b8ae372e48
This removes two minor OnceLock tests which test private methods. The rest of the tests should be more than enough to catch mistakes in those private methods. Also makes ReentrantLock::try_lock public. And finally it makes the mpmc tests actually run.
417 lines
14 KiB
Rust
417 lines
14 KiB
Rust
use cfg_if::cfg_if;
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use crate::cell::UnsafeCell;
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use crate::fmt;
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use crate::ops::Deref;
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use crate::panic::{RefUnwindSafe, UnwindSafe};
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use crate::sys::sync as sys;
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use crate::thread::{ThreadId, current_id};
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/// A re-entrant mutual exclusion lock
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///
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/// This lock will block *other* threads waiting for the lock to become
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/// available. The thread which has already locked the mutex can lock it
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/// multiple times without blocking, preventing a common source of deadlocks.
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///
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/// # Examples
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///
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/// Allow recursively calling a function needing synchronization from within
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/// a callback (this is how [`StdoutLock`](crate::io::StdoutLock) is currently
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/// implemented):
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///
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/// ```
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/// #![feature(reentrant_lock)]
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///
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/// use std::cell::RefCell;
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/// use std::sync::ReentrantLock;
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///
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/// pub struct Log {
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/// data: RefCell<String>,
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/// }
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///
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/// impl Log {
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/// pub fn append(&self, msg: &str) {
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/// self.data.borrow_mut().push_str(msg);
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/// }
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/// }
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///
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/// static LOG: ReentrantLock<Log> = ReentrantLock::new(Log { data: RefCell::new(String::new()) });
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///
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/// pub fn with_log<R>(f: impl FnOnce(&Log) -> R) -> R {
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/// let log = LOG.lock();
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/// f(&*log)
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/// }
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///
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/// with_log(|log| {
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/// log.append("Hello");
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/// with_log(|log| log.append(" there!"));
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/// });
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/// ```
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///
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// # Implementation details
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//
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// The 'owner' field tracks which thread has locked the mutex.
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//
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// We use thread::current_id() as the thread identifier, which is just the
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// current thread's ThreadId, so it's unique across the process lifetime.
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//
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// If `owner` is set to the identifier of the current thread,
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// we assume the mutex is already locked and instead of locking it again,
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// we increment `lock_count`.
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//
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// When unlocking, we decrement `lock_count`, and only unlock the mutex when
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// it reaches zero.
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//
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// `lock_count` is protected by the mutex and only accessed by the thread that has
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// locked the mutex, so needs no synchronization.
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//
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// `owner` can be checked by other threads that want to see if they already
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// hold the lock, so needs to be atomic. If it compares equal, we're on the
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// same thread that holds the mutex and memory access can use relaxed ordering
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// since we're not dealing with multiple threads. If it's not equal,
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// synchronization is left to the mutex, making relaxed memory ordering for
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// the `owner` field fine in all cases.
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//
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// On systems without 64 bit atomics we also store the address of a TLS variable
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// along the 64-bit TID. We then first check that address against the address
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// of that variable on the current thread, and only if they compare equal do we
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// compare the actual TIDs. Because we only ever read the TID on the same thread
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// that it was written on (or a thread sharing the TLS block with that writer thread),
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// we don't need to further synchronize the TID accesses, so they can be regular 64-bit
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// non-atomic accesses.
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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pub struct ReentrantLock<T: ?Sized> {
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mutex: sys::Mutex,
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owner: Tid,
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lock_count: UnsafeCell<u32>,
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data: T,
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}
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cfg_if!(
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if #[cfg(target_has_atomic = "64")] {
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use crate::sync::atomic::{AtomicU64, Ordering::Relaxed};
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struct Tid(AtomicU64);
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impl Tid {
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const fn new() -> Self {
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Self(AtomicU64::new(0))
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}
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#[inline]
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fn contains(&self, owner: ThreadId) -> bool {
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owner.as_u64().get() == self.0.load(Relaxed)
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}
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#[inline]
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// This is just unsafe to match the API of the Tid type below.
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unsafe fn set(&self, tid: Option<ThreadId>) {
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let value = tid.map_or(0, |tid| tid.as_u64().get());
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self.0.store(value, Relaxed);
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}
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}
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} else {
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/// Returns the address of a TLS variable. This is guaranteed to
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/// be unique across all currently alive threads.
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fn tls_addr() -> usize {
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thread_local! { static X: u8 = const { 0u8 } };
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X.with(|p| <*const u8>::addr(p))
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}
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use crate::sync::atomic::{
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AtomicUsize,
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Ordering,
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};
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struct Tid {
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// When a thread calls `set()`, this value gets updated to
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// the address of a thread local on that thread. This is
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// used as a first check in `contains()`; if the `tls_addr`
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// doesn't match the TLS address of the current thread, then
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// the ThreadId also can't match. Only if the TLS addresses do
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// match do we read out the actual TID.
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// Note also that we can use relaxed atomic operations here, because
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// we only ever read from the tid if `tls_addr` matches the current
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// TLS address. In that case, either the tid has been set by
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// the current thread, or by a thread that has terminated before
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// the current thread was created. In either case, no further
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// synchronization is needed (as per <https://github.com/rust-lang/miri/issues/3450>)
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tls_addr: AtomicUsize,
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tid: UnsafeCell<u64>,
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}
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unsafe impl Send for Tid {}
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unsafe impl Sync for Tid {}
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impl Tid {
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const fn new() -> Self {
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Self { tls_addr: AtomicUsize::new(0), tid: UnsafeCell::new(0) }
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}
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#[inline]
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// NOTE: This assumes that `owner` is the ID of the current
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// thread, and may spuriously return `false` if that's not the case.
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fn contains(&self, owner: ThreadId) -> bool {
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// SAFETY: See the comments in the struct definition.
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self.tls_addr.load(Ordering::Relaxed) == tls_addr()
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&& unsafe { *self.tid.get() } == owner.as_u64().get()
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}
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#[inline]
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// This may only be called by one thread at a time, and can lead to
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// race conditions otherwise.
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unsafe fn set(&self, tid: Option<ThreadId>) {
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// It's important that we set `self.tls_addr` to 0 if the tid is
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// cleared. Otherwise, there might be race conditions between
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// `set()` and `get()`.
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let tls_addr = if tid.is_some() { tls_addr() } else { 0 };
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let value = tid.map_or(0, |tid| tid.as_u64().get());
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self.tls_addr.store(tls_addr, Ordering::Relaxed);
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unsafe { *self.tid.get() = value };
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}
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}
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}
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);
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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unsafe impl<T: Send + ?Sized> Send for ReentrantLock<T> {}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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unsafe impl<T: Send + ?Sized> Sync for ReentrantLock<T> {}
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// Because of the `UnsafeCell`, these traits are not implemented automatically
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: UnwindSafe + ?Sized> UnwindSafe for ReentrantLock<T> {}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: RefUnwindSafe + ?Sized> RefUnwindSafe for ReentrantLock<T> {}
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/// An RAII implementation of a "scoped lock" of a re-entrant lock. When this
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/// structure is dropped (falls out of scope), the lock will be unlocked.
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///
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/// The data protected by the mutex can be accessed through this guard via its
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/// [`Deref`] implementation.
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///
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/// This structure is created by the [`lock`](ReentrantLock::lock) method on
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/// [`ReentrantLock`].
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///
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/// # Mutability
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///
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/// Unlike [`MutexGuard`](super::MutexGuard), `ReentrantLockGuard` does not
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/// implement [`DerefMut`](crate::ops::DerefMut), because implementation of
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/// the trait would violate Rust’s reference aliasing rules. Use interior
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/// mutability (usually [`RefCell`](crate::cell::RefCell)) in order to mutate
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/// the guarded data.
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#[must_use = "if unused the ReentrantLock will immediately unlock"]
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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pub struct ReentrantLockGuard<'a, T: ?Sized + 'a> {
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lock: &'a ReentrantLock<T>,
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}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: ?Sized> !Send for ReentrantLockGuard<'_, T> {}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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unsafe impl<T: ?Sized + Sync> Sync for ReentrantLockGuard<'_, T> {}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T> ReentrantLock<T> {
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/// Creates a new re-entrant lock in an unlocked state ready for use.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(reentrant_lock)]
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/// use std::sync::ReentrantLock;
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///
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/// let lock = ReentrantLock::new(0);
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/// ```
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pub const fn new(t: T) -> ReentrantLock<T> {
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ReentrantLock {
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mutex: sys::Mutex::new(),
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owner: Tid::new(),
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lock_count: UnsafeCell::new(0),
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data: t,
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}
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}
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/// Consumes this lock, returning the underlying data.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(reentrant_lock)]
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///
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/// use std::sync::ReentrantLock;
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///
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/// let lock = ReentrantLock::new(0);
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/// assert_eq!(lock.into_inner(), 0);
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/// ```
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pub fn into_inner(self) -> T {
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self.data
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}
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}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: ?Sized> ReentrantLock<T> {
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/// Acquires the lock, blocking the current thread until it is able to do
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/// so.
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///
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/// This function will block the caller until it is available to acquire
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/// the lock. Upon returning, the thread is the only thread with the lock
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/// held. When the thread calling this method already holds the lock, the
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/// call succeeds without blocking.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(reentrant_lock)]
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/// use std::cell::Cell;
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/// use std::sync::{Arc, ReentrantLock};
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/// use std::thread;
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///
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/// let lock = Arc::new(ReentrantLock::new(Cell::new(0)));
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/// let c_lock = Arc::clone(&lock);
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///
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/// thread::spawn(move || {
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/// c_lock.lock().set(10);
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/// }).join().expect("thread::spawn failed");
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/// assert_eq!(lock.lock().get(), 10);
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/// ```
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pub fn lock(&self) -> ReentrantLockGuard<'_, T> {
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let this_thread = current_id();
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// Safety: We only touch lock_count when we own the inner mutex.
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// Additionally, we only call `self.owner.set()` while holding
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// the inner mutex, so no two threads can call it concurrently.
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unsafe {
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if self.owner.contains(this_thread) {
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self.increment_lock_count().expect("lock count overflow in reentrant mutex");
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} else {
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self.mutex.lock();
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self.owner.set(Some(this_thread));
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debug_assert_eq!(*self.lock_count.get(), 0);
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*self.lock_count.get() = 1;
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}
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}
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ReentrantLockGuard { lock: self }
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}
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/// Returns a mutable reference to the underlying data.
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///
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/// Since this call borrows the `ReentrantLock` mutably, no actual locking
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/// needs to take place -- the mutable borrow statically guarantees no locks
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/// exist.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(reentrant_lock)]
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/// use std::sync::ReentrantLock;
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///
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/// let mut lock = ReentrantLock::new(0);
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/// *lock.get_mut() = 10;
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/// assert_eq!(*lock.lock(), 10);
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/// ```
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pub fn get_mut(&mut self) -> &mut T {
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&mut self.data
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}
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/// Attempts to acquire this lock.
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///
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/// If the lock could not be acquired at this time, then `None` is returned.
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/// Otherwise, an RAII guard is returned.
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///
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/// This function does not block.
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// FIXME maybe make it a public part of the API?
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#[unstable(issue = "none", feature = "std_internals")]
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#[doc(hidden)]
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pub fn try_lock(&self) -> Option<ReentrantLockGuard<'_, T>> {
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let this_thread = current_id();
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// Safety: We only touch lock_count when we own the inner mutex.
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// Additionally, we only call `self.owner.set()` while holding
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// the inner mutex, so no two threads can call it concurrently.
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unsafe {
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if self.owner.contains(this_thread) {
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self.increment_lock_count()?;
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Some(ReentrantLockGuard { lock: self })
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} else if self.mutex.try_lock() {
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self.owner.set(Some(this_thread));
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debug_assert_eq!(*self.lock_count.get(), 0);
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*self.lock_count.get() = 1;
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Some(ReentrantLockGuard { lock: self })
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} else {
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None
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}
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}
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}
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unsafe fn increment_lock_count(&self) -> Option<()> {
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unsafe {
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*self.lock_count.get() = (*self.lock_count.get()).checked_add(1)?;
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}
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Some(())
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}
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}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: fmt::Debug + ?Sized> fmt::Debug for ReentrantLock<T> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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let mut d = f.debug_struct("ReentrantLock");
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match self.try_lock() {
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Some(v) => d.field("data", &&*v),
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None => d.field("data", &format_args!("<locked>")),
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};
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d.finish_non_exhaustive()
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}
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}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: Default> Default for ReentrantLock<T> {
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fn default() -> Self {
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Self::new(T::default())
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}
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}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T> From<T> for ReentrantLock<T> {
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fn from(t: T) -> Self {
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Self::new(t)
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}
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}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: ?Sized> Deref for ReentrantLockGuard<'_, T> {
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type Target = T;
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fn deref(&self) -> &T {
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&self.lock.data
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}
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}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: fmt::Debug + ?Sized> fmt::Debug for ReentrantLockGuard<'_, T> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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(**self).fmt(f)
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}
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}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: fmt::Display + ?Sized> fmt::Display for ReentrantLockGuard<'_, T> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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(**self).fmt(f)
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}
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}
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#[unstable(feature = "reentrant_lock", issue = "121440")]
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impl<T: ?Sized> Drop for ReentrantLockGuard<'_, T> {
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#[inline]
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fn drop(&mut self) {
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// Safety: We own the lock.
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unsafe {
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*self.lock.lock_count.get() -= 1;
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if *self.lock.lock_count.get() == 0 {
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self.lock.owner.set(None);
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self.lock.mutex.unlock();
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}
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}
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}
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}
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