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#![doc = include_str!("../readme.md")]
use std::{collections::VecDeque, ops::Deref, ptr};
#[cfg(not(loom))]
use std::{
cell::Cell,
sync::{
atomic::{AtomicPtr, AtomicU64, Ordering},
Arc, Mutex, MutexGuard,
},
};
#[cfg(loom)]
use loom::{
cell::Cell,
sync::{
atomic::{AtomicPtr, AtomicU64, Ordering},
Arc, Mutex, MutexGuard,
},
};
/// Represents a read-copy-update for a specific value.
///
/// This is the write side, new read handles can be constructed by calling [`Rcu::reader`].
#[derive(Debug)]
pub struct Rcu<T> {
epoch: u64,
shared: Arc<Shared<T>>,
}
/// The reader handle for a value stored in an [`Rcu`].
///
/// Specific values can be read using [`Reader::read`]. Readers are `!Sync` and expected to be used
/// only on a single thread.
#[derive(Debug)]
pub struct Reader<T: 'static> {
cache: Cell<&'static StampedValue<T>>,
refs: Cell<usize>,
state: ReaderState,
shared: Arc<Shared<T>>,
}
#[derive(Debug)]
struct Shared<T> {
ptr: Pointer<T>,
reclaim: Mutex<VecDeque<Box<StampedValue<T>>>>,
readers: Mutex<Vec<ReaderState>>,
}
#[derive(Debug)]
struct StampedValue<T> {
value: T,
epoch: u64,
}
#[derive(Debug, Clone)]
struct ReaderState(Arc<AtomicU64>);
#[derive(Debug)]
struct Pointer<T>(AtomicPtr<StampedValue<T>>);
#[derive(Debug)]
pub struct Guard<'a, T: 'static> {
cache: &'a StampedValue<T>,
reader: &'a Reader<T>,
}
impl<T: 'static> Default for Rcu<T>
where
T: Default,
{
fn default() -> Self {
Self::new(T::default())
}
}
impl<T: 'static> Rcu<T> {
/// Constructs a new RCU with an initial value.
pub fn new(value: T) -> Self {
Self {
epoch: 1,
shared: Arc::new(Shared {
ptr: Pointer::new(StampedValue { value, epoch: 1 }),
reclaim: Mutex::new(VecDeque::new()),
readers: Mutex::new(Vec::new()),
}),
}
}
/// Registers a new [`Reader`] allowing values to be read.
pub fn reader(&mut self) -> Reader<T> {
Reader::new(self.shared.clone())
}
/// Write a new value making it available to all readers.
///
/// Previously written values will be reclaimed when they are no longer accesed.
pub fn write(&mut self, value: T) {
// Records a new epoch associated with this value, not allowed to wrap around.
self.epoch += 1;
// Publish the value, we will attempt to reclaim the previous value.
let next = StampedValue {
epoch: self.epoch,
value,
};
let prev = self.shared.ptr.swap(next);
self.reclaim_queue().push_back(prev);
// Immediately attempt to reclaim.
self.try_reclaim();
}
/// Try and reclaim any values which are no longer in-use.
///
/// Returns the number of values still waiting to be reclaimed.
pub fn try_reclaim(&mut self) -> usize {
let mut readers = self.shared.readers.lock().unwrap();
// Trim readers which have been removed.
readers.retain(|reader| reader.get() > ReaderState::NOT_IN_USE);
let mut reclaim = self.reclaim_queue();
// If there are no readers, we can reclaim everything.
if readers.is_empty() {
reclaim.clear();
}
// Check the minimum epoch across all active threads, removing records
// that are below the minimum epoch.
let min_epoch = readers
.iter()
.map(|r| r.get())
.min()
.unwrap_or(ReaderState::NOT_IN_USE);
while let Some(candidate) = reclaim.pop_front() {
if min_epoch > candidate.epoch {
drop(candidate);
} else {
reclaim.push_front(candidate);
// We short circuit, no point checking others with a higher epoch.
return reclaim.len();
}
}
0
}
fn reclaim_queue(&self) -> MutexGuard<'_, VecDeque<Box<StampedValue<T>>>> {
// The reclaimer must be shared so we can drop any remaining values when the 'Arc<Shared>'
// drops, but access to it should only be from this function. As a result we protect it with
// a mutex but only rely on try_lock().
self.shared
.reclaim
.try_lock()
.expect("invalid shared reclaimer access")
}
}
impl<T: 'static> Reader<T> {
fn new(shared: Arc<Shared<T>>) -> Self {
let value = shared.ptr.load();
let mut readers = shared.readers.lock().unwrap();
let state = ReaderState::new(value.epoch);
readers.push(state.clone());
Reader {
shared: shared.clone(),
refs: Cell::new(0),
cache: Cell::new(value),
state,
}
}
/// Reads the latest value guarded to ensure that the pointer will not be reclaimed while the
/// current reader has access.
pub fn read(&self) -> Guard<'_, T> {
// The read method provides a guard that allows deref access to one of the values written
// by the writer previously. The invariant this method maintains, using ref-counts, is that
// the epoch stamped on the current thread is always less than or equal to the epoch of the
// last used value. As soon as the reclaimer sees an epoch for a specific thread, it can be
// sure that no references with epochs 'below' the available epoch exist on that thread.
let cache = if self.refs.get() == 0 {
let value = self.shared.ptr.load();
// Update the epoch to note that we are currently using this value. This uses release
// ordering to ensure that loads when reclaiming will be ordered after this operation.
self.state.set(value.epoch);
// Cache the pointer in the current reader.
self.cache.set(value);
value
} else {
self.cache.get()
};
self.refs.set(self.refs.get() + 1);
Guard {
reader: self,
cache,
}
}
}
impl<'a, T> Deref for Guard<'a, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
&self.cache.value
}
}
impl<'a, T> Drop for Guard<'a, T> {
fn drop(&mut self) {
self.reader.refs.replace(self.reader.refs.get() - 1);
}
}
impl<T> Drop for Reader<T> {
fn drop(&mut self) {
self.state.mark_dropped();
}
}
impl<T> Pointer<T> {
fn new(value: StampedValue<T>) -> Self {
Self(AtomicPtr::new(Box::leak(Box::new(value))))
}
fn swap(&self, value: StampedValue<T>) -> Box<StampedValue<T>> {
let ptr = Box::leak(Box::new(value));
let prev = self.0.swap(ptr, Ordering::AcqRel);
unsafe { Box::from_raw(prev) }
}
fn load(&self) -> &'static StampedValue<T> {
unsafe { &*self.0.load(Ordering::Relaxed) }
}
}
impl<T> Drop for Pointer<T> {
fn drop(&mut self) {
let prev = self.0.swap(ptr::null_mut(), Ordering::AcqRel);
let _ = unsafe { Box::from_raw(prev) };
}
}
impl ReaderState {
const NOT_IN_USE: u64 = 0;
fn new(epoch: u64) -> Self {
Self(Arc::new(AtomicU64::new(epoch)))
}
fn mark_dropped(&self) {
self.set(Self::NOT_IN_USE)
}
fn set(&self, epoch: u64) {
self.0.store(epoch, Ordering::Release)
}
fn get(&self) -> u64 {
self.0.load(Ordering::Acquire)
}
}
/// Provides thread-local storage to read [`Rcu`] values.
///
/// When a new thread is initialized a new [`Reader`] will be created and stored in a slot for the
/// provided thread. Values will be published to the thread-local and access will be cheap
#[cfg(feature = "thread-local")]
pub struct ThreadLocal<T: Send + Sync + 'static> {
shared: Arc<Shared<T>>,
thread_local: thread_local::ThreadLocal<Reader<T>>,
}
#[cfg(feature = "thread-local")]
impl<T: Send + Sync + 'static> ThreadLocal<T> {
pub fn new(rcu: &Rcu<T>) -> Self {
Self {
shared: rcu.shared.clone(),
thread_local: thread_local::ThreadLocal::new(),
}
}
/// Returns the element for the current thread, if it exists,
pub fn get(&self) -> Option<Guard<'_, T>> {
self.thread_local.get().map(|r| r.read())
}
/// Returns the element for the current thread, or creates it if it doesn't exist.
pub fn get_or_init(&self) -> Guard<'_, T> {
self.thread_local
.get_or(|| Reader::new(self.shared.clone()))
.read()
}
}
#[cfg(test)]
#[cfg(loom)]
mod loom_tests {
use loom::thread;
use super::*;
#[test]
fn nested() {
loom::model(|| {
let mut rcu = Rcu::new(10);
let rdr = rcu.reader();
{
let g = rdr.read();
assert_eq!(10, *g);
rcu.write(20);
{
let g = rdr.read();
assert_eq!(10, *g);
}
}
});
}
#[test]
fn thread_nested() {
loom::model(|| {
let n = 2;
let mut rcu = Rcu::new(0);
let rdr = rcu.reader();
let h = thread::spawn(move || {
let v = rdr.read();
assert!(*v < n);
{
let g = rdr.read();
assert!(*g < n);
}
});
for i in 0..n {
rcu.write(i);
loom::thread::yield_now();
}
h.join().unwrap();
});
}
#[test]
fn thread() {
loom::model(|| {
let n = 2;
let mut rcu = Rcu::new(0);
let rdr = rcu.reader();
let h = thread::spawn(move || {
for _ in 0..n {
let v = rdr.read();
assert!(*v < n);
loom::thread::yield_now();
}
});
for i in 0..n {
rcu.write(i);
loom::thread::yield_now();
}
h.join().unwrap();
});
}
#[test]
fn thread_detached() {
loom::model(|| {
let n = 2;
let mut rcu = Rcu::new(0);
let rdr = rcu.reader();
thread::spawn(move || {
for _ in 0..n {
let v = rdr.read();
assert!(*v < n);
loom::thread::yield_now();
}
});
for i in 0..n {
rcu.write(i);
loom::thread::yield_now();
}
});
}
}
#[cfg(test)]
#[cfg(not(loom))]
mod tests {
use std::{
sync::{atomic::AtomicUsize, Condvar},
thread,
time::Duration,
};
use super::*;
thread_local! {
static REFS: AtomicUsize = AtomicUsize::new(0);
}
struct RefsCheck;
impl RefsCheck {
fn new() -> Self {
REFS.with(|refs| {
assert_eq!(refs.load(Ordering::SeqCst), 0);
});
Self
}
}
impl Drop for RefsCheck {
fn drop(&mut self) {
REFS.with(|refs| {
assert_eq!(refs.load(Ordering::SeqCst), 0);
});
}
}
#[derive(Debug)]
struct RecordDrop(u32);
impl RecordDrop {
fn new(v: u32) -> Self {
REFS.with(|refs| {
refs.fetch_add(1, Ordering::SeqCst);
});
Self(v)
}
}
impl Drop for RecordDrop {
fn drop(&mut self) {
REFS.with(|refs| {
refs.fetch_sub(1, Ordering::SeqCst);
});
}
}
#[cfg(feature = "thread-local")]
#[test]
fn thread_local() {
let mut rcu = Rcu::new(10);
let tls = ThreadLocal::new(&rcu);
thread::scope(|s| {
s.spawn(|| {
let _val = tls.get_or_init();
assert!(tls.get().is_some());
});
s.spawn(|| {
let _val = tls.get_or_init();
assert!(tls.get().is_some());
});
});
rcu.write(1);
}
#[test]
fn send_check() {
let mut rcu = Rcu::new(10);
let rdr = rcu.reader();
thread::spawn(move || {
assert_eq!(10, *rdr.read());
});
}
#[test]
fn single_value() {
let _refs = RefsCheck::new();
let mut rcu = Rcu::new(RecordDrop::new(10));
let rdr = rcu.reader();
assert_eq!(10, rdr.read().0);
}
#[test]
fn old_value() {
let _refs = RefsCheck::new();
let mut rcu = Rcu::new(RecordDrop::new(10));
let rdr1 = rcu.reader();
assert_eq!(10, rdr1.read().0);
let rdr2 = rcu.reader();
assert_eq!(10, rdr2.read().0);
for i in 11..=20 {
rcu.write(RecordDrop::new(i));
assert_eq!(i, rdr1.read().0);
}
// because of the limitations of the current design, all values will
// not be dropped until this point.
}
#[test]
fn remove_readers() {
let _refs = RefsCheck::new();
let mut rcu = Rcu::new(RecordDrop::new(10));
let rdr1 = rcu.reader();
let rdr2 = rcu.reader();
for i in 11..=20 {
rcu.write(RecordDrop::new(i));
}
drop(rdr1);
drop(rdr2);
rcu.write(RecordDrop::new(30));
}
#[test]
fn nested() {
let _refs = RefsCheck::new();
let mut rcu = Rcu::new(RecordDrop::new(10));
let rdr = rcu.reader();
{
let handle = rdr.read();
assert_eq!(10, handle.0);
rcu.write(RecordDrop::new(20));
{
let handle = rdr.read();
assert_eq!(10, handle.0);
}
let handle2 = rdr.read();
assert_eq!(10, handle.0);
assert_eq!(10, handle2.0);
}
assert_eq!(20, rdr.read().0);
}
#[test]
fn nested_multi_threaded() {
let _refs = RefsCheck::new();
let notify = Arc::new((Mutex::new(false), Condvar::new()));
let mut rcu = Rcu::new(RecordDrop::new(10));
let rdr = rcu.reader();
assert_eq!(10, rdr.read().0);
let handles: Vec<_> = (0..10)
.map(|_| {
let rdr = rcu.reader();
let notify = notify.clone();
std::thread::spawn(move || {
let _refs = RefsCheck::new();
assert_eq!(10, rdr.read().0);
{
let handle = rdr.read();
assert_eq!(10, handle.0);
}
let (lock, cvar) = &*notify;
let mut started = lock.lock().unwrap();
while !*started {
started = cvar.wait(started).unwrap();
}
assert_eq!(20, rdr.read().0);
{
let handle = rdr.read();
assert_eq!(20, handle.0);
}
})
})
.collect();
thread::sleep(Duration::from_millis(10));
rcu.write(RecordDrop::new(20));
{
let (lock, cvar) = &*notify;
*lock.lock().unwrap() = true;
cvar.notify_all();
}
handles.into_iter().for_each(|h| h.join().unwrap());
}
}