538 lines
15 KiB
Rust
538 lines
15 KiB
Rust
//! A ticket-based mutex.
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//!
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//! Waiting threads take a 'ticket' from the lock in the order they arrive and gain access to the lock when their
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//! ticket is next in the queue. Best-case latency is slightly worse than a regular spinning mutex, but worse-case
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//! latency is infinitely better. Waiting threads simply need to wait for all threads that come before them in the
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//! queue to finish.
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use crate::{
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atomic::{AtomicUsize, Ordering},
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RelaxStrategy, Spin,
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};
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use core::{
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cell::UnsafeCell,
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fmt,
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marker::PhantomData,
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ops::{Deref, DerefMut},
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};
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/// A spin-based [ticket lock](https://en.wikipedia.org/wiki/Ticket_lock) providing mutually exclusive access to data.
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///
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/// A ticket lock is analogous to a queue management system for lock requests. When a thread tries to take a lock, it
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/// is assigned a 'ticket'. It then spins until its ticket becomes next in line. When the lock guard is released, the
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/// next ticket will be processed.
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///
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/// Ticket locks significantly reduce the worse-case performance of locking at the cost of slightly higher average-time
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/// overhead.
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///
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/// # Example
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///
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/// ```
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/// use spin;
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///
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/// let lock = spin::mutex::TicketMutex::<_>::new(0);
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///
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/// // Modify the data
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/// *lock.lock() = 2;
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///
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/// // Read the data
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/// let answer = *lock.lock();
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/// assert_eq!(answer, 2);
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/// ```
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///
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/// # Thread safety example
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///
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/// ```
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/// use spin;
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/// use std::sync::{Arc, Barrier};
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///
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/// let thread_count = 1000;
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/// let spin_mutex = Arc::new(spin::mutex::TicketMutex::<_>::new(0));
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///
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/// // We use a barrier to ensure the readout happens after all writing
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/// let barrier = Arc::new(Barrier::new(thread_count + 1));
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///
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/// for _ in (0..thread_count) {
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/// let my_barrier = barrier.clone();
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/// let my_lock = spin_mutex.clone();
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/// std::thread::spawn(move || {
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/// let mut guard = my_lock.lock();
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/// *guard += 1;
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///
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/// // Release the lock to prevent a deadlock
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/// drop(guard);
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/// my_barrier.wait();
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/// });
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/// }
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///
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/// barrier.wait();
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///
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/// let answer = { *spin_mutex.lock() };
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/// assert_eq!(answer, thread_count);
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/// ```
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pub struct TicketMutex<T: ?Sized, R = Spin> {
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phantom: PhantomData<R>,
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next_ticket: AtomicUsize,
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next_serving: AtomicUsize,
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data: UnsafeCell<T>,
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}
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/// A guard that protects some data.
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///
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/// When the guard is dropped, the next ticket will be processed.
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pub struct TicketMutexGuard<'a, T: ?Sized + 'a> {
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next_serving: &'a AtomicUsize,
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ticket: usize,
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data: &'a mut T,
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}
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unsafe impl<T: ?Sized + Send, R> Sync for TicketMutex<T, R> {}
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unsafe impl<T: ?Sized + Send, R> Send for TicketMutex<T, R> {}
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impl<T, R> TicketMutex<T, R> {
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/// Creates a new [`TicketMutex`] wrapping the supplied data.
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///
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/// # Example
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///
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/// ```
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/// use spin::mutex::TicketMutex;
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///
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/// static MUTEX: TicketMutex<()> = TicketMutex::<_>::new(());
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///
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/// fn demo() {
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/// let lock = MUTEX.lock();
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/// // do something with lock
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/// drop(lock);
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/// }
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/// ```
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#[inline(always)]
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pub const fn new(data: T) -> Self {
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Self {
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phantom: PhantomData,
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next_ticket: AtomicUsize::new(0),
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next_serving: AtomicUsize::new(0),
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data: UnsafeCell::new(data),
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}
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}
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/// Consumes this [`TicketMutex`] and unwraps the underlying data.
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///
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/// # Example
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///
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/// ```
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/// let lock = spin::mutex::TicketMutex::<_>::new(42);
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/// assert_eq!(42, lock.into_inner());
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/// ```
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#[inline(always)]
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pub fn into_inner(self) -> T {
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self.data.into_inner()
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}
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/// Returns a mutable pointer to the underying data.
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///
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/// This is mostly meant to be used for applications which require manual unlocking, but where
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/// storing both the lock and the pointer to the inner data gets inefficient.
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///
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/// # Example
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/// ```
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/// let lock = spin::mutex::SpinMutex::<_>::new(42);
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///
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/// unsafe {
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/// core::mem::forget(lock.lock());
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///
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/// assert_eq!(lock.as_mut_ptr().read(), 42);
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/// lock.as_mut_ptr().write(58);
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///
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/// lock.force_unlock();
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/// }
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///
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/// assert_eq!(*lock.lock(), 58);
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///
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/// ```
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#[inline(always)]
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pub fn as_mut_ptr(&self) -> *mut T {
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self.data.get()
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}
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}
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impl<T: ?Sized + fmt::Debug, R> fmt::Debug for TicketMutex<T, R> {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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match self.try_lock() {
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Some(guard) => write!(f, "Mutex {{ data: ")
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.and_then(|()| (&*guard).fmt(f))
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.and_then(|()| write!(f, "}}")),
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None => write!(f, "Mutex {{ <locked> }}"),
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}
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}
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}
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impl<T: ?Sized, R: RelaxStrategy> TicketMutex<T, R> {
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/// Locks the [`TicketMutex`] and returns a guard that permits access to the inner data.
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///
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/// The returned data may be dereferenced for data access
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/// and the lock will be dropped when the guard falls out of scope.
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///
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/// ```
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/// let lock = spin::mutex::TicketMutex::<_>::new(0);
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/// {
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/// let mut data = lock.lock();
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/// // The lock is now locked and the data can be accessed
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/// *data += 1;
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/// // The lock is implicitly dropped at the end of the scope
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/// }
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/// ```
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#[inline(always)]
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pub fn lock(&self) -> TicketMutexGuard<T> {
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let ticket = self.next_ticket.fetch_add(1, Ordering::Relaxed);
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while self.next_serving.load(Ordering::Acquire) != ticket {
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R::relax();
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}
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TicketMutexGuard {
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next_serving: &self.next_serving,
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ticket,
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// Safety
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// We know that we are the next ticket to be served,
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// so there's no other thread accessing the data.
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//
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// Every other thread has another ticket number so it's
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// definitely stuck in the spin loop above.
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data: unsafe { &mut *self.data.get() },
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}
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}
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}
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impl<T: ?Sized, R> TicketMutex<T, R> {
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/// Returns `true` if the lock is currently held.
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///
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/// # Safety
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///
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/// This function provides no synchronization guarantees and so its result should be considered 'out of date'
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/// the instant it is called. Do not use it for synchronization purposes. However, it may be useful as a heuristic.
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#[inline(always)]
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pub fn is_locked(&self) -> bool {
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let ticket = self.next_ticket.load(Ordering::Relaxed);
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self.next_serving.load(Ordering::Relaxed) != ticket
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}
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/// Force unlock this [`TicketMutex`], by serving the next ticket.
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///
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/// # Safety
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///
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/// This is *extremely* unsafe if the lock is not held by the current
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/// thread. However, this can be useful in some instances for exposing the
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/// lock to FFI that doesn't know how to deal with RAII.
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#[inline(always)]
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pub unsafe fn force_unlock(&self) {
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self.next_serving.fetch_add(1, Ordering::Release);
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}
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/// Try to lock this [`TicketMutex`], returning a lock guard if successful.
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///
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/// # Example
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///
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/// ```
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/// let lock = spin::mutex::TicketMutex::<_>::new(42);
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///
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/// let maybe_guard = lock.try_lock();
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/// assert!(maybe_guard.is_some());
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///
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/// // `maybe_guard` is still held, so the second call fails
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/// let maybe_guard2 = lock.try_lock();
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/// assert!(maybe_guard2.is_none());
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/// ```
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#[inline(always)]
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pub fn try_lock(&self) -> Option<TicketMutexGuard<T>> {
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let ticket = self
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.next_ticket
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.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |ticket| {
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if self.next_serving.load(Ordering::Acquire) == ticket {
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Some(ticket + 1)
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} else {
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None
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}
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});
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ticket.ok().map(|ticket| TicketMutexGuard {
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next_serving: &self.next_serving,
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ticket,
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// Safety
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// We have a ticket that is equal to the next_serving ticket, so we know:
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// - that no other thread can have the same ticket id as this thread
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// - that we are the next one to be served so we have exclusive access to the data
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data: unsafe { &mut *self.data.get() },
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})
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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 [`TicketMutex`] mutably, and a mutable reference is guaranteed to be exclusive in
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/// Rust, no actual locking needs to take place -- the mutable borrow statically guarantees no locks exist. As
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/// such, this is a 'zero-cost' operation.
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///
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/// # Example
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///
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/// ```
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/// let mut lock = spin::mutex::TicketMutex::<_>::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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#[inline(always)]
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pub fn get_mut(&mut self) -> &mut T {
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// Safety:
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// We know that there are no other references to `self`,
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// so it's safe to return a exclusive reference to the data.
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unsafe { &mut *self.data.get() }
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}
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}
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impl<T: ?Sized + Default, R> Default for TicketMutex<T, R> {
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fn default() -> Self {
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Self::new(Default::default())
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}
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}
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impl<T, R> From<T> for TicketMutex<T, R> {
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fn from(data: T) -> Self {
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Self::new(data)
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}
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}
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impl<'a, T: ?Sized> TicketMutexGuard<'a, T> {
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/// Leak the lock guard, yielding a mutable reference to the underlying data.
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///
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/// Note that this function will permanently lock the original [`TicketMutex`].
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///
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/// ```
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/// let mylock = spin::mutex::TicketMutex::<_>::new(0);
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///
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/// let data: &mut i32 = spin::mutex::TicketMutexGuard::leak(mylock.lock());
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///
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/// *data = 1;
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/// assert_eq!(*data, 1);
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/// ```
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#[inline(always)]
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pub fn leak(this: Self) -> &'a mut T {
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let data = this.data as *mut _; // Keep it in pointer form temporarily to avoid double-aliasing
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core::mem::forget(this);
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unsafe { &mut *data }
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}
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}
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impl<'a, T: ?Sized + fmt::Debug> fmt::Debug for TicketMutexGuard<'a, T> {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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fmt::Debug::fmt(&**self, f)
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}
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}
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impl<'a, T: ?Sized + fmt::Display> fmt::Display for TicketMutexGuard<'a, T> {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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fmt::Display::fmt(&**self, f)
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}
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}
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impl<'a, T: ?Sized> Deref for TicketMutexGuard<'a, T> {
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type Target = T;
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fn deref(&self) -> &T {
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self.data
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}
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}
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impl<'a, T: ?Sized> DerefMut for TicketMutexGuard<'a, T> {
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fn deref_mut(&mut self) -> &mut T {
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self.data
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}
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}
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impl<'a, T: ?Sized> Drop for TicketMutexGuard<'a, T> {
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fn drop(&mut self) {
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let new_ticket = self.ticket + 1;
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self.next_serving.store(new_ticket, Ordering::Release);
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}
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}
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#[cfg(feature = "lock_api")]
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unsafe impl<R: RelaxStrategy> lock_api_crate::RawMutex for TicketMutex<(), R> {
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type GuardMarker = lock_api_crate::GuardSend;
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const INIT: Self = Self::new(());
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fn lock(&self) {
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// Prevent guard destructor running
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core::mem::forget(Self::lock(self));
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}
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fn try_lock(&self) -> bool {
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// Prevent guard destructor running
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Self::try_lock(self).map(core::mem::forget).is_some()
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}
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unsafe fn unlock(&self) {
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self.force_unlock();
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}
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fn is_locked(&self) -> bool {
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Self::is_locked(self)
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}
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}
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#[cfg(test)]
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mod tests {
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use std::prelude::v1::*;
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use std::sync::atomic::{AtomicUsize, Ordering};
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use std::sync::mpsc::channel;
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use std::sync::Arc;
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use std::thread;
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type TicketMutex<T> = super::TicketMutex<T>;
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#[derive(Eq, PartialEq, Debug)]
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struct NonCopy(i32);
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#[test]
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fn smoke() {
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let m = TicketMutex::<_>::new(());
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drop(m.lock());
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drop(m.lock());
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}
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#[test]
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fn lots_and_lots() {
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static M: TicketMutex<()> = TicketMutex::<_>::new(());
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static mut CNT: u32 = 0;
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const J: u32 = 1000;
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const K: u32 = 3;
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fn inc() {
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for _ in 0..J {
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unsafe {
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let _g = M.lock();
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CNT += 1;
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}
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}
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}
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let (tx, rx) = channel();
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for _ in 0..K {
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let tx2 = tx.clone();
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thread::spawn(move || {
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inc();
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tx2.send(()).unwrap();
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});
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let tx2 = tx.clone();
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thread::spawn(move || {
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inc();
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tx2.send(()).unwrap();
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});
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}
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drop(tx);
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for _ in 0..2 * K {
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rx.recv().unwrap();
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}
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assert_eq!(unsafe { CNT }, J * K * 2);
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}
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#[test]
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fn try_lock() {
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let mutex = TicketMutex::<_>::new(42);
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// First lock succeeds
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let a = mutex.try_lock();
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assert_eq!(a.as_ref().map(|r| **r), Some(42));
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// Additional lock fails
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let b = mutex.try_lock();
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assert!(b.is_none());
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// After dropping lock, it succeeds again
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::core::mem::drop(a);
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let c = mutex.try_lock();
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assert_eq!(c.as_ref().map(|r| **r), Some(42));
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}
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#[test]
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fn test_into_inner() {
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let m = TicketMutex::<_>::new(NonCopy(10));
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assert_eq!(m.into_inner(), NonCopy(10));
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}
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#[test]
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fn test_into_inner_drop() {
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struct Foo(Arc<AtomicUsize>);
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impl Drop for Foo {
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fn drop(&mut self) {
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self.0.fetch_add(1, Ordering::SeqCst);
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}
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}
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let num_drops = Arc::new(AtomicUsize::new(0));
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let m = TicketMutex::<_>::new(Foo(num_drops.clone()));
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assert_eq!(num_drops.load(Ordering::SeqCst), 0);
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{
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let _inner = m.into_inner();
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assert_eq!(num_drops.load(Ordering::SeqCst), 0);
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}
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assert_eq!(num_drops.load(Ordering::SeqCst), 1);
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}
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#[test]
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fn test_mutex_arc_nested() {
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// Tests nested mutexes and access
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// to underlying data.
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let arc = Arc::new(TicketMutex::<_>::new(1));
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let arc2 = Arc::new(TicketMutex::<_>::new(arc));
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let (tx, rx) = channel();
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let _t = thread::spawn(move || {
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let lock = arc2.lock();
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let lock2 = lock.lock();
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assert_eq!(*lock2, 1);
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tx.send(()).unwrap();
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});
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rx.recv().unwrap();
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}
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#[test]
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fn test_mutex_arc_access_in_unwind() {
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let arc = Arc::new(TicketMutex::<_>::new(1));
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let arc2 = arc.clone();
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let _ = thread::spawn(move || -> () {
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struct Unwinder {
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i: Arc<TicketMutex<i32>>,
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}
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impl Drop for Unwinder {
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fn drop(&mut self) {
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*self.i.lock() += 1;
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}
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}
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let _u = Unwinder { i: arc2 };
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panic!();
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})
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.join();
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let lock = arc.lock();
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assert_eq!(*lock, 2);
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}
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#[test]
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fn test_mutex_unsized() {
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let mutex: &TicketMutex<[i32]> = &TicketMutex::<_>::new([1, 2, 3]);
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{
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let b = &mut *mutex.lock();
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b[0] = 4;
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b[2] = 5;
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}
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let comp: &[i32] = &[4, 2, 5];
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assert_eq!(&*mutex.lock(), comp);
|
|
}
|
|
|
|
#[test]
|
|
fn is_locked() {
|
|
let mutex = TicketMutex::<_>::new(());
|
|
assert!(!mutex.is_locked());
|
|
let lock = mutex.lock();
|
|
assert!(mutex.is_locked());
|
|
drop(lock);
|
|
assert!(!mutex.is_locked());
|
|
}
|
|
}
|