async_lock/rwlock/raw.rs
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//! Raw, unsafe reader-writer locking implementation,
//! doesn't depend on the data protected by the lock.
//! [`RwLock`](super::RwLock) is implemented in terms of this.
//!
//! Splitting the implementation this way allows instantiating
//! the locking code only once, and also lets us make
//! [`RwLockReadGuard`](super::RwLockReadGuard) covariant in `T`.
use core::marker::PhantomPinned;
use core::mem::forget;
use core::pin::Pin;
use core::task::Poll;
use crate::sync::atomic::{AtomicUsize, Ordering};
use event_listener::{Event, EventListener};
use event_listener_strategy::{EventListenerFuture, Strategy};
use crate::futures::Lock;
use crate::Mutex;
const WRITER_BIT: usize = 1;
const ONE_READER: usize = 2;
/// A "raw" RwLock that doesn't hold any data.
pub(super) struct RawRwLock {
/// Acquired by the writer.
mutex: Mutex<()>,
/// Event triggered when the last reader is dropped.
no_readers: Event,
/// Event triggered when the writer is dropped.
no_writer: Event,
/// Current state of the lock.
///
/// The least significant bit (`WRITER_BIT`) is set to 1 when a writer is holding the lock or
/// trying to acquire it.
///
/// The upper bits contain the number of currently active readers. Each active reader
/// increments the state by `ONE_READER`.
state: AtomicUsize,
}
impl RawRwLock {
const_fn! {
const_if: #[cfg(not(loom))];
#[inline]
pub(super) const fn new() -> Self {
RawRwLock {
mutex: Mutex::new(()),
no_readers: Event::new(),
no_writer: Event::new(),
state: AtomicUsize::new(0),
}
}
}
/// Returns `true` iff a read lock was successfully acquired.
pub(super) fn try_read(&self) -> bool {
let mut state = self.state.load(Ordering::Acquire);
loop {
// If there's a writer holding the lock or attempting to acquire it, we cannot acquire
// a read lock here.
if state & WRITER_BIT != 0 {
return false;
}
// Make sure the number of readers doesn't overflow.
if state > core::isize::MAX as usize {
crate::abort();
}
// Increment the number of readers.
match self.state.compare_exchange(
state,
state + ONE_READER,
Ordering::AcqRel,
Ordering::Acquire,
) {
Ok(_) => return true,
Err(s) => state = s,
}
}
}
#[inline]
pub(super) fn read(&self) -> RawRead<'_> {
RawRead {
lock: self,
state: self.state.load(Ordering::Acquire),
listener: None,
_pin: PhantomPinned,
}
}
/// Returns `true` iff an upgradable read lock was successfully acquired.
pub(super) fn try_upgradable_read(&self) -> bool {
// First try grabbing the mutex.
let lock = if let Some(lock) = self.mutex.try_lock() {
lock
} else {
return false;
};
forget(lock);
let mut state = self.state.load(Ordering::Acquire);
// Make sure the number of readers doesn't overflow.
if state > core::isize::MAX as usize {
crate::abort();
}
// Increment the number of readers.
loop {
match self.state.compare_exchange(
state,
state + ONE_READER,
Ordering::AcqRel,
Ordering::Acquire,
) {
Ok(_) => return true,
Err(s) => state = s,
}
}
}
#[inline]
pub(super) fn upgradable_read(&self) -> RawUpgradableRead<'_> {
RawUpgradableRead {
lock: self,
acquire: self.mutex.lock(),
}
}
/// Returs `true` iff a write lock was successfully acquired.
pub(super) fn try_write(&self) -> bool {
// First try grabbing the mutex.
let lock = if let Some(lock) = self.mutex.try_lock() {
lock
} else {
return false;
};
// If there are no readers, grab the write lock.
if self
.state
.compare_exchange(0, WRITER_BIT, Ordering::AcqRel, Ordering::Acquire)
.is_ok()
{
forget(lock);
true
} else {
drop(lock);
false
}
}
#[inline]
pub(super) fn write(&self) -> RawWrite<'_> {
RawWrite {
lock: self,
no_readers: None,
state: WriteState::Acquiring {
lock: self.mutex.lock(),
},
}
}
/// Returns `true` iff a the upgradable read lock was successfully upgraded to a write lock.
///
/// # Safety
///
/// Caller must hold an upgradable read lock.
/// This will attempt to upgrade it to a write lock.
pub(super) unsafe fn try_upgrade(&self) -> bool {
self.state
.compare_exchange(ONE_READER, WRITER_BIT, Ordering::AcqRel, Ordering::Acquire)
.is_ok()
}
/// # Safety
///
/// Caller must hold an upgradable read lock.
/// This will upgrade it to a write lock.
pub(super) unsafe fn upgrade(&self) -> RawUpgrade<'_> {
// Set `WRITER_BIT` and decrement the number of readers at the same time.
self.state
.fetch_sub(ONE_READER - WRITER_BIT, Ordering::SeqCst);
RawUpgrade {
lock: Some(self),
listener: None,
_pin: PhantomPinned,
}
}
/// # Safety
///
/// Caller must hold an upgradable read lock.
/// This will downgrade it to a stadard read lock.
#[inline]
pub(super) unsafe fn downgrade_upgradable_read(&self) {
self.mutex.unlock_unchecked();
}
/// # Safety
///
/// Caller must hold a write lock.
/// This will downgrade it to a read lock.
pub(super) unsafe fn downgrade_write(&self) {
// Atomically downgrade state.
self.state
.fetch_add(ONE_READER - WRITER_BIT, Ordering::SeqCst);
// Release the writer mutex.
self.mutex.unlock_unchecked();
// Trigger the "no writer" event.
self.no_writer.notify(1);
}
/// # Safety
///
/// Caller must hold a write lock.
/// This will downgrade it to an upgradable read lock.
pub(super) unsafe fn downgrade_to_upgradable(&self) {
// Atomically downgrade state.
self.state
.fetch_add(ONE_READER - WRITER_BIT, Ordering::SeqCst);
}
/// # Safety
///
/// Caller must hold a read lock .
/// This will unlock that lock.
pub(super) unsafe fn read_unlock(&self) {
// Decrement the number of readers.
if self.state.fetch_sub(ONE_READER, Ordering::SeqCst) & !WRITER_BIT == ONE_READER {
// If this was the last reader, trigger the "no readers" event.
self.no_readers.notify(1);
}
}
/// # Safety
///
/// Caller must hold an upgradable read lock.
/// This will unlock that lock.
pub(super) unsafe fn upgradable_read_unlock(&self) {
// Decrement the number of readers.
if self.state.fetch_sub(ONE_READER, Ordering::SeqCst) & !WRITER_BIT == ONE_READER {
// If this was the last reader, trigger the "no readers" event.
self.no_readers.notify(1);
}
// SAFETY: upgradable read guards acquire the writer mutex upon creation.
self.mutex.unlock_unchecked();
}
/// # Safety
///
/// Caller must hold a write lock.
/// This will unlock that lock.
pub(super) unsafe fn write_unlock(&self) {
// Unset `WRITER_BIT`.
self.state.fetch_and(!WRITER_BIT, Ordering::SeqCst);
// Trigger the "no writer" event.
self.no_writer.notify(1);
// Release the writer lock.
// SAFETY: `RwLockWriteGuard` always holds a lock on writer mutex.
self.mutex.unlock_unchecked();
}
}
pin_project_lite::pin_project! {
/// The future returned by [`RawRwLock::read`].
pub(super) struct RawRead<'a> {
// The lock that is being acquired.
pub(super) lock: &'a RawRwLock,
// The last-observed state of the lock.
state: usize,
// The listener for the "no writers" event.
listener: Option<EventListener>,
// Making this type `!Unpin` enables future optimizations.
#[pin]
_pin: PhantomPinned
}
}
impl<'a> EventListenerFuture for RawRead<'a> {
type Output = ();
fn poll_with_strategy<'x, S: Strategy<'x>>(
self: Pin<&mut Self>,
strategy: &mut S,
cx: &mut S::Context,
) -> Poll<()> {
let this = self.project();
loop {
if *this.state & WRITER_BIT == 0 {
// Make sure the number of readers doesn't overflow.
if *this.state > core::isize::MAX as usize {
crate::abort();
}
// If nobody is holding a write lock or attempting to acquire it, increment the
// number of readers.
match this.lock.state.compare_exchange(
*this.state,
*this.state + ONE_READER,
Ordering::AcqRel,
Ordering::Acquire,
) {
Ok(_) => return Poll::Ready(()),
Err(s) => *this.state = s,
}
} else {
// Start listening for "no writer" events.
let load_ordering = if this.listener.is_none() {
*this.listener = Some(this.lock.no_writer.listen());
// Make sure there really is no writer.
Ordering::SeqCst
} else {
// Wait for the writer to finish.
ready!(strategy.poll(this.listener, cx));
// Notify the next reader waiting in list.
this.lock.no_writer.notify(1);
// Check the state again.
Ordering::Acquire
};
// Reload the state.
*this.state = this.lock.state.load(load_ordering);
}
}
}
}
pin_project_lite::pin_project! {
/// The future returned by [`RawRwLock::upgradable_read`].
pub(super) struct RawUpgradableRead<'a> {
// The lock that is being acquired.
pub(super) lock: &'a RawRwLock,
// The mutex we are trying to acquire.
#[pin]
acquire: Lock<'a, ()>,
}
}
impl<'a> EventListenerFuture for RawUpgradableRead<'a> {
type Output = ();
fn poll_with_strategy<'x, S: Strategy<'x>>(
self: Pin<&mut Self>,
strategy: &mut S,
cx: &mut S::Context,
) -> Poll<()> {
let this = self.project();
// Acquire the mutex.
let mutex_guard = ready!(this.acquire.poll_with_strategy(strategy, cx));
forget(mutex_guard);
// Load the current state.
let mut state = this.lock.state.load(Ordering::Acquire);
// Make sure the number of readers doesn't overflow.
if state > core::isize::MAX as usize {
crate::abort();
}
// Increment the number of readers.
loop {
match this.lock.state.compare_exchange(
state,
state + ONE_READER,
Ordering::AcqRel,
Ordering::Acquire,
) {
Ok(_) => {
return Poll::Ready(());
}
Err(s) => state = s,
}
}
}
}
pin_project_lite::pin_project! {
/// The future returned by [`RawRwLock::write`].
pub(super) struct RawWrite<'a> {
// The lock that is being acquired.
pub(super) lock: &'a RawRwLock,
// Our listener for the "no readers" event.
no_readers: Option<EventListener>,
// Current state fof this future.
#[pin]
state: WriteState<'a>,
}
impl PinnedDrop for RawWrite<'_> {
fn drop(this: Pin<&mut Self>) {
let this = this.project();
if matches!(this.state.project(), WriteStateProj::WaitingReaders) {
// Safety: we hold a write lock, more or less.
unsafe {
this.lock.write_unlock();
}
}
}
}
}
pin_project_lite::pin_project! {
#[project = WriteStateProj]
#[project_replace = WriteStateProjReplace]
enum WriteState<'a> {
// We are currently acquiring the inner mutex.
Acquiring { #[pin] lock: Lock<'a, ()> },
// We are currently waiting for readers to finish.
WaitingReaders,
// The future has completed.
Acquired,
}
}
impl<'a> EventListenerFuture for RawWrite<'a> {
type Output = ();
fn poll_with_strategy<'x, S: Strategy<'x>>(
self: Pin<&mut Self>,
strategy: &mut S,
cx: &mut S::Context,
) -> Poll<()> {
let mut this = self.project();
loop {
match this.state.as_mut().project() {
WriteStateProj::Acquiring { lock } => {
// First grab the mutex.
let mutex_guard = ready!(lock.poll_with_strategy(strategy, cx));
forget(mutex_guard);
// Set `WRITER_BIT` and create a guard that unsets it in case this future is canceled.
let new_state = this.lock.state.fetch_or(WRITER_BIT, Ordering::SeqCst);
// If we just acquired the lock, return.
if new_state == WRITER_BIT {
this.state.as_mut().set(WriteState::Acquired);
return Poll::Ready(());
}
// Start waiting for the readers to finish.
*this.no_readers = Some(this.lock.no_readers.listen());
this.state.as_mut().set(WriteState::WaitingReaders);
}
WriteStateProj::WaitingReaders => {
let load_ordering = if this.no_readers.is_some() {
Ordering::Acquire
} else {
Ordering::SeqCst
};
// Check the state again.
if this.lock.state.load(load_ordering) == WRITER_BIT {
// We are the only ones holding the lock, return `Ready`.
this.state.as_mut().set(WriteState::Acquired);
return Poll::Ready(());
}
// Wait for the readers to finish.
if this.no_readers.is_none() {
// Register a listener.
*this.no_readers = Some(this.lock.no_readers.listen());
} else {
// Wait for the readers to finish.
ready!(strategy.poll(this.no_readers, cx));
};
}
WriteStateProj::Acquired => panic!("Write lock already acquired"),
}
}
}
}
pin_project_lite::pin_project! {
/// The future returned by [`RawRwLock::upgrade`].
pub(super) struct RawUpgrade<'a> {
lock: Option<&'a RawRwLock>,
// The event listener we are waiting on.
listener: Option<EventListener>,
// Keeping this future `!Unpin` enables future optimizations.
#[pin]
_pin: PhantomPinned
}
impl PinnedDrop for RawUpgrade<'_> {
fn drop(this: Pin<&mut Self>) {
let this = this.project();
if let Some(lock) = this.lock {
// SAFETY: we are dropping the future that would give us a write lock,
// so we don't need said lock anymore.
unsafe {
lock.write_unlock();
}
}
}
}
}
impl<'a> EventListenerFuture for RawUpgrade<'a> {
type Output = &'a RawRwLock;
fn poll_with_strategy<'x, S: Strategy<'x>>(
self: Pin<&mut Self>,
strategy: &mut S,
cx: &mut S::Context,
) -> Poll<&'a RawRwLock> {
let this = self.project();
let lock = this.lock.expect("cannot poll future after completion");
// If there are readers, we need to wait for them to finish.
loop {
let load_ordering = if this.listener.is_some() {
Ordering::Acquire
} else {
Ordering::SeqCst
};
// See if the number of readers is zero.
let state = lock.state.load(load_ordering);
if state == WRITER_BIT {
break;
}
// If there are readers, wait for them to finish.
if this.listener.is_none() {
// Start listening for "no readers" events.
*this.listener = Some(lock.no_readers.listen());
} else {
// Wait for the readers to finish.
ready!(strategy.poll(this.listener, cx));
};
}
// We are done.
Poll::Ready(this.lock.take().unwrap())
}
}
impl<'a> RawUpgrade<'a> {
/// Whether the future returned `Poll::Ready(..)` at some point.
#[inline]
pub(super) fn is_ready(&self) -> bool {
self.lock.is_none()
}
}