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// Copyright 2016 Amanieu d'Antras
//
// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
// http://opensource.org/licenses/MIT>, at your option. This file may not be
// copied, modified, or distributed except according to those terms.
use crate::raw_fair_mutex::RawFairMutex;
use lock_api;
/// A mutual exclusive primitive that is always fair, useful for protecting shared data
///
/// This mutex will block threads waiting for the lock to become available. The
/// mutex can also be statically initialized or created via a `new`
/// constructor. Each mutex has a type parameter which represents the data that
/// it is protecting. The data can only be accessed through the RAII guards
/// returned from `lock` and `try_lock`, which guarantees that the data is only
/// ever accessed when the mutex is locked.
///
/// The regular mutex provided by `parking_lot` uses eventual locking fairness
/// (after some time it will default to the fair algorithm), but eventual
/// fairness does not provide the same garantees a always fair method would.
/// Fair mutexes are generally slower, but sometimes needed. This wrapper was
/// created to avoid using a unfair protocol when it's forbidden by mistake.
///
/// In a fair mutex the lock is provided to whichever thread asked first,
/// they form a queue and always follow the first-in first-out order. This
/// means some thread in the queue won't be able to steal the lock and use it fast
/// to increase throughput, at the cost of latency. Since the response time will grow
/// for some threads that are waiting for the lock and losing to faster but later ones,
/// but it may make sending more responses possible.
///
/// A fair mutex may not be interesting if threads have different priorities (this is known as
/// priority inversion).
///
/// # Differences from the standard library `Mutex`
///
/// - No poisoning, the lock is released normally on panic.
/// - Only requires 1 byte of space, whereas the standard library boxes the
/// `FairMutex` due to platform limitations.
/// - Can be statically constructed (requires the `const_fn` nightly feature).
/// - Does not require any drop glue when dropped.
/// - Inline fast path for the uncontended case.
/// - Efficient handling of micro-contention using adaptive spinning.
/// - Allows raw locking & unlocking without a guard.
///
/// # Examples
///
/// ```
/// use parking_lot::FairMutex;
/// use std::sync::{Arc, mpsc::channel};
/// use std::thread;
///
/// const N: usize = 10;
///
/// // Spawn a few threads to increment a shared variable (non-atomically), and
/// // let the main thread know once all increments are done.
/// //
/// // Here we're using an Arc to share memory among threads, and the data inside
/// // the Arc is protected with a mutex.
/// let data = Arc::new(FairMutex::new(0));
///
/// let (tx, rx) = channel();
/// for _ in 0..10 {
/// let (data, tx) = (Arc::clone(&data), tx.clone());
/// thread::spawn(move || {
/// // The shared state can only be accessed once the lock is held.
/// // Our non-atomic increment is safe because we're the only thread
/// // which can access the shared state when the lock is held.
/// let mut data = data.lock();
/// *data += 1;
/// if *data == N {
/// tx.send(()).unwrap();
/// }
/// // the lock is unlocked here when `data` goes out of scope.
/// });
/// }
///
/// rx.recv().unwrap();
/// ```
pub type FairMutex<T> = lock_api::Mutex<RawFairMutex, T>;
/// Creates a new fair mutex in an unlocked state ready for use.
///
/// This allows creating a fair mutex in a constant context on stable Rust.
pub const fn const_fair_mutex<T>(val: T) -> FairMutex<T> {
FairMutex::const_new(<RawFairMutex as lock_api::RawMutex>::INIT, val)
}
/// An RAII implementation of a "scoped lock" of a mutex. 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` and `DerefMut` implementations.
pub type FairMutexGuard<'a, T> = lock_api::MutexGuard<'a, RawFairMutex, T>;
/// An RAII mutex guard returned by `FairMutexGuard::map`, which can point to a
/// subfield of the protected data.
///
/// The main difference between `MappedFairMutexGuard` and `FairMutexGuard` is that the
/// former doesn't support temporarily unlocking and re-locking, since that
/// could introduce soundness issues if the locked object is modified by another
/// thread.
pub type MappedFairMutexGuard<'a, T> = lock_api::MappedMutexGuard<'a, RawFairMutex, T>;
#[cfg(test)]
mod tests {
use crate::FairMutex;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::sync::mpsc::channel;
use std::sync::Arc;
use std::thread;
#[cfg(feature = "serde")]
use bincode::{deserialize, serialize};
#[derive(Eq, PartialEq, Debug)]
struct NonCopy(i32);
#[test]
fn smoke() {
let m = FairMutex::new(());
drop(m.lock());
drop(m.lock());
}
#[test]
fn lots_and_lots() {
const J: u32 = 1000;
const K: u32 = 3;
let m = Arc::new(FairMutex::new(0));
fn inc(m: &FairMutex<u32>) {
for _ in 0..J {
*m.lock() += 1;
}
}
let (tx, rx) = channel();
for _ in 0..K {
let tx2 = tx.clone();
let m2 = m.clone();
thread::spawn(move || {
inc(&m2);
tx2.send(()).unwrap();
});
let tx2 = tx.clone();
let m2 = m.clone();
thread::spawn(move || {
inc(&m2);
tx2.send(()).unwrap();
});
}
drop(tx);
for _ in 0..2 * K {
rx.recv().unwrap();
}
assert_eq!(*m.lock(), J * K * 2);
}
#[test]
fn try_lock() {
let m = FairMutex::new(());
*m.try_lock().unwrap() = ();
}
#[test]
fn test_into_inner() {
let m = FairMutex::new(NonCopy(10));
assert_eq!(m.into_inner(), NonCopy(10));
}
#[test]
fn test_into_inner_drop() {
struct Foo(Arc<AtomicUsize>);
impl Drop for Foo {
fn drop(&mut self) {
self.0.fetch_add(1, Ordering::SeqCst);
}
}
let num_drops = Arc::new(AtomicUsize::new(0));
let m = FairMutex::new(Foo(num_drops.clone()));
assert_eq!(num_drops.load(Ordering::SeqCst), 0);
{
let _inner = m.into_inner();
assert_eq!(num_drops.load(Ordering::SeqCst), 0);
}
assert_eq!(num_drops.load(Ordering::SeqCst), 1);
}
#[test]
fn test_get_mut() {
let mut m = FairMutex::new(NonCopy(10));
*m.get_mut() = NonCopy(20);
assert_eq!(m.into_inner(), NonCopy(20));
}
#[test]
fn test_mutex_arc_nested() {
// Tests nested mutexes and access
// to underlying data.
let arc = Arc::new(FairMutex::new(1));
let arc2 = Arc::new(FairMutex::new(arc));
let (tx, rx) = channel();
let _t = thread::spawn(move || {
let lock = arc2.lock();
let lock2 = lock.lock();
assert_eq!(*lock2, 1);
tx.send(()).unwrap();
});
rx.recv().unwrap();
}
#[test]
fn test_mutex_arc_access_in_unwind() {
let arc = Arc::new(FairMutex::new(1));
let arc2 = arc.clone();
let _ = thread::spawn(move || {
struct Unwinder {
i: Arc<FairMutex<i32>>,
}
impl Drop for Unwinder {
fn drop(&mut self) {
*self.i.lock() += 1;
}
}
let _u = Unwinder { i: arc2 };
panic!();
})
.join();
let lock = arc.lock();
assert_eq!(*lock, 2);
}
#[test]
fn test_mutex_unsized() {
let mutex: &FairMutex<[i32]> = &FairMutex::new([1, 2, 3]);
{
let b = &mut *mutex.lock();
b[0] = 4;
b[2] = 5;
}
let comp: &[i32] = &[4, 2, 5];
assert_eq!(&*mutex.lock(), comp);
}
#[test]
fn test_mutexguard_sync() {
fn sync<T: Sync>(_: T) {}
let mutex = FairMutex::new(());
sync(mutex.lock());
}
#[test]
fn test_mutex_debug() {
let mutex = FairMutex::new(vec![0u8, 10]);
assert_eq!(format!("{:?}", mutex), "Mutex { data: [0, 10] }");
let _lock = mutex.lock();
assert_eq!(format!("{:?}", mutex), "Mutex { data: <locked> }");
}
#[cfg(feature = "serde")]
#[test]
fn test_serde() {
let contents: Vec<u8> = vec![0, 1, 2];
let mutex = FairMutex::new(contents.clone());
let serialized = serialize(&mutex).unwrap();
let deserialized: FairMutex<Vec<u8>> = deserialize(&serialized).unwrap();
assert_eq!(*(mutex.lock()), *(deserialized.lock()));
assert_eq!(contents, *(deserialized.lock()));
}
}