1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
15 use ops::{Deref, DerefMut};
17 use sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
18 use sys_common::rwlock as sys;
20 /// A reader-writer lock
22 /// This type of lock allows a number of readers or at most one writer at any
23 /// point in time. The write portion of this lock typically allows modification
24 /// of the underlying data (exclusive access) and the read portion of this lock
25 /// typically allows for read-only access (shared access).
27 /// The priority policy of the lock is dependent on the underlying operating
28 /// system's implementation, and this type does not guarantee that any
29 /// particular policy will be used.
31 /// The type parameter `T` represents the data that this lock protects. It is
32 /// required that `T` satisfies `Send` to be shared across threads and `Sync` to
33 /// allow concurrent access through readers. The RAII guards returned from the
34 /// locking methods implement `Deref` (and `DerefMut` for the `write` methods)
35 /// to allow access to the contained of the lock.
39 /// An `RwLock`, like `Mutex`, will become poisoned on a panic. Note, however,
40 /// that an `RwLock` may only be poisoned if a panic occurs while it is locked
41 /// exclusively (write mode). If a panic occurs in any reader, then the lock
42 /// will not be poisoned.
47 /// use std::sync::RwLock;
49 /// let lock = RwLock::new(5);
51 /// // many reader locks can be held at once
53 /// let r1 = lock.read().unwrap();
54 /// let r2 = lock.read().unwrap();
55 /// assert_eq!(*r1, 5);
56 /// assert_eq!(*r2, 5);
57 /// } // read locks are dropped at this point
59 /// // only one write lock may be held, however
61 /// let mut w = lock.write().unwrap();
63 /// assert_eq!(*w, 6);
64 /// } // write lock is dropped here
66 #[stable(feature = "rust1", since = "1.0.0")]
67 pub struct RwLock<T: ?Sized> {
68 inner: Box<sys::RWLock>,
73 #[stable(feature = "rust1", since = "1.0.0")]
74 unsafe impl<T: ?Sized + Send + Sync> Send for RwLock<T> {}
75 #[stable(feature = "rust1", since = "1.0.0")]
76 unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
78 /// RAII structure used to release the shared read access of a lock when
81 /// This structure is created by the [`read`] and [`try_read`] methods on
84 /// [`read`]: struct.RwLock.html#method.read
85 /// [`try_read`]: struct.RwLock.html#method.try_read
86 /// [`RwLock`]: struct.RwLock.html
88 #[stable(feature = "rust1", since = "1.0.0")]
89 pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
90 __lock: &'a RwLock<T>,
93 #[stable(feature = "rust1", since = "1.0.0")]
94 impl<'a, T: ?Sized> !marker::Send for RwLockReadGuard<'a, T> {}
96 /// RAII structure used to release the exclusive write access of a lock when
99 /// This structure is created by the [`write`] and [`try_write`] methods
102 /// [`write`]: struct.RwLock.html#method.write
103 /// [`try_write`]: struct.RwLock.html#method.try_write
104 /// [`RwLock`]: struct.RwLock.html
106 #[stable(feature = "rust1", since = "1.0.0")]
107 pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
108 __lock: &'a RwLock<T>,
109 __poison: poison::Guard,
112 #[stable(feature = "rust1", since = "1.0.0")]
113 impl<'a, T: ?Sized> !marker::Send for RwLockWriteGuard<'a, T> {}
116 /// Creates a new instance of an `RwLock<T>` which is unlocked.
121 /// use std::sync::RwLock;
123 /// let lock = RwLock::new(5);
125 #[stable(feature = "rust1", since = "1.0.0")]
126 pub fn new(t: T) -> RwLock<T> {
128 inner: box sys::RWLock::new(),
129 poison: poison::Flag::new(),
130 data: UnsafeCell::new(t),
135 impl<T: ?Sized> RwLock<T> {
136 /// Locks this rwlock with shared read access, blocking the current thread
137 /// until it can be acquired.
139 /// The calling thread will be blocked until there are no more writers which
140 /// hold the lock. There may be other readers currently inside the lock when
141 /// this method returns. This method does not provide any guarantees with
142 /// respect to the ordering of whether contentious readers or writers will
143 /// acquire the lock first.
145 /// Returns an RAII guard which will release this thread's shared access
146 /// once it is dropped.
150 /// This function will return an error if the RwLock is poisoned. An RwLock
151 /// is poisoned whenever a writer panics while holding an exclusive lock.
152 /// The failure will occur immediately after the lock has been acquired.
156 /// This function might panic when called if the lock is already held by the current thread.
158 #[stable(feature = "rust1", since = "1.0.0")]
159 pub fn read(&self) -> LockResult<RwLockReadGuard<T>> {
162 RwLockReadGuard::new(self)
166 /// Attempts to acquire this rwlock with shared read access.
168 /// If the access could not be granted at this time, then `Err` is returned.
169 /// Otherwise, an RAII guard is returned which will release the shared access
170 /// when it is dropped.
172 /// This function does not block.
174 /// This function does not provide any guarantees with respect to the ordering
175 /// of whether contentious readers or writers will acquire the lock first.
179 /// This function will return an error if the RwLock is poisoned. An RwLock
180 /// is poisoned whenever a writer panics while holding an exclusive lock. An
181 /// error will only be returned if the lock would have otherwise been
184 #[stable(feature = "rust1", since = "1.0.0")]
185 pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<T>> {
187 if self.inner.try_read() {
188 Ok(RwLockReadGuard::new(self)?)
190 Err(TryLockError::WouldBlock)
195 /// Locks this rwlock with exclusive write access, blocking the current
196 /// thread until it can be acquired.
198 /// This function will not return while other writers or other readers
199 /// currently have access to the lock.
201 /// Returns an RAII guard which will drop the write access of this rwlock
206 /// This function will return an error if the RwLock is poisoned. An RwLock
207 /// is poisoned whenever a writer panics while holding an exclusive lock.
208 /// An error will be returned when the lock is acquired.
212 /// This function might panic when called if the lock is already held by the current thread.
214 #[stable(feature = "rust1", since = "1.0.0")]
215 pub fn write(&self) -> LockResult<RwLockWriteGuard<T>> {
218 RwLockWriteGuard::new(self)
222 /// Attempts to lock this rwlock with exclusive write access.
224 /// If the lock could not be acquired at this time, then `Err` is returned.
225 /// Otherwise, an RAII guard is returned which will release the lock when
228 /// This function does not block.
230 /// This function does not provide any guarantees with respect to the ordering
231 /// of whether contentious readers or writers will acquire the lock first.
235 /// This function will return an error if the RwLock is poisoned. An RwLock
236 /// is poisoned whenever a writer panics while holding an exclusive lock. An
237 /// error will only be returned if the lock would have otherwise been
240 #[stable(feature = "rust1", since = "1.0.0")]
241 pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<T>> {
243 if self.inner.try_write() {
244 Ok(RwLockWriteGuard::new(self)?)
246 Err(TryLockError::WouldBlock)
251 /// Determines whether the lock is poisoned.
253 /// If another thread is active, the lock can still become poisoned at any
254 /// time. You should not trust a `false` value for program correctness
255 /// without additional synchronization.
257 #[stable(feature = "sync_poison", since = "1.2.0")]
258 pub fn is_poisoned(&self) -> bool {
262 /// Consumes this `RwLock`, returning the underlying data.
266 /// This function will return an error if the RwLock is poisoned. An RwLock
267 /// is poisoned whenever a writer panics while holding an exclusive lock. An
268 /// error will only be returned if the lock would have otherwise been
270 #[stable(feature = "rwlock_into_inner", since = "1.6.0")]
271 pub fn into_inner(self) -> LockResult<T> where T: Sized {
272 // We know statically that there are no outstanding references to
273 // `self` so there's no need to lock the inner lock.
275 // To get the inner value, we'd like to call `data.into_inner()`,
276 // but because `RwLock` impl-s `Drop`, we can't move out of it, so
277 // we'll have to destructure it manually instead.
279 // Like `let RwLock { inner, poison, data } = self`.
280 let (inner, poison, data) = {
281 let RwLock { ref inner, ref poison, ref data } = self;
282 (ptr::read(inner), ptr::read(poison), ptr::read(data))
285 inner.destroy(); // Keep in sync with the `Drop` impl.
288 poison::map_result(poison.borrow(), |_| data.into_inner())
292 /// Returns a mutable reference to the underlying data.
294 /// Since this call borrows the `RwLock` mutably, no actual locking needs to
295 /// take place---the mutable borrow statically guarantees no locks exist.
299 /// This function will return an error if the RwLock is poisoned. An RwLock
300 /// is poisoned whenever a writer panics while holding an exclusive lock. An
301 /// error will only be returned if the lock would have otherwise been
303 #[stable(feature = "rwlock_get_mut", since = "1.6.0")]
304 pub fn get_mut(&mut self) -> LockResult<&mut T> {
305 // We know statically that there are no other references to `self`, so
306 // there's no need to lock the inner lock.
307 let data = unsafe { &mut *self.data.get() };
308 poison::map_result(self.poison.borrow(), |_| data)
312 #[stable(feature = "rust1", since = "1.0.0")]
313 unsafe impl<#[may_dangle] T: ?Sized> Drop for RwLock<T> {
315 // IMPORTANT: This code needs to be kept in sync with `RwLock::into_inner`.
316 unsafe { self.inner.destroy() }
320 #[stable(feature = "rust1", since = "1.0.0")]
321 impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
322 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
323 match self.try_read() {
324 Ok(guard) => write!(f, "RwLock {{ data: {:?} }}", &*guard),
325 Err(TryLockError::Poisoned(err)) => {
326 write!(f, "RwLock {{ data: Poisoned({:?}) }}", &**err.get_ref())
328 Err(TryLockError::WouldBlock) => write!(f, "RwLock {{ <locked> }}")
333 #[stable(feature = "rw_lock_default", since = "1.9.0")]
334 impl<T: Default> Default for RwLock<T> {
335 /// Creates a new `RwLock<T>`, with the `Default` value for T.
336 fn default() -> RwLock<T> {
337 RwLock::new(Default::default())
341 impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
342 unsafe fn new(lock: &'rwlock RwLock<T>)
343 -> LockResult<RwLockReadGuard<'rwlock, T>> {
344 poison::map_result(lock.poison.borrow(), |_| {
352 impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
353 unsafe fn new(lock: &'rwlock RwLock<T>)
354 -> LockResult<RwLockWriteGuard<'rwlock, T>> {
355 poison::map_result(lock.poison.borrow(), |guard| {
364 #[stable(feature = "std_debug", since = "1.16.0")]
365 impl<'a, T: fmt::Debug> fmt::Debug for RwLockReadGuard<'a, T> {
366 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
367 f.debug_struct("RwLockReadGuard")
368 .field("lock", &self.__lock)
373 #[stable(feature = "std_debug", since = "1.16.0")]
374 impl<'a, T: fmt::Debug> fmt::Debug for RwLockWriteGuard<'a, T> {
375 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
376 f.debug_struct("RwLockWriteGuard")
377 .field("lock", &self.__lock)
382 #[stable(feature = "rust1", since = "1.0.0")]
383 impl<'rwlock, T: ?Sized> Deref for RwLockReadGuard<'rwlock, T> {
386 fn deref(&self) -> &T {
387 unsafe { &*self.__lock.data.get() }
391 #[stable(feature = "rust1", since = "1.0.0")]
392 impl<'rwlock, T: ?Sized> Deref for RwLockWriteGuard<'rwlock, T> {
395 fn deref(&self) -> &T {
396 unsafe { &*self.__lock.data.get() }
400 #[stable(feature = "rust1", since = "1.0.0")]
401 impl<'rwlock, T: ?Sized> DerefMut for RwLockWriteGuard<'rwlock, T> {
402 fn deref_mut(&mut self) -> &mut T {
403 unsafe { &mut *self.__lock.data.get() }
407 #[stable(feature = "rust1", since = "1.0.0")]
408 impl<'a, T: ?Sized> Drop for RwLockReadGuard<'a, T> {
410 unsafe { self.__lock.inner.read_unlock(); }
414 #[stable(feature = "rust1", since = "1.0.0")]
415 impl<'a, T: ?Sized> Drop for RwLockWriteGuard<'a, T> {
417 self.__lock.poison.done(&self.__poison);
418 unsafe { self.__lock.inner.write_unlock(); }
422 #[cfg(all(test, not(target_os = "emscripten")))]
424 #![allow(deprecated)] // rand
426 use rand::{self, Rng};
427 use sync::mpsc::channel;
429 use sync::{Arc, RwLock, TryLockError};
430 use sync::atomic::{AtomicUsize, Ordering};
432 #[derive(Eq, PartialEq, Debug)]
437 let l = RwLock::new(());
438 drop(l.read().unwrap());
439 drop(l.write().unwrap());
440 drop((l.read().unwrap(), l.read().unwrap()));
441 drop(l.write().unwrap());
447 const M: usize = 1000;
449 let r = Arc::new(RwLock::new(()));
451 let (tx, rx) = channel::<()>();
455 thread::spawn(move || {
456 let mut rng = rand::thread_rng();
458 if rng.gen_weighted_bool(N) {
459 drop(r.write().unwrap());
461 drop(r.read().unwrap());
472 fn test_rw_arc_poison_wr() {
473 let arc = Arc::new(RwLock::new(1));
474 let arc2 = arc.clone();
475 let _: Result<(), _> = thread::spawn(move|| {
476 let _lock = arc2.write().unwrap();
479 assert!(arc.read().is_err());
483 fn test_rw_arc_poison_ww() {
484 let arc = Arc::new(RwLock::new(1));
485 assert!(!arc.is_poisoned());
486 let arc2 = arc.clone();
487 let _: Result<(), _> = thread::spawn(move|| {
488 let _lock = arc2.write().unwrap();
491 assert!(arc.write().is_err());
492 assert!(arc.is_poisoned());
496 fn test_rw_arc_no_poison_rr() {
497 let arc = Arc::new(RwLock::new(1));
498 let arc2 = arc.clone();
499 let _: Result<(), _> = thread::spawn(move|| {
500 let _lock = arc2.read().unwrap();
503 let lock = arc.read().unwrap();
504 assert_eq!(*lock, 1);
507 fn test_rw_arc_no_poison_rw() {
508 let arc = Arc::new(RwLock::new(1));
509 let arc2 = arc.clone();
510 let _: Result<(), _> = thread::spawn(move|| {
511 let _lock = arc2.read().unwrap();
514 let lock = arc.write().unwrap();
515 assert_eq!(*lock, 1);
520 let arc = Arc::new(RwLock::new(0));
521 let arc2 = arc.clone();
522 let (tx, rx) = channel();
524 thread::spawn(move|| {
525 let mut lock = arc2.write().unwrap();
532 tx.send(()).unwrap();
535 // Readers try to catch the writer in the act
536 let mut children = Vec::new();
538 let arc3 = arc.clone();
539 children.push(thread::spawn(move|| {
540 let lock = arc3.read().unwrap();
545 // Wait for children to pass their asserts
547 assert!(r.join().is_ok());
550 // Wait for writer to finish
552 let lock = arc.read().unwrap();
553 assert_eq!(*lock, 10);
557 fn test_rw_arc_access_in_unwind() {
558 let arc = Arc::new(RwLock::new(1));
559 let arc2 = arc.clone();
560 let _ = thread::spawn(move|| -> () {
562 i: Arc<RwLock<isize>>,
564 impl Drop for Unwinder {
566 let mut lock = self.i.write().unwrap();
570 let _u = Unwinder { i: arc2 };
573 let lock = arc.read().unwrap();
574 assert_eq!(*lock, 2);
578 fn test_rwlock_unsized() {
579 let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
581 let b = &mut *rw.write().unwrap();
585 let comp: &[i32] = &[4, 2, 5];
586 assert_eq!(&*rw.read().unwrap(), comp);
590 fn test_rwlock_try_write() {
591 let lock = RwLock::new(0isize);
592 let read_guard = lock.read().unwrap();
594 let write_result = lock.try_write();
596 Err(TryLockError::WouldBlock) => (),
597 Ok(_) => assert!(false, "try_write should not succeed while read_guard is in scope"),
598 Err(_) => assert!(false, "unexpected error"),
605 fn test_into_inner() {
606 let m = RwLock::new(NonCopy(10));
607 assert_eq!(m.into_inner().unwrap(), NonCopy(10));
611 fn test_into_inner_drop() {
612 struct Foo(Arc<AtomicUsize>);
615 self.0.fetch_add(1, Ordering::SeqCst);
618 let num_drops = Arc::new(AtomicUsize::new(0));
619 let m = RwLock::new(Foo(num_drops.clone()));
620 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
622 let _inner = m.into_inner().unwrap();
623 assert_eq!(num_drops.load(Ordering::SeqCst), 0);
625 assert_eq!(num_drops.load(Ordering::SeqCst), 1);
629 fn test_into_inner_poison() {
630 let m = Arc::new(RwLock::new(NonCopy(10)));
632 let _ = thread::spawn(move || {
633 let _lock = m2.write().unwrap();
634 panic!("test panic in inner thread to poison RwLock");
637 assert!(m.is_poisoned());
638 match Arc::try_unwrap(m).unwrap().into_inner() {
639 Err(e) => assert_eq!(e.into_inner(), NonCopy(10)),
640 Ok(x) => panic!("into_inner of poisoned RwLock is Ok: {:?}", x),
646 let mut m = RwLock::new(NonCopy(10));
647 *m.get_mut().unwrap() = NonCopy(20);
648 assert_eq!(m.into_inner().unwrap(), NonCopy(20));
652 fn test_get_mut_poison() {
653 let m = Arc::new(RwLock::new(NonCopy(10)));
655 let _ = thread::spawn(move || {
656 let _lock = m2.write().unwrap();
657 panic!("test panic in inner thread to poison RwLock");
660 assert!(m.is_poisoned());
661 match Arc::try_unwrap(m).unwrap().get_mut() {
662 Err(e) => assert_eq!(*e.into_inner(), NonCopy(10)),
663 Ok(x) => panic!("get_mut of poisoned RwLock is Ok: {:?}", x),