1 use crate::cell::UnsafeCell;
4 use crate::ops::{Deref, DerefMut};
6 use crate::sys_common::mutex as sys;
7 use crate::sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
9 /// A mutual exclusion primitive useful for protecting shared data
11 /// This mutex will block threads waiting for the lock to become available. The
12 /// mutex can also be statically initialized or created via a [`new`]
13 /// constructor. Each mutex has a type parameter which represents the data that
14 /// it is protecting. The data can only be accessed through the RAII guards
15 /// returned from [`lock`] and [`try_lock`], which guarantees that the data is only
16 /// 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`]: #method.new
37 /// [`lock`]: #method.lock
38 /// [`try_lock`]: #method.try_lock
39 /// [`Result`]: ../../std/result/enum.Result.html
40 /// [`unwrap()`]: ../../std/result/enum.Result.html#method.unwrap
41 /// [`PoisonError`]: ../../std/sync/struct.PoisonError.html
42 /// [`into_inner`]: ../../std/sync/struct.PoisonError.html#method.into_inner
47 /// use std::sync::{Arc, Mutex};
49 /// use std::sync::mpsc::channel;
51 /// const N: usize = 10;
53 /// // Spawn a few threads to increment a shared variable (non-atomically), and
54 /// // let the main thread know once all increments are done.
56 /// // Here we're using an Arc to share memory among threads, and the data inside
57 /// // the Arc is protected with a mutex.
58 /// let data = Arc::new(Mutex::new(0));
60 /// let (tx, rx) = channel();
62 /// let (data, tx) = (Arc::clone(&data), tx.clone());
63 /// thread::spawn(move || {
64 /// // The shared state can only be accessed once the lock is held.
65 /// // Our non-atomic increment is safe because we're the only thread
66 /// // which can access the shared state when the lock is held.
68 /// // We unwrap() the return value to assert that we are not expecting
69 /// // threads to ever fail while holding the lock.
70 /// let mut data = data.lock().unwrap();
73 /// tx.send(()).unwrap();
75 /// // the lock is unlocked here when `data` goes out of scope.
79 /// rx.recv().unwrap();
82 /// To recover from a poisoned mutex:
85 /// use std::sync::{Arc, Mutex};
88 /// let lock = Arc::new(Mutex::new(0_u32));
89 /// let lock2 = lock.clone();
91 /// let _ = thread::spawn(move || -> () {
92 /// // This thread will acquire the mutex first, unwrapping the result of
93 /// // `lock` because the lock has not been poisoned.
94 /// let _guard = lock2.lock().unwrap();
96 /// // This panic while holding the lock (`_guard` is in scope) will poison
101 /// // The lock is poisoned by this point, but the returned result can be
102 /// // pattern matched on to return the underlying guard on both branches.
103 /// let mut guard = match lock.lock() {
104 /// Ok(guard) => guard,
105 /// Err(poisoned) => poisoned.into_inner(),
111 /// It is sometimes a good idea (or even necessary) to manually drop the mutex
112 /// to unlock it as soon as possible. If you need the resource until the end of
113 /// the scope, this is not needed.
116 /// use std::sync::{Arc, Mutex};
119 /// const N: usize = 3;
121 /// // Some data to work with in multiple threads.
122 /// let data_mutex = Arc::new(Mutex::new(vec![1, 2, 3, 4]));
123 /// // The result of all the work across all threads.
124 /// let res_mutex = Arc::new(Mutex::new(0));
126 /// // Threads other than the main thread.
127 /// let mut threads = Vec::with_capacity(N);
128 /// (0..N).for_each(|_| {
129 /// // Getting clones for the mutexes.
130 /// let data_mutex_clone = Arc::clone(&data_mutex);
131 /// let res_mutex_clone = Arc::clone(&res_mutex);
133 /// threads.push(thread::spawn(move || {
134 /// let mut data = data_mutex_clone.lock().unwrap();
135 /// // This is the result of some important and long-ish work.
136 /// let result = data.iter().fold(0, |acc, x| acc + x * 2);
137 /// data.push(result);
138 /// // We drop the `data` explicitely because it's not necessary anymore
139 /// // and the thread still has work to do. This allow other threads to
140 /// // start working on the data immediately, without waiting
141 /// // for the rest of the unrelated work to be done here.
143 /// *res_mutex_clone.lock().unwrap() += result;
147 /// let mut data = data_mutex.lock().unwrap();
148 /// // This is the result of some important and long-ish work.
149 /// let result = data.iter().fold(0, |acc, x| acc + x * 2);
150 /// data.push(result);
151 /// // We drop the `data` explicitely because it's not necessary anymore
152 /// // and the thread still has work to do. This allow other threads to
153 /// // start working on the data immediately, without waiting
154 /// // for the rest of the unrelated work to be done here.
156 /// // It's even more important here because we `.join` the threads after that.
157 /// // If we had not dropped the lock, a thread could be waiting forever for
158 /// // it, causing a deadlock.
160 /// // Here the lock is not assigned to a variable and so, even if the scope
161 /// // does not end after this line, the mutex is still released:
162 /// // there is no deadlock.
163 /// *res_mutex.lock().unwrap() += result;
165 /// threads.into_iter().for_each(|thread| {
168 /// .expect("The thread creating or execution failed !")
171 /// assert_eq!(*res_mutex.lock().unwrap(), 800);
173 #[stable(feature = "rust1", since = "1.0.0")]
174 #[cfg_attr(not(test), rustc_diagnostic_item = "mutex_type")]
175 pub struct Mutex<T: ?Sized> {
176 // Note that this mutex is in a *box*, not inlined into the struct itself.
177 // Once a native mutex has been used once, its address can never change (it
178 // can't be moved). This mutex type can be safely moved at any time, so to
179 // ensure that the native mutex is used correctly we box the inner mutex to
180 // give it a constant address.
181 inner: Box<sys::Mutex>,
182 poison: poison::Flag,
186 // these are the only places where `T: Send` matters; all other
187 // functionality works fine on a single thread.
188 #[stable(feature = "rust1", since = "1.0.0")]
189 unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
190 #[stable(feature = "rust1", since = "1.0.0")]
191 unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
193 /// An RAII implementation of a "scoped lock" of a mutex. When this structure is
194 /// dropped (falls out of scope), the lock will be unlocked.
196 /// The data protected by the mutex can be accessed through this guard via its
197 /// [`Deref`] and [`DerefMut`] implementations.
199 /// This structure is created by the [`lock`] and [`try_lock`] methods on
202 /// [`Deref`]: ../../std/ops/trait.Deref.html
203 /// [`DerefMut`]: ../../std/ops/trait.DerefMut.html
204 /// [`lock`]: struct.Mutex.html#method.lock
205 /// [`try_lock`]: struct.Mutex.html#method.try_lock
206 /// [`Mutex`]: struct.Mutex.html
207 #[must_use = "if unused the Mutex will immediately unlock"]
208 #[stable(feature = "rust1", since = "1.0.0")]
209 pub struct MutexGuard<'a, T: ?Sized + 'a> {
211 poison: poison::Guard,
214 #[stable(feature = "rust1", since = "1.0.0")]
215 impl<T: ?Sized> !Send for MutexGuard<'_, T> {}
216 #[stable(feature = "mutexguard", since = "1.19.0")]
217 unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}
220 /// Creates a new mutex in an unlocked state ready for use.
225 /// use std::sync::Mutex;
227 /// let mutex = Mutex::new(0);
229 #[stable(feature = "rust1", since = "1.0.0")]
230 pub fn new(t: T) -> Mutex<T> {
232 inner: box sys::Mutex::new(),
233 poison: poison::Flag::new(),
234 data: UnsafeCell::new(t),
243 impl<T: ?Sized> Mutex<T> {
244 /// Acquires a mutex, blocking the current thread until it is able to do so.
246 /// This function will block the local thread until it is available to acquire
247 /// the mutex. Upon returning, the thread is the only thread with the lock
248 /// held. An RAII guard is returned to allow scoped unlock of the lock. When
249 /// the guard goes out of scope, the mutex will be unlocked.
251 /// The exact behavior on locking a mutex in the thread which already holds
252 /// the lock is left unspecified. However, this function will not return on
253 /// the second call (it might panic or deadlock, for example).
257 /// If another user of this mutex panicked while holding the mutex, then
258 /// this call will return an error once the mutex is acquired.
262 /// This function might panic when called if the lock is already held by
263 /// the current thread.
268 /// use std::sync::{Arc, Mutex};
271 /// let mutex = Arc::new(Mutex::new(0));
272 /// let c_mutex = mutex.clone();
274 /// thread::spawn(move || {
275 /// *c_mutex.lock().unwrap() = 10;
276 /// }).join().expect("thread::spawn failed");
277 /// assert_eq!(*mutex.lock().unwrap(), 10);
279 #[stable(feature = "rust1", since = "1.0.0")]
280 pub fn lock(&self) -> LockResult<MutexGuard<'_, T>> {
282 self.inner.raw_lock();
283 MutexGuard::new(self)
287 /// Attempts to acquire this lock.
289 /// If the lock could not be acquired at this time, then [`Err`] is returned.
290 /// Otherwise, an RAII guard is returned. The lock will be unlocked when the
291 /// guard is dropped.
293 /// This function does not block.
297 /// If another user of this mutex panicked while holding the mutex, then
298 /// this call will return failure if the mutex would otherwise be
301 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
306 /// use std::sync::{Arc, Mutex};
309 /// let mutex = Arc::new(Mutex::new(0));
310 /// let c_mutex = mutex.clone();
312 /// thread::spawn(move || {
313 /// let mut lock = c_mutex.try_lock();
314 /// if let Ok(ref mut mutex) = lock {
317 /// println!("try_lock failed");
319 /// }).join().expect("thread::spawn failed");
320 /// assert_eq!(*mutex.lock().unwrap(), 10);
322 #[stable(feature = "rust1", since = "1.0.0")]
323 pub fn try_lock(&self) -> TryLockResult<MutexGuard<'_, T>> {
325 if self.inner.try_lock() {
326 Ok(MutexGuard::new(self)?)
328 Err(TryLockError::WouldBlock)
333 /// Determines whether the mutex is poisoned.
335 /// If another thread is active, the mutex can still become poisoned at any
336 /// time. You should not trust a `false` value for program correctness
337 /// without additional synchronization.
342 /// use std::sync::{Arc, Mutex};
345 /// let mutex = Arc::new(Mutex::new(0));
346 /// let c_mutex = mutex.clone();
348 /// let _ = thread::spawn(move || {
349 /// let _lock = c_mutex.lock().unwrap();
350 /// panic!(); // the mutex gets poisoned
352 /// assert_eq!(mutex.is_poisoned(), true);
355 #[stable(feature = "sync_poison", since = "1.2.0")]
356 pub fn is_poisoned(&self) -> bool {
360 /// Consumes this mutex, returning the underlying data.
364 /// If another user of this mutex panicked while holding the mutex, then
365 /// this call will return an error instead.
370 /// use std::sync::Mutex;
372 /// let mutex = Mutex::new(0);
373 /// assert_eq!(mutex.into_inner().unwrap(), 0);
375 #[stable(feature = "mutex_into_inner", since = "1.6.0")]
376 pub fn into_inner(self) -> LockResult<T>
380 // We know statically that there are no outstanding references to
381 // `self` so there's no need to lock the inner mutex.
383 // To get the inner value, we'd like to call `data.into_inner()`,
384 // but because `Mutex` impl-s `Drop`, we can't move out of it, so
385 // we'll have to destructure it manually instead.
387 // Like `let Mutex { inner, poison, data } = self`.
388 let (inner, poison, data) = {
389 let Mutex { ref inner, ref poison, ref data } = self;
390 (ptr::read(inner), ptr::read(poison), ptr::read(data))
393 inner.destroy(); // Keep in sync with the `Drop` impl.
396 poison::map_result(poison.borrow(), |_| data.into_inner())
400 /// Returns a mutable reference to the underlying data.
402 /// Since this call borrows the `Mutex` mutably, no actual locking needs to
403 /// take place -- the mutable borrow statically guarantees no locks exist.
407 /// If another user of this mutex panicked while holding the mutex, then
408 /// this call will return an error instead.
413 /// use std::sync::Mutex;
415 /// let mut mutex = Mutex::new(0);
416 /// *mutex.get_mut().unwrap() = 10;
417 /// assert_eq!(*mutex.lock().unwrap(), 10);
419 #[stable(feature = "mutex_get_mut", since = "1.6.0")]
420 pub fn get_mut(&mut self) -> LockResult<&mut T> {
421 // We know statically that there are no other references to `self`, so
422 // there's no need to lock the inner mutex.
423 let data = unsafe { &mut *self.data.get() };
424 poison::map_result(self.poison.borrow(), |_| data)
428 #[stable(feature = "rust1", since = "1.0.0")]
429 unsafe impl<#[may_dangle] T: ?Sized> Drop for Mutex<T> {
431 // This is actually safe b/c we know that there is no further usage of
432 // this mutex (it's up to the user to arrange for a mutex to get
433 // dropped, that's not our job)
435 // IMPORTANT: This code must be kept in sync with `Mutex::into_inner`.
436 unsafe { self.inner.destroy() }
440 #[stable(feature = "mutex_from", since = "1.24.0")]
441 impl<T> From<T> for Mutex<T> {
442 /// Creates a new mutex in an unlocked state ready for use.
443 /// This is equivalent to [`Mutex::new`].
445 /// [`Mutex::new`]: ../../std/sync/struct.Mutex.html#method.new
446 fn from(t: T) -> Self {
451 #[stable(feature = "mutex_default", since = "1.10.0")]
452 impl<T: ?Sized + Default> Default for Mutex<T> {
453 /// Creates a `Mutex<T>`, with the `Default` value for T.
454 fn default() -> Mutex<T> {
455 Mutex::new(Default::default())
459 #[stable(feature = "rust1", since = "1.0.0")]
460 impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
461 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
462 match self.try_lock() {
463 Ok(guard) => f.debug_struct("Mutex").field("data", &&*guard).finish(),
464 Err(TryLockError::Poisoned(err)) => {
465 f.debug_struct("Mutex").field("data", &&**err.get_ref()).finish()
467 Err(TryLockError::WouldBlock) => {
468 struct LockedPlaceholder;
469 impl fmt::Debug for LockedPlaceholder {
470 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
471 f.write_str("<locked>")
475 f.debug_struct("Mutex").field("data", &LockedPlaceholder).finish()
481 impl<'mutex, T: ?Sized> MutexGuard<'mutex, T> {
482 unsafe fn new(lock: &'mutex Mutex<T>) -> LockResult<MutexGuard<'mutex, T>> {
483 poison::map_result(lock.poison.borrow(), |guard| MutexGuard { lock, poison: guard })
487 #[stable(feature = "rust1", since = "1.0.0")]
488 impl<T: ?Sized> Deref for MutexGuard<'_, T> {
491 fn deref(&self) -> &T {
492 unsafe { &*self.lock.data.get() }
496 #[stable(feature = "rust1", since = "1.0.0")]
497 impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
498 fn deref_mut(&mut self) -> &mut T {
499 unsafe { &mut *self.lock.data.get() }
503 #[stable(feature = "rust1", since = "1.0.0")]
504 impl<T: ?Sized> Drop for MutexGuard<'_, T> {
508 self.lock.poison.done(&self.poison);
509 self.lock.inner.raw_unlock();
514 #[stable(feature = "std_debug", since = "1.16.0")]
515 impl<T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, T> {
516 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
517 fmt::Debug::fmt(&**self, f)
521 #[stable(feature = "std_guard_impls", since = "1.20.0")]
522 impl<T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'_, T> {
523 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
528 pub fn guard_lock<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a sys::Mutex {
532 pub fn guard_poison<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a poison::Flag {
536 #[cfg(all(test, not(target_os = "emscripten")))]
538 use crate::sync::atomic::{AtomicUsize, Ordering};
539 use crate::sync::mpsc::channel;
540 use crate::sync::{Arc, Condvar, Mutex};
543 struct Packet<T>(Arc<(Mutex<T>, Condvar)>);
545 #[derive(Eq, PartialEq, Debug)]
550 let m = Mutex::new(());
551 drop(m.lock().unwrap());
552 drop(m.lock().unwrap());
560 let m = Arc::new(Mutex::new(0));
562 fn inc(m: &Mutex<u32>) {
564 *m.lock().unwrap() += 1;
568 let (tx, rx) = channel();
570 let tx2 = tx.clone();
572 thread::spawn(move || {
574 tx2.send(()).unwrap();
576 let tx2 = tx.clone();
578 thread::spawn(move || {
580 tx2.send(()).unwrap();
588 assert_eq!(*m.lock().unwrap(), J * K * 2);
593 let m = Mutex::new(());
594 *m.try_lock().unwrap() = ();
598 fn test_into_inner() {
599 let m = Mutex::new(NonCopy(10));
600 assert_eq!(m.into_inner().unwrap(), NonCopy(10));
604 fn test_into_inner_drop() {
605 struct Foo(Arc<AtomicUsize>);
608 self.0.fetch_add(1, Ordering::SeqCst);
611 let num_drops = Arc::new(AtomicUsize::new(0));
612 let m = Mutex::new(Foo(num_drops.clone()));
613 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
615 let _inner = m.into_inner().unwrap();
616 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
618 assert_eq!(num_drops.load(Ordering::SeqCst), 1);
622 fn test_into_inner_poison() {
623 let m = Arc::new(Mutex::new(NonCopy(10)));
625 let _ = thread::spawn(move || {
626 let _lock = m2.lock().unwrap();
627 panic!("test panic in inner thread to poison mutex");
631 assert!(m.is_poisoned());
632 match Arc::try_unwrap(m).unwrap().into_inner() {
633 Err(e) => assert_eq!(e.into_inner(), NonCopy(10)),
634 Ok(x) => panic!("into_inner of poisoned Mutex is Ok: {:?}", x),
640 let mut m = Mutex::new(NonCopy(10));
641 *m.get_mut().unwrap() = NonCopy(20);
642 assert_eq!(m.into_inner().unwrap(), NonCopy(20));
646 fn test_get_mut_poison() {
647 let m = Arc::new(Mutex::new(NonCopy(10)));
649 let _ = thread::spawn(move || {
650 let _lock = m2.lock().unwrap();
651 panic!("test panic in inner thread to poison mutex");
655 assert!(m.is_poisoned());
656 match Arc::try_unwrap(m).unwrap().get_mut() {
657 Err(e) => assert_eq!(*e.into_inner(), NonCopy(10)),
658 Ok(x) => panic!("get_mut of poisoned Mutex is Ok: {:?}", x),
663 fn test_mutex_arc_condvar() {
664 let packet = Packet(Arc::new((Mutex::new(false), Condvar::new())));
665 let packet2 = Packet(packet.0.clone());
666 let (tx, rx) = channel();
667 let _t = thread::spawn(move || {
668 // wait until parent gets in
670 let &(ref lock, ref cvar) = &*packet2.0;
671 let mut lock = lock.lock().unwrap();
676 let &(ref lock, ref cvar) = &*packet.0;
677 let mut lock = lock.lock().unwrap();
678 tx.send(()).unwrap();
681 lock = cvar.wait(lock).unwrap();
686 fn test_arc_condvar_poison() {
687 let packet = Packet(Arc::new((Mutex::new(1), Condvar::new())));
688 let packet2 = Packet(packet.0.clone());
689 let (tx, rx) = channel();
691 let _t = thread::spawn(move || -> () {
693 let &(ref lock, ref cvar) = &*packet2.0;
694 let _g = lock.lock().unwrap();
696 // Parent should fail when it wakes up.
700 let &(ref lock, ref cvar) = &*packet.0;
701 let mut lock = lock.lock().unwrap();
702 tx.send(()).unwrap();
704 match cvar.wait(lock) {
707 assert_eq!(*lock, 1);
715 fn test_mutex_arc_poison() {
716 let arc = Arc::new(Mutex::new(1));
717 assert!(!arc.is_poisoned());
718 let arc2 = arc.clone();
719 let _ = thread::spawn(move || {
720 let lock = arc2.lock().unwrap();
721 assert_eq!(*lock, 2);
724 assert!(arc.lock().is_err());
725 assert!(arc.is_poisoned());
729 fn test_mutex_arc_nested() {
730 // Tests nested mutexes and access
731 // to underlying data.
732 let arc = Arc::new(Mutex::new(1));
733 let arc2 = Arc::new(Mutex::new(arc));
734 let (tx, rx) = channel();
735 let _t = thread::spawn(move || {
736 let lock = arc2.lock().unwrap();
737 let lock2 = lock.lock().unwrap();
738 assert_eq!(*lock2, 1);
739 tx.send(()).unwrap();
745 fn test_mutex_arc_access_in_unwind() {
746 let arc = Arc::new(Mutex::new(1));
747 let arc2 = arc.clone();
748 let _ = thread::spawn(move || -> () {
752 impl Drop for Unwinder {
754 *self.i.lock().unwrap() += 1;
757 let _u = Unwinder { i: arc2 };
761 let lock = arc.lock().unwrap();
762 assert_eq!(*lock, 2);
766 fn test_mutex_unsized() {
767 let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
769 let b = &mut *mutex.lock().unwrap();
773 let comp: &[i32] = &[4, 2, 5];
774 assert_eq!(&*mutex.lock().unwrap(), comp);