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([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 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 /// // We drop the `data` explicitely because it's not necessary anymore
138 /// // and the thread still has work to do. This allow other threads to
139 /// // start working on the data immediately, without waiting
140 /// // for the rest of the unrelated work to be done here.
141 /// std::mem::drop(data);
142 /// *res_mutex_clone.lock().unwrap() += result;
146 /// let data = *data_mutex.lock().unwrap();
147 /// // This is the result of some important and long-ish work.
148 /// let result = data.iter().fold(0, |acc, x| acc + x * 2);
149 /// // We drop the `data` explicitely because it's not necessary anymore
150 /// // and the thread still has work to do. This allow other threads to
151 /// // start working on the data immediately, without waiting
152 /// // for the rest of the unrelated work to be done here.
154 /// // It's even more important here because we `.join` the threads after that.
155 /// // If we had not dropped the lock, a thread could be waiting forever for
156 /// // it, causing a deadlock.
157 /// std::mem::drop(data);
158 /// // Here the lock is not assigned to a variable and so, even if the scope
159 /// // does not end after this line, the mutex is still released:
160 /// // there is no deadlock.
161 /// *res_mutex.lock().unwrap() += result;
163 /// threads.into_iter().for_each(|thread| {
166 /// .expect("The thread creating or execution failed !")
169 /// assert_eq!(*res_mutex.lock().unwrap(), 80);
171 #[stable(feature = "rust1", since = "1.0.0")]
172 #[cfg_attr(not(test), rustc_diagnostic_item = "mutex_type")]
173 pub struct Mutex<T: ?Sized> {
174 // Note that this mutex is in a *box*, not inlined into the struct itself.
175 // Once a native mutex has been used once, its address can never change (it
176 // can't be moved). This mutex type can be safely moved at any time, so to
177 // ensure that the native mutex is used correctly we box the inner mutex to
178 // give it a constant address.
179 inner: Box<sys::Mutex>,
180 poison: poison::Flag,
184 // these are the only places where `T: Send` matters; all other
185 // functionality works fine on a single thread.
186 #[stable(feature = "rust1", since = "1.0.0")]
187 unsafe impl<T: ?Sized + Send> Send for Mutex<T> {}
188 #[stable(feature = "rust1", since = "1.0.0")]
189 unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
191 /// An RAII implementation of a "scoped lock" of a mutex. When this structure is
192 /// dropped (falls out of scope), the lock will be unlocked.
194 /// The data protected by the mutex can be accessed through this guard via its
195 /// [`Deref`] and [`DerefMut`] implementations.
197 /// This structure is created by the [`lock`] and [`try_lock`] methods on
200 /// [`Deref`]: ../../std/ops/trait.Deref.html
201 /// [`DerefMut`]: ../../std/ops/trait.DerefMut.html
202 /// [`lock`]: struct.Mutex.html#method.lock
203 /// [`try_lock`]: struct.Mutex.html#method.try_lock
204 /// [`Mutex`]: struct.Mutex.html
205 #[must_use = "if unused the Mutex will immediately unlock"]
206 #[stable(feature = "rust1", since = "1.0.0")]
207 pub struct MutexGuard<'a, T: ?Sized + 'a> {
209 poison: poison::Guard,
212 #[stable(feature = "rust1", since = "1.0.0")]
213 impl<T: ?Sized> !Send for MutexGuard<'_, T> {}
214 #[stable(feature = "mutexguard", since = "1.19.0")]
215 unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}
218 /// Creates a new mutex in an unlocked state ready for use.
223 /// use std::sync::Mutex;
225 /// let mutex = Mutex::new(0);
227 #[stable(feature = "rust1", since = "1.0.0")]
228 pub fn new(t: T) -> Mutex<T> {
230 inner: box sys::Mutex::new(),
231 poison: poison::Flag::new(),
232 data: UnsafeCell::new(t),
241 impl<T: ?Sized> Mutex<T> {
242 /// Acquires a mutex, blocking the current thread until it is able to do so.
244 /// This function will block the local thread until it is available to acquire
245 /// the mutex. Upon returning, the thread is the only thread with the lock
246 /// held. An RAII guard is returned to allow scoped unlock of the lock. When
247 /// the guard goes out of scope, the mutex will be unlocked.
249 /// The exact behavior on locking a mutex in the thread which already holds
250 /// the lock is left unspecified. However, this function will not return on
251 /// the second call (it might panic or deadlock, for example).
255 /// If another user of this mutex panicked while holding the mutex, then
256 /// this call will return an error once the mutex is acquired.
260 /// This function might panic when called if the lock is already held by
261 /// the current thread.
266 /// use std::sync::{Arc, Mutex};
269 /// let mutex = Arc::new(Mutex::new(0));
270 /// let c_mutex = mutex.clone();
272 /// thread::spawn(move || {
273 /// *c_mutex.lock().unwrap() = 10;
274 /// }).join().expect("thread::spawn failed");
275 /// assert_eq!(*mutex.lock().unwrap(), 10);
277 #[stable(feature = "rust1", since = "1.0.0")]
278 pub fn lock(&self) -> LockResult<MutexGuard<'_, T>> {
280 self.inner.raw_lock();
281 MutexGuard::new(self)
285 /// Attempts to acquire this lock.
287 /// If the lock could not be acquired at this time, then [`Err`] is returned.
288 /// Otherwise, an RAII guard is returned. The lock will be unlocked when the
289 /// guard is dropped.
291 /// This function does not block.
295 /// If another user of this mutex panicked while holding the mutex, then
296 /// this call will return failure if the mutex would otherwise be
299 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
304 /// use std::sync::{Arc, Mutex};
307 /// let mutex = Arc::new(Mutex::new(0));
308 /// let c_mutex = mutex.clone();
310 /// thread::spawn(move || {
311 /// let mut lock = c_mutex.try_lock();
312 /// if let Ok(ref mut mutex) = lock {
315 /// println!("try_lock failed");
317 /// }).join().expect("thread::spawn failed");
318 /// assert_eq!(*mutex.lock().unwrap(), 10);
320 #[stable(feature = "rust1", since = "1.0.0")]
321 pub fn try_lock(&self) -> TryLockResult<MutexGuard<'_, T>> {
323 if self.inner.try_lock() {
324 Ok(MutexGuard::new(self)?)
326 Err(TryLockError::WouldBlock)
331 /// Determines whether the mutex is poisoned.
333 /// If another thread is active, the mutex can still become poisoned at any
334 /// time. You should not trust a `false` value for program correctness
335 /// without additional synchronization.
340 /// use std::sync::{Arc, Mutex};
343 /// let mutex = Arc::new(Mutex::new(0));
344 /// let c_mutex = mutex.clone();
346 /// let _ = thread::spawn(move || {
347 /// let _lock = c_mutex.lock().unwrap();
348 /// panic!(); // the mutex gets poisoned
350 /// assert_eq!(mutex.is_poisoned(), true);
353 #[stable(feature = "sync_poison", since = "1.2.0")]
354 pub fn is_poisoned(&self) -> bool {
358 /// Consumes this mutex, returning the underlying data.
362 /// If another user of this mutex panicked while holding the mutex, then
363 /// this call will return an error instead.
368 /// use std::sync::Mutex;
370 /// let mutex = Mutex::new(0);
371 /// assert_eq!(mutex.into_inner().unwrap(), 0);
373 #[stable(feature = "mutex_into_inner", since = "1.6.0")]
374 pub fn into_inner(self) -> LockResult<T>
378 // We know statically that there are no outstanding references to
379 // `self` so there's no need to lock the inner mutex.
381 // To get the inner value, we'd like to call `data.into_inner()`,
382 // but because `Mutex` impl-s `Drop`, we can't move out of it, so
383 // we'll have to destructure it manually instead.
385 // Like `let Mutex { inner, poison, data } = self`.
386 let (inner, poison, data) = {
387 let Mutex { ref inner, ref poison, ref data } = self;
388 (ptr::read(inner), ptr::read(poison), ptr::read(data))
391 inner.destroy(); // Keep in sync with the `Drop` impl.
394 poison::map_result(poison.borrow(), |_| data.into_inner())
398 /// Returns a mutable reference to the underlying data.
400 /// Since this call borrows the `Mutex` mutably, no actual locking needs to
401 /// take place -- the mutable borrow statically guarantees no locks exist.
405 /// If another user of this mutex panicked while holding the mutex, then
406 /// this call will return an error instead.
411 /// use std::sync::Mutex;
413 /// let mut mutex = Mutex::new(0);
414 /// *mutex.get_mut().unwrap() = 10;
415 /// assert_eq!(*mutex.lock().unwrap(), 10);
417 #[stable(feature = "mutex_get_mut", since = "1.6.0")]
418 pub fn get_mut(&mut self) -> LockResult<&mut T> {
419 // We know statically that there are no other references to `self`, so
420 // there's no need to lock the inner mutex.
421 let data = unsafe { &mut *self.data.get() };
422 poison::map_result(self.poison.borrow(), |_| data)
426 #[stable(feature = "rust1", since = "1.0.0")]
427 unsafe impl<#[may_dangle] T: ?Sized> Drop for Mutex<T> {
429 // This is actually safe b/c we know that there is no further usage of
430 // this mutex (it's up to the user to arrange for a mutex to get
431 // dropped, that's not our job)
433 // IMPORTANT: This code must be kept in sync with `Mutex::into_inner`.
434 unsafe { self.inner.destroy() }
438 #[stable(feature = "mutex_from", since = "1.24.0")]
439 impl<T> From<T> for Mutex<T> {
440 /// Creates a new mutex in an unlocked state ready for use.
441 /// This is equivalent to [`Mutex::new`].
443 /// [`Mutex::new`]: ../../std/sync/struct.Mutex.html#method.new
444 fn from(t: T) -> Self {
449 #[stable(feature = "mutex_default", since = "1.10.0")]
450 impl<T: ?Sized + Default> Default for Mutex<T> {
451 /// Creates a `Mutex<T>`, with the `Default` value for T.
452 fn default() -> Mutex<T> {
453 Mutex::new(Default::default())
457 #[stable(feature = "rust1", since = "1.0.0")]
458 impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
459 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
460 match self.try_lock() {
461 Ok(guard) => f.debug_struct("Mutex").field("data", &&*guard).finish(),
462 Err(TryLockError::Poisoned(err)) => {
463 f.debug_struct("Mutex").field("data", &&**err.get_ref()).finish()
465 Err(TryLockError::WouldBlock) => {
466 struct LockedPlaceholder;
467 impl fmt::Debug for LockedPlaceholder {
468 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
469 f.write_str("<locked>")
473 f.debug_struct("Mutex").field("data", &LockedPlaceholder).finish()
479 impl<'mutex, T: ?Sized> MutexGuard<'mutex, T> {
480 unsafe fn new(lock: &'mutex Mutex<T>) -> LockResult<MutexGuard<'mutex, T>> {
481 poison::map_result(lock.poison.borrow(), |guard| MutexGuard { lock, poison: guard })
485 #[stable(feature = "rust1", since = "1.0.0")]
486 impl<T: ?Sized> Deref for MutexGuard<'_, T> {
489 fn deref(&self) -> &T {
490 unsafe { &*self.lock.data.get() }
494 #[stable(feature = "rust1", since = "1.0.0")]
495 impl<T: ?Sized> DerefMut for MutexGuard<'_, T> {
496 fn deref_mut(&mut self) -> &mut T {
497 unsafe { &mut *self.lock.data.get() }
501 #[stable(feature = "rust1", since = "1.0.0")]
502 impl<T: ?Sized> Drop for MutexGuard<'_, T> {
506 self.lock.poison.done(&self.poison);
507 self.lock.inner.raw_unlock();
512 #[stable(feature = "std_debug", since = "1.16.0")]
513 impl<T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'_, T> {
514 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
515 fmt::Debug::fmt(&**self, f)
519 #[stable(feature = "std_guard_impls", since = "1.20.0")]
520 impl<T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'_, T> {
521 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
526 pub fn guard_lock<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a sys::Mutex {
530 pub fn guard_poison<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a poison::Flag {
534 #[cfg(all(test, not(target_os = "emscripten")))]
536 use crate::sync::atomic::{AtomicUsize, Ordering};
537 use crate::sync::mpsc::channel;
538 use crate::sync::{Arc, Condvar, Mutex};
541 struct Packet<T>(Arc<(Mutex<T>, Condvar)>);
543 #[derive(Eq, PartialEq, Debug)]
548 let m = Mutex::new(());
549 drop(m.lock().unwrap());
550 drop(m.lock().unwrap());
558 let m = Arc::new(Mutex::new(0));
560 fn inc(m: &Mutex<u32>) {
562 *m.lock().unwrap() += 1;
566 let (tx, rx) = channel();
568 let tx2 = tx.clone();
570 thread::spawn(move || {
572 tx2.send(()).unwrap();
574 let tx2 = tx.clone();
576 thread::spawn(move || {
578 tx2.send(()).unwrap();
586 assert_eq!(*m.lock().unwrap(), J * K * 2);
591 let m = Mutex::new(());
592 *m.try_lock().unwrap() = ();
596 fn test_into_inner() {
597 let m = Mutex::new(NonCopy(10));
598 assert_eq!(m.into_inner().unwrap(), NonCopy(10));
602 fn test_into_inner_drop() {
603 struct Foo(Arc<AtomicUsize>);
606 self.0.fetch_add(1, Ordering::SeqCst);
609 let num_drops = Arc::new(AtomicUsize::new(0));
610 let m = Mutex::new(Foo(num_drops.clone()));
611 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
613 let _inner = m.into_inner().unwrap();
614 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
616 assert_eq!(num_drops.load(Ordering::SeqCst), 1);
620 fn test_into_inner_poison() {
621 let m = Arc::new(Mutex::new(NonCopy(10)));
623 let _ = thread::spawn(move || {
624 let _lock = m2.lock().unwrap();
625 panic!("test panic in inner thread to poison mutex");
629 assert!(m.is_poisoned());
630 match Arc::try_unwrap(m).unwrap().into_inner() {
631 Err(e) => assert_eq!(e.into_inner(), NonCopy(10)),
632 Ok(x) => panic!("into_inner of poisoned Mutex is Ok: {:?}", x),
638 let mut m = Mutex::new(NonCopy(10));
639 *m.get_mut().unwrap() = NonCopy(20);
640 assert_eq!(m.into_inner().unwrap(), NonCopy(20));
644 fn test_get_mut_poison() {
645 let m = Arc::new(Mutex::new(NonCopy(10)));
647 let _ = thread::spawn(move || {
648 let _lock = m2.lock().unwrap();
649 panic!("test panic in inner thread to poison mutex");
653 assert!(m.is_poisoned());
654 match Arc::try_unwrap(m).unwrap().get_mut() {
655 Err(e) => assert_eq!(*e.into_inner(), NonCopy(10)),
656 Ok(x) => panic!("get_mut of poisoned Mutex is Ok: {:?}", x),
661 fn test_mutex_arc_condvar() {
662 let packet = Packet(Arc::new((Mutex::new(false), Condvar::new())));
663 let packet2 = Packet(packet.0.clone());
664 let (tx, rx) = channel();
665 let _t = thread::spawn(move || {
666 // wait until parent gets in
668 let &(ref lock, ref cvar) = &*packet2.0;
669 let mut lock = lock.lock().unwrap();
674 let &(ref lock, ref cvar) = &*packet.0;
675 let mut lock = lock.lock().unwrap();
676 tx.send(()).unwrap();
679 lock = cvar.wait(lock).unwrap();
684 fn test_arc_condvar_poison() {
685 let packet = Packet(Arc::new((Mutex::new(1), Condvar::new())));
686 let packet2 = Packet(packet.0.clone());
687 let (tx, rx) = channel();
689 let _t = thread::spawn(move || -> () {
691 let &(ref lock, ref cvar) = &*packet2.0;
692 let _g = lock.lock().unwrap();
694 // Parent should fail when it wakes up.
698 let &(ref lock, ref cvar) = &*packet.0;
699 let mut lock = lock.lock().unwrap();
700 tx.send(()).unwrap();
702 match cvar.wait(lock) {
705 assert_eq!(*lock, 1);
713 fn test_mutex_arc_poison() {
714 let arc = Arc::new(Mutex::new(1));
715 assert!(!arc.is_poisoned());
716 let arc2 = arc.clone();
717 let _ = thread::spawn(move || {
718 let lock = arc2.lock().unwrap();
719 assert_eq!(*lock, 2);
722 assert!(arc.lock().is_err());
723 assert!(arc.is_poisoned());
727 fn test_mutex_arc_nested() {
728 // Tests nested mutexes and access
729 // to underlying data.
730 let arc = Arc::new(Mutex::new(1));
731 let arc2 = Arc::new(Mutex::new(arc));
732 let (tx, rx) = channel();
733 let _t = thread::spawn(move || {
734 let lock = arc2.lock().unwrap();
735 let lock2 = lock.lock().unwrap();
736 assert_eq!(*lock2, 1);
737 tx.send(()).unwrap();
743 fn test_mutex_arc_access_in_unwind() {
744 let arc = Arc::new(Mutex::new(1));
745 let arc2 = arc.clone();
746 let _ = thread::spawn(move || -> () {
750 impl Drop for Unwinder {
752 *self.i.lock().unwrap() += 1;
755 let _u = Unwinder { i: arc2 };
759 let lock = arc.lock().unwrap();
760 assert_eq!(*lock, 2);
764 fn test_mutex_unsized() {
765 let mutex: &Mutex<[i32]> = &Mutex::new([1, 2, 3]);
767 let b = &mut *mutex.lock().unwrap();
771 let comp: &[i32] = &[4, 2, 5];
772 assert_eq!(&*mutex.lock().unwrap(), comp);