std/sync/reentrant_lock.rs
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#[cfg(all(test, not(target_os = "emscripten")))]
mod tests;
use cfg_if::cfg_if;
use crate::cell::UnsafeCell;
use crate::fmt;
use crate::ops::Deref;
use crate::panic::{RefUnwindSafe, UnwindSafe};
use crate::sys::sync as sys;
use crate::thread::{current_id, ThreadId};
/// A re-entrant mutual exclusion lock
///
/// This lock will block *other* threads waiting for the lock to become
/// available. The thread which has already locked the mutex can lock it
/// multiple times without blocking, preventing a common source of deadlocks.
///
/// # Examples
///
/// Allow recursively calling a function needing synchronization from within
/// a callback (this is how [`StdoutLock`](crate::io::StdoutLock) is currently
/// implemented):
///
/// ```
/// #![feature(reentrant_lock)]
///
/// use std::cell::RefCell;
/// use std::sync::ReentrantLock;
///
/// pub struct Log {
/// data: RefCell<String>,
/// }
///
/// impl Log {
/// pub fn append(&self, msg: &str) {
/// self.data.borrow_mut().push_str(msg);
/// }
/// }
///
/// static LOG: ReentrantLock<Log> = ReentrantLock::new(Log { data: RefCell::new(String::new()) });
///
/// pub fn with_log<R>(f: impl FnOnce(&Log) -> R) -> R {
/// let log = LOG.lock();
/// f(&*log)
/// }
///
/// with_log(|log| {
/// log.append("Hello");
/// with_log(|log| log.append(" there!"));
/// });
/// ```
///
// # Implementation details
//
// The 'owner' field tracks which thread has locked the mutex.
//
// We use thread::current_id() as the thread identifier, which is just the
// current thread's ThreadId, so it's unique across the process lifetime.
//
// If `owner` is set to the identifier of the current thread,
// we assume the mutex is already locked and instead of locking it again,
// we increment `lock_count`.
//
// When unlocking, we decrement `lock_count`, and only unlock the mutex when
// it reaches zero.
//
// `lock_count` is protected by the mutex and only accessed by the thread that has
// locked the mutex, so needs no synchronization.
//
// `owner` can be checked by other threads that want to see if they already
// hold the lock, so needs to be atomic. If it compares equal, we're on the
// same thread that holds the mutex and memory access can use relaxed ordering
// since we're not dealing with multiple threads. If it's not equal,
// synchronization is left to the mutex, making relaxed memory ordering for
// the `owner` field fine in all cases.
//
// On systems without 64 bit atomics we also store the address of a TLS variable
// along the 64-bit TID. We then first check that address against the address
// of that variable on the current thread, and only if they compare equal do we
// compare the actual TIDs. Because we only ever read the TID on the same thread
// that it was written on (or a thread sharing the TLS block with that writer thread),
// we don't need to further synchronize the TID accesses, so they can be regular 64-bit
// non-atomic accesses.
#[unstable(feature = "reentrant_lock", issue = "121440")]
pub struct ReentrantLock<T: ?Sized> {
mutex: sys::Mutex,
owner: Tid,
lock_count: UnsafeCell<u32>,
data: T,
}
cfg_if!(
if #[cfg(target_has_atomic = "64")] {
use crate::sync::atomic::{AtomicU64, Ordering::Relaxed};
struct Tid(AtomicU64);
impl Tid {
const fn new() -> Self {
Self(AtomicU64::new(0))
}
#[inline]
fn contains(&self, owner: ThreadId) -> bool {
owner.as_u64().get() == self.0.load(Relaxed)
}
#[inline]
// This is just unsafe to match the API of the Tid type below.
unsafe fn set(&self, tid: Option<ThreadId>) {
let value = tid.map_or(0, |tid| tid.as_u64().get());
self.0.store(value, Relaxed);
}
}
} else {
/// Returns the address of a TLS variable. This is guaranteed to
/// be unique across all currently alive threads.
fn tls_addr() -> usize {
thread_local! { static X: u8 = const { 0u8 } };
X.with(|p| <*const u8>::addr(p))
}
use crate::sync::atomic::{
AtomicUsize,
Ordering,
};
struct Tid {
// When a thread calls `set()`, this value gets updated to
// the address of a thread local on that thread. This is
// used as a first check in `contains()`; if the `tls_addr`
// doesn't match the TLS address of the current thread, then
// the ThreadId also can't match. Only if the TLS addresses do
// match do we read out the actual TID.
// Note also that we can use relaxed atomic operations here, because
// we only ever read from the tid if `tls_addr` matches the current
// TLS address. In that case, either the tid has been set by
// the current thread, or by a thread that has terminated before
// the current thread was created. In either case, no further
// synchronization is needed (as per <https://github.com/rust-lang/miri/issues/3450>)
tls_addr: AtomicUsize,
tid: UnsafeCell<u64>,
}
unsafe impl Send for Tid {}
unsafe impl Sync for Tid {}
impl Tid {
const fn new() -> Self {
Self { tls_addr: AtomicUsize::new(0), tid: UnsafeCell::new(0) }
}
#[inline]
// NOTE: This assumes that `owner` is the ID of the current
// thread, and may spuriously return `false` if that's not the case.
fn contains(&self, owner: ThreadId) -> bool {
// SAFETY: See the comments in the struct definition.
self.tls_addr.load(Ordering::Relaxed) == tls_addr()
&& unsafe { *self.tid.get() } == owner.as_u64().get()
}
#[inline]
// This may only be called by one thread at a time, and can lead to
// race conditions otherwise.
unsafe fn set(&self, tid: Option<ThreadId>) {
// It's important that we set `self.tls_addr` to 0 if the tid is
// cleared. Otherwise, there might be race conditions between
// `set()` and `get()`.
let tls_addr = if tid.is_some() { tls_addr() } else { 0 };
let value = tid.map_or(0, |tid| tid.as_u64().get());
self.tls_addr.store(tls_addr, Ordering::Relaxed);
unsafe { *self.tid.get() = value };
}
}
}
);
#[unstable(feature = "reentrant_lock", issue = "121440")]
unsafe impl<T: Send + ?Sized> Send for ReentrantLock<T> {}
#[unstable(feature = "reentrant_lock", issue = "121440")]
unsafe impl<T: Send + ?Sized> Sync for ReentrantLock<T> {}
// Because of the `UnsafeCell`, these traits are not implemented automatically
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: UnwindSafe + ?Sized> UnwindSafe for ReentrantLock<T> {}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: RefUnwindSafe + ?Sized> RefUnwindSafe for ReentrantLock<T> {}
/// An RAII implementation of a "scoped lock" of a re-entrant lock. When this
/// structure is dropped (falls out of scope), the lock will be unlocked.
///
/// The data protected by the mutex can be accessed through this guard via its
/// [`Deref`] implementation.
///
/// This structure is created by the [`lock`](ReentrantLock::lock) method on
/// [`ReentrantLock`].
///
/// # Mutability
///
/// Unlike [`MutexGuard`](super::MutexGuard), `ReentrantLockGuard` does not
/// implement [`DerefMut`](crate::ops::DerefMut), because implementation of
/// the trait would violate Rust’s reference aliasing rules. Use interior
/// mutability (usually [`RefCell`](crate::cell::RefCell)) in order to mutate
/// the guarded data.
#[must_use = "if unused the ReentrantLock will immediately unlock"]
#[unstable(feature = "reentrant_lock", issue = "121440")]
pub struct ReentrantLockGuard<'a, T: ?Sized + 'a> {
lock: &'a ReentrantLock<T>,
}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: ?Sized> !Send for ReentrantLockGuard<'_, T> {}
#[unstable(feature = "reentrant_lock", issue = "121440")]
unsafe impl<T: ?Sized + Sync> Sync for ReentrantLockGuard<'_, T> {}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T> ReentrantLock<T> {
/// Creates a new re-entrant lock in an unlocked state ready for use.
///
/// # Examples
///
/// ```
/// #![feature(reentrant_lock)]
/// use std::sync::ReentrantLock;
///
/// let lock = ReentrantLock::new(0);
/// ```
pub const fn new(t: T) -> ReentrantLock<T> {
ReentrantLock {
mutex: sys::Mutex::new(),
owner: Tid::new(),
lock_count: UnsafeCell::new(0),
data: t,
}
}
/// Consumes this lock, returning the underlying data.
///
/// # Examples
///
/// ```
/// #![feature(reentrant_lock)]
///
/// use std::sync::ReentrantLock;
///
/// let lock = ReentrantLock::new(0);
/// assert_eq!(lock.into_inner(), 0);
/// ```
pub fn into_inner(self) -> T {
self.data
}
}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: ?Sized> ReentrantLock<T> {
/// Acquires the lock, blocking the current thread until it is able to do
/// so.
///
/// This function will block the caller until it is available to acquire
/// the lock. Upon returning, the thread is the only thread with the lock
/// held. When the thread calling this method already holds the lock, the
/// call succeeds without blocking.
///
/// # Examples
///
/// ```
/// #![feature(reentrant_lock)]
/// use std::cell::Cell;
/// use std::sync::{Arc, ReentrantLock};
/// use std::thread;
///
/// let lock = Arc::new(ReentrantLock::new(Cell::new(0)));
/// let c_lock = Arc::clone(&lock);
///
/// thread::spawn(move || {
/// c_lock.lock().set(10);
/// }).join().expect("thread::spawn failed");
/// assert_eq!(lock.lock().get(), 10);
/// ```
pub fn lock(&self) -> ReentrantLockGuard<'_, T> {
let this_thread = current_id();
// Safety: We only touch lock_count when we own the inner mutex.
// Additionally, we only call `self.owner.set()` while holding
// the inner mutex, so no two threads can call it concurrently.
unsafe {
if self.owner.contains(this_thread) {
self.increment_lock_count().expect("lock count overflow in reentrant mutex");
} else {
self.mutex.lock();
self.owner.set(Some(this_thread));
debug_assert_eq!(*self.lock_count.get(), 0);
*self.lock_count.get() = 1;
}
}
ReentrantLockGuard { lock: self }
}
/// Returns a mutable reference to the underlying data.
///
/// Since this call borrows the `ReentrantLock` mutably, no actual locking
/// needs to take place -- the mutable borrow statically guarantees no locks
/// exist.
///
/// # Examples
///
/// ```
/// #![feature(reentrant_lock)]
/// use std::sync::ReentrantLock;
///
/// let mut lock = ReentrantLock::new(0);
/// *lock.get_mut() = 10;
/// assert_eq!(*lock.lock(), 10);
/// ```
pub fn get_mut(&mut self) -> &mut T {
&mut self.data
}
/// Attempts to acquire this lock.
///
/// If the lock could not be acquired at this time, then `None` is returned.
/// Otherwise, an RAII guard is returned.
///
/// This function does not block.
pub(crate) fn try_lock(&self) -> Option<ReentrantLockGuard<'_, T>> {
let this_thread = current_id();
// Safety: We only touch lock_count when we own the inner mutex.
// Additionally, we only call `self.owner.set()` while holding
// the inner mutex, so no two threads can call it concurrently.
unsafe {
if self.owner.contains(this_thread) {
self.increment_lock_count()?;
Some(ReentrantLockGuard { lock: self })
} else if self.mutex.try_lock() {
self.owner.set(Some(this_thread));
debug_assert_eq!(*self.lock_count.get(), 0);
*self.lock_count.get() = 1;
Some(ReentrantLockGuard { lock: self })
} else {
None
}
}
}
unsafe fn increment_lock_count(&self) -> Option<()> {
unsafe {
*self.lock_count.get() = (*self.lock_count.get()).checked_add(1)?;
}
Some(())
}
}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: fmt::Debug + ?Sized> fmt::Debug for ReentrantLock<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let mut d = f.debug_struct("ReentrantLock");
match self.try_lock() {
Some(v) => d.field("data", &&*v),
None => d.field("data", &format_args!("<locked>")),
};
d.finish_non_exhaustive()
}
}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: Default> Default for ReentrantLock<T> {
fn default() -> Self {
Self::new(T::default())
}
}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T> From<T> for ReentrantLock<T> {
fn from(t: T) -> Self {
Self::new(t)
}
}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: ?Sized> Deref for ReentrantLockGuard<'_, T> {
type Target = T;
fn deref(&self) -> &T {
&self.lock.data
}
}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: fmt::Debug + ?Sized> fmt::Debug for ReentrantLockGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: fmt::Display + ?Sized> fmt::Display for ReentrantLockGuard<'_, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
(**self).fmt(f)
}
}
#[unstable(feature = "reentrant_lock", issue = "121440")]
impl<T: ?Sized> Drop for ReentrantLockGuard<'_, T> {
#[inline]
fn drop(&mut self) {
// Safety: We own the lock.
unsafe {
*self.lock.lock_count.get() -= 1;
if *self.lock.lock_count.get() == 0 {
self.lock.owner.set(None);
self.lock.mutex.unlock();
}
}
}
}