1use cfg_if::cfg_if;
2
3use crate::cell::UnsafeCell;
4use crate::fmt;
5use crate::ops::Deref;
6use crate::panic::{RefUnwindSafe, UnwindSafe};
7use crate::sys::sync as sys;
8use crate::thread::{ThreadId, current_id};
9
10/// A re-entrant mutual exclusion lock
11///
12/// This lock will block *other* threads waiting for the lock to become
13/// available. The thread which has already locked the mutex can lock it
14/// multiple times without blocking, preventing a common source of deadlocks.
15///
16/// # Examples
17///
18/// Allow recursively calling a function needing synchronization from within
19/// a callback (this is how [`StdoutLock`](crate::io::StdoutLock) is currently
20/// implemented):
21///
22/// ```
23/// #![feature(reentrant_lock)]
24///
25/// use std::cell::RefCell;
26/// use std::sync::ReentrantLock;
27///
28/// pub struct Log {
29/// data: RefCell<String>,
30/// }
31///
32/// impl Log {
33/// pub fn append(&self, msg: &str) {
34/// self.data.borrow_mut().push_str(msg);
35/// }
36/// }
37///
38/// static LOG: ReentrantLock<Log> = ReentrantLock::new(Log { data: RefCell::new(String::new()) });
39///
40/// pub fn with_log<R>(f: impl FnOnce(&Log) -> R) -> R {
41/// let log = LOG.lock();
42/// f(&*log)
43/// }
44///
45/// with_log(|log| {
46/// log.append("Hello");
47/// with_log(|log| log.append(" there!"));
48/// });
49/// ```
50///
51// # Implementation details
52//
53// The 'owner' field tracks which thread has locked the mutex.
54//
55// We use thread::current_id() as the thread identifier, which is just the
56// current thread's ThreadId, so it's unique across the process lifetime.
57//
58// If `owner` is set to the identifier of the current thread,
59// we assume the mutex is already locked and instead of locking it again,
60// we increment `lock_count`.
61//
62// When unlocking, we decrement `lock_count`, and only unlock the mutex when
63// it reaches zero.
64//
65// `lock_count` is protected by the mutex and only accessed by the thread that has
66// locked the mutex, so needs no synchronization.
67//
68// `owner` can be checked by other threads that want to see if they already
69// hold the lock, so needs to be atomic. If it compares equal, we're on the
70// same thread that holds the mutex and memory access can use relaxed ordering
71// since we're not dealing with multiple threads. If it's not equal,
72// synchronization is left to the mutex, making relaxed memory ordering for
73// the `owner` field fine in all cases.
74//
75// On systems without 64 bit atomics we also store the address of a TLS variable
76// along the 64-bit TID. We then first check that address against the address
77// of that variable on the current thread, and only if they compare equal do we
78// compare the actual TIDs. Because we only ever read the TID on the same thread
79// that it was written on (or a thread sharing the TLS block with that writer thread),
80// we don't need to further synchronize the TID accesses, so they can be regular 64-bit
81// non-atomic accesses.
82#[unstable(feature = "reentrant_lock", issue = "121440")]
83pub struct ReentrantLock<T: ?Sized> {
84 mutex: sys::Mutex,
85 owner: Tid,
86 lock_count: UnsafeCell<u32>,
87 data: T,
88}
89
90cfg_if!(
91 if #[cfg(target_has_atomic = "64")] {
92 use crate::sync::atomic::{AtomicU64, Ordering::Relaxed};
93
94 struct Tid(AtomicU64);
95
96 impl Tid {
97 const fn new() -> Self {
98 Self(AtomicU64::new(0))
99 }
100
101 #[inline]
102 fn contains(&self, owner: ThreadId) -> bool {
103 owner.as_u64().get() == self.0.load(Relaxed)
104 }
105
106 #[inline]
107 // This is just unsafe to match the API of the Tid type below.
108 unsafe fn set(&self, tid: Option<ThreadId>) {
109 let value = tid.map_or(0, |tid| tid.as_u64().get());
110 self.0.store(value, Relaxed);
111 }
112 }
113 } else {
114 /// Returns the address of a TLS variable. This is guaranteed to
115 /// be unique across all currently alive threads.
116 fn tls_addr() -> usize {
117 thread_local! { static X: u8 = const { 0u8 } };
118
119 X.with(|p| <*const u8>::addr(p))
120 }
121
122 use crate::sync::atomic::{
123 AtomicUsize,
124 Ordering,
125 };
126
127 struct Tid {
128 // When a thread calls `set()`, this value gets updated to
129 // the address of a thread local on that thread. This is
130 // used as a first check in `contains()`; if the `tls_addr`
131 // doesn't match the TLS address of the current thread, then
132 // the ThreadId also can't match. Only if the TLS addresses do
133 // match do we read out the actual TID.
134 // Note also that we can use relaxed atomic operations here, because
135 // we only ever read from the tid if `tls_addr` matches the current
136 // TLS address. In that case, either the tid has been set by
137 // the current thread, or by a thread that has terminated before
138 // the current thread was created. In either case, no further
139 // synchronization is needed (as per <https://github.com/rust-lang/miri/issues/3450>)
140 tls_addr: AtomicUsize,
141 tid: UnsafeCell<u64>,
142 }
143
144 unsafe impl Send for Tid {}
145 unsafe impl Sync for Tid {}
146
147 impl Tid {
148 const fn new() -> Self {
149 Self { tls_addr: AtomicUsize::new(0), tid: UnsafeCell::new(0) }
150 }
151
152 #[inline]
153 // NOTE: This assumes that `owner` is the ID of the current
154 // thread, and may spuriously return `false` if that's not the case.
155 fn contains(&self, owner: ThreadId) -> bool {
156 // SAFETY: See the comments in the struct definition.
157 self.tls_addr.load(Ordering::Relaxed) == tls_addr()
158 && unsafe { *self.tid.get() } == owner.as_u64().get()
159 }
160
161 #[inline]
162 // This may only be called by one thread at a time, and can lead to
163 // race conditions otherwise.
164 unsafe fn set(&self, tid: Option<ThreadId>) {
165 // It's important that we set `self.tls_addr` to 0 if the tid is
166 // cleared. Otherwise, there might be race conditions between
167 // `set()` and `get()`.
168 let tls_addr = if tid.is_some() { tls_addr() } else { 0 };
169 let value = tid.map_or(0, |tid| tid.as_u64().get());
170 self.tls_addr.store(tls_addr, Ordering::Relaxed);
171 unsafe { *self.tid.get() = value };
172 }
173 }
174 }
175);
176
177#[unstable(feature = "reentrant_lock", issue = "121440")]
178unsafe impl<T: Send + ?Sized> Send for ReentrantLock<T> {}
179#[unstable(feature = "reentrant_lock", issue = "121440")]
180unsafe impl<T: Send + ?Sized> Sync for ReentrantLock<T> {}
181
182// Because of the `UnsafeCell`, these traits are not implemented automatically
183#[unstable(feature = "reentrant_lock", issue = "121440")]
184impl<T: UnwindSafe + ?Sized> UnwindSafe for ReentrantLock<T> {}
185#[unstable(feature = "reentrant_lock", issue = "121440")]
186impl<T: RefUnwindSafe + ?Sized> RefUnwindSafe for ReentrantLock<T> {}
187
188/// An RAII implementation of a "scoped lock" of a re-entrant lock. When this
189/// structure is dropped (falls out of scope), the lock will be unlocked.
190///
191/// The data protected by the mutex can be accessed through this guard via its
192/// [`Deref`] implementation.
193///
194/// This structure is created by the [`lock`](ReentrantLock::lock) method on
195/// [`ReentrantLock`].
196///
197/// # Mutability
198///
199/// Unlike [`MutexGuard`](super::MutexGuard), `ReentrantLockGuard` does not
200/// implement [`DerefMut`](crate::ops::DerefMut), because implementation of
201/// the trait would violate Rust’s reference aliasing rules. Use interior
202/// mutability (usually [`RefCell`](crate::cell::RefCell)) in order to mutate
203/// the guarded data.
204#[must_use = "if unused the ReentrantLock will immediately unlock"]
205#[unstable(feature = "reentrant_lock", issue = "121440")]
206pub struct ReentrantLockGuard<'a, T: ?Sized + 'a> {
207 lock: &'a ReentrantLock<T>,
208}
209
210#[unstable(feature = "reentrant_lock", issue = "121440")]
211impl<T: ?Sized> !Send for ReentrantLockGuard<'_, T> {}
212
213#[unstable(feature = "reentrant_lock", issue = "121440")]
214unsafe impl<T: ?Sized + Sync> Sync for ReentrantLockGuard<'_, T> {}
215
216#[unstable(feature = "reentrant_lock", issue = "121440")]
217impl<T> ReentrantLock<T> {
218 /// Creates a new re-entrant lock in an unlocked state ready for use.
219 ///
220 /// # Examples
221 ///
222 /// ```
223 /// #![feature(reentrant_lock)]
224 /// use std::sync::ReentrantLock;
225 ///
226 /// let lock = ReentrantLock::new(0);
227 /// ```
228 pub const fn new(t: T) -> ReentrantLock<T> {
229 ReentrantLock {
230 mutex: sys::Mutex::new(),
231 owner: Tid::new(),
232 lock_count: UnsafeCell::new(0),
233 data: t,
234 }
235 }
236
237 /// Consumes this lock, returning the underlying data.
238 ///
239 /// # Examples
240 ///
241 /// ```
242 /// #![feature(reentrant_lock)]
243 ///
244 /// use std::sync::ReentrantLock;
245 ///
246 /// let lock = ReentrantLock::new(0);
247 /// assert_eq!(lock.into_inner(), 0);
248 /// ```
249 pub fn into_inner(self) -> T {
250 self.data
251 }
252}
253
254#[unstable(feature = "reentrant_lock", issue = "121440")]
255impl<T: ?Sized> ReentrantLock<T> {
256 /// Acquires the lock, blocking the current thread until it is able to do
257 /// so.
258 ///
259 /// This function will block the caller until it is available to acquire
260 /// the lock. Upon returning, the thread is the only thread with the lock
261 /// held. When the thread calling this method already holds the lock, the
262 /// call succeeds without blocking.
263 ///
264 /// # Examples
265 ///
266 /// ```
267 /// #![feature(reentrant_lock)]
268 /// use std::cell::Cell;
269 /// use std::sync::{Arc, ReentrantLock};
270 /// use std::thread;
271 ///
272 /// let lock = Arc::new(ReentrantLock::new(Cell::new(0)));
273 /// let c_lock = Arc::clone(&lock);
274 ///
275 /// thread::spawn(move || {
276 /// c_lock.lock().set(10);
277 /// }).join().expect("thread::spawn failed");
278 /// assert_eq!(lock.lock().get(), 10);
279 /// ```
280 pub fn lock(&self) -> ReentrantLockGuard<'_, T> {
281 let this_thread = current_id();
282 // Safety: We only touch lock_count when we own the inner mutex.
283 // Additionally, we only call `self.owner.set()` while holding
284 // the inner mutex, so no two threads can call it concurrently.
285 unsafe {
286 if self.owner.contains(this_thread) {
287 self.increment_lock_count().expect("lock count overflow in reentrant mutex");
288 } else {
289 self.mutex.lock();
290 self.owner.set(Some(this_thread));
291 debug_assert_eq!(*self.lock_count.get(), 0);
292 *self.lock_count.get() = 1;
293 }
294 }
295 ReentrantLockGuard { lock: self }
296 }
297
298 /// Returns a mutable reference to the underlying data.
299 ///
300 /// Since this call borrows the `ReentrantLock` mutably, no actual locking
301 /// needs to take place -- the mutable borrow statically guarantees no locks
302 /// exist.
303 ///
304 /// # Examples
305 ///
306 /// ```
307 /// #![feature(reentrant_lock)]
308 /// use std::sync::ReentrantLock;
309 ///
310 /// let mut lock = ReentrantLock::new(0);
311 /// *lock.get_mut() = 10;
312 /// assert_eq!(*lock.lock(), 10);
313 /// ```
314 pub fn get_mut(&mut self) -> &mut T {
315 &mut self.data
316 }
317
318 /// Attempts to acquire this lock.
319 ///
320 /// If the lock could not be acquired at this time, then `None` is returned.
321 /// Otherwise, an RAII guard is returned.
322 ///
323 /// This function does not block.
324 // FIXME maybe make it a public part of the API?
325 #[unstable(issue = "none", feature = "std_internals")]
326 #[doc(hidden)]
327 pub fn try_lock(&self) -> Option<ReentrantLockGuard<'_, T>> {
328 let this_thread = current_id();
329 // Safety: We only touch lock_count when we own the inner mutex.
330 // Additionally, we only call `self.owner.set()` while holding
331 // the inner mutex, so no two threads can call it concurrently.
332 unsafe {
333 if self.owner.contains(this_thread) {
334 self.increment_lock_count()?;
335 Some(ReentrantLockGuard { lock: self })
336 } else if self.mutex.try_lock() {
337 self.owner.set(Some(this_thread));
338 debug_assert_eq!(*self.lock_count.get(), 0);
339 *self.lock_count.get() = 1;
340 Some(ReentrantLockGuard { lock: self })
341 } else {
342 None
343 }
344 }
345 }
346
347 unsafe fn increment_lock_count(&self) -> Option<()> {
348 unsafe {
349 *self.lock_count.get() = (*self.lock_count.get()).checked_add(1)?;
350 }
351 Some(())
352 }
353}
354
355#[unstable(feature = "reentrant_lock", issue = "121440")]
356impl<T: fmt::Debug + ?Sized> fmt::Debug for ReentrantLock<T> {
357 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
358 let mut d: DebugStruct<'_, '_> = f.debug_struct(name:"ReentrantLock");
359 match self.try_lock() {
360 Some(v: ReentrantLockGuard<'_, T>) => d.field(name:"data", &&*v),
361 None => d.field(name:"data", &format_args!("<locked>")),
362 };
363 d.finish_non_exhaustive()
364 }
365}
366
367#[unstable(feature = "reentrant_lock", issue = "121440")]
368impl<T: Default> Default for ReentrantLock<T> {
369 fn default() -> Self {
370 Self::new(T::default())
371 }
372}
373
374#[unstable(feature = "reentrant_lock", issue = "121440")]
375impl<T> From<T> for ReentrantLock<T> {
376 fn from(t: T) -> Self {
377 Self::new(t)
378 }
379}
380
381#[unstable(feature = "reentrant_lock", issue = "121440")]
382impl<T: ?Sized> Deref for ReentrantLockGuard<'_, T> {
383 type Target = T;
384
385 fn deref(&self) -> &T {
386 &self.lock.data
387 }
388}
389
390#[unstable(feature = "reentrant_lock", issue = "121440")]
391impl<T: fmt::Debug + ?Sized> fmt::Debug for ReentrantLockGuard<'_, T> {
392 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
393 (**self).fmt(f)
394 }
395}
396
397#[unstable(feature = "reentrant_lock", issue = "121440")]
398impl<T: fmt::Display + ?Sized> fmt::Display for ReentrantLockGuard<'_, T> {
399 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
400 (**self).fmt(f)
401 }
402}
403
404#[unstable(feature = "reentrant_lock", issue = "121440")]
405impl<T: ?Sized> Drop for ReentrantLockGuard<'_, T> {
406 #[inline]
407 fn drop(&mut self) {
408 // Safety: We own the lock.
409 unsafe {
410 *self.lock.lock_count.get() -= 1;
411 if *self.lock.lock_count.get() == 0 {
412 self.lock.owner.set(tid:None);
413 self.lock.mutex.unlock();
414 }
415 }
416 }
417}
418