::new(0);
420 /// assert_eq!(mutex.into_inner().unwrap(), 0);
422 #[stable(feature = "mutex_into_inner", since = "1.6.0")]
423 pub fn into_inner(self) -> LockResult<T>
427 let data = self.data.into_inner();
428 poison::map_result(self.poison.borrow(), |()| data)
431 /// Returns a mutable reference to the underlying data.
433 /// Since this call borrows the `Mutex` mutably, no actual locking needs to
434 /// take place -- the mutable borrow statically guarantees no locks exist.
438 /// If another user of this mutex panicked while holding the mutex, then
439 /// this call will return an error instead.
444 /// use std::sync::Mutex;
446 /// let mut mutex = Mutex::new(0);
447 /// *mutex.get_mut().unwrap() = 10;
448 /// assert_eq!(*mutex.lock().unwrap(), 10);
450 #[stable(feature = "mutex_get_mut", since = "1.6.0")]
451 pub fn get_mut(&mut self) -> LockResult<&mut T> {
452 let data = self.data.get_mut();
453 poison::map_result(self.poison.borrow(), |()| data)
457 #[stable(feature = "mutex_from", since = "1.24.0")]
458 impl<T> From<T> for Mutex<T> {
459 /// Creates a new mutex in an unlocked state ready for use.
460 /// This is equivalent to [`Mutex::new`].
461 fn from(t: T) -> Self {
466 #[stable(feature = "mutex_default", since = "1.10.0")]
467 impl<T: ?Sized + Default> Default for Mutex<T> {
468 /// Creates a `Mutex<T>`, with the `Default` value for T.
469 fn default() -> Mutex<T> {
470 Mutex::new(Default::default())
474 #[stable(feature = "rust1", since = "1.0.0")]
475 impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
476 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
477 let mut d = f.debug_struct("Mutex");
478 match self.try_lock() {
480 d.field("data", &&*guard);
482 Err(TryLockError::Poisoned(err)) => {
483 d.field("data", &&**err.get_ref());
485 Err(TryLockError::WouldBlock) => {
486 struct LockedPlaceholder;
487 impl fmt::Debug for LockedPlaceholder {
488 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
489 f.write_str("<locked>")
492 d.field("data", &LockedPlaceholder);
495 d.field("poisoned", &self.poison.get());
496 d.finish_non_exhaustive()
500 impl<'mutex, T: ?Sized> MutexGuard<'mutex, T> {
501 unsafe fn new(lock: &'mutex Mutex<T>) -> LockResult<MutexGuard<'mutex, T>> {
502 poison::map_result(lock.poison.guard(), |guard| MutexGuard { lock, poison: guard })
506 #[stable(feature = "rust1", since = "1.0.0")]
507 impl<T: ?Sized> Deref for MutexGuard<'_, T> {
510 fn deref(&self) -> &T {
511 unsafe { &*self.lock.data.get() }
515 #[stable(feature = "rust1", since = "1.0.0")]
516 impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
517 fn deref_mut(&mut self) -> &mut T {
518 unsafe { &mut *self.lock.data.get() }
522 #[stable(feature = "rust1", since = "1.0.0")]
523 impl<T: ?Sized> Drop for MutexGuard<'_, T> {
527 self.lock.poison.done(&self.poison);
528 self.lock.inner.raw_unlock();
533 #[stable(feature = "std_debug", since = "1.16.0")]
534 impl<T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, T> {
535 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
536 fmt::Debug::fmt(&**self, f)
540 #[stable(feature = "std_guard_impls", since = "1.20.0")]
541 impl<T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'_, T> {
542 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
547 pub fn guard_lock<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a sys::MovableMutex {
551 pub fn guard_poison<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a poison::Flag {