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#![cfg_attr(not(feature = "full"), allow(dead_code))]
use crate::loom::sync::atomic::AtomicUsize;
use crate::loom::sync::{Arc, Condvar, Mutex};
use std::sync::atomic::Ordering::SeqCst;
use std::time::Duration;
#[derive(Debug)]
pub(crate) struct ParkThread {
inner: Arc<Inner>,
}
/// Unblocks a thread that was blocked by `ParkThread`.
#[derive(Clone, Debug)]
pub(crate) struct UnparkThread {
inner: Arc<Inner>,
}
#[derive(Debug)]
struct Inner {
state: AtomicUsize,
mutex: Mutex<()>,
condvar: Condvar,
}
const EMPTY: usize = 0;
const PARKED: usize = 1;
const NOTIFIED: usize = 2;
tokio_thread_local! {
static CURRENT_PARKER: ParkThread = ParkThread::new();
}
// Bit of a hack, but it is only for loom
#[cfg(loom)]
tokio_thread_local! {
pub(crate) static CURRENT_THREAD_PARK_COUNT: AtomicUsize = AtomicUsize::new(0);
}
// ==== impl ParkThread ====
impl ParkThread {
pub(crate) fn new() -> Self {
Self {
inner: Arc::new(Inner {
state: AtomicUsize::new(EMPTY),
mutex: Mutex::new(()),
condvar: Condvar::new(),
}),
}
}
pub(crate) fn unpark(&self) -> UnparkThread {
let inner = self.inner.clone();
UnparkThread { inner }
}
pub(crate) fn park(&mut self) {
#[cfg(loom)]
CURRENT_THREAD_PARK_COUNT.with(|count| count.fetch_add(1, SeqCst));
self.inner.park();
}
pub(crate) fn park_timeout(&mut self, duration: Duration) {
#[cfg(loom)]
CURRENT_THREAD_PARK_COUNT.with(|count| count.fetch_add(1, SeqCst));
// Wasm doesn't have threads, so just sleep.
#[cfg(not(target_family = "wasm"))]
self.inner.park_timeout(duration);
#[cfg(target_family = "wasm")]
std::thread::sleep(duration);
}
pub(crate) fn shutdown(&mut self) {
self.inner.shutdown();
}
}
// ==== impl Inner ====
impl Inner {
fn park(&self) {
// If we were previously notified then we consume this notification and
// return quickly.
if self
.state
.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
.is_ok()
{
return;
}
// Otherwise we need to coordinate going to sleep
let mut m = self.mutex.lock();
match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
Err(NOTIFIED) => {
// We must read here, even though we know it will be `NOTIFIED`.
// This is because `unpark` may have been called again since we read
// `NOTIFIED` in the `compare_exchange` above. We must perform an
// acquire operation that synchronizes with that `unpark` to observe
// any writes it made before the call to unpark. To do that we must
// read from the write it made to `state`.
let old = self.state.swap(EMPTY, SeqCst);
debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
}
Err(actual) => panic!("inconsistent park state; actual = {}", actual),
}
loop {
m = self.condvar.wait(m).unwrap();
if self
.state
.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
.is_ok()
{
// got a notification
return;
}
// spurious wakeup, go back to sleep
}
}
/// Parks the current thread for at most `dur`.
fn park_timeout(&self, dur: Duration) {
// Like `park` above we have a fast path for an already-notified thread,
// and afterwards we start coordinating for a sleep. Return quickly.
if self
.state
.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
.is_ok()
{
return;
}
if dur == Duration::from_millis(0) {
return;
}
let m = self.mutex.lock();
match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
Err(NOTIFIED) => {
// We must read again here, see `park`.
let old = self.state.swap(EMPTY, SeqCst);
debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
}
Err(actual) => panic!("inconsistent park_timeout state; actual = {}", actual),
}
// Wait with a timeout, and if we spuriously wake up or otherwise wake up
// from a notification, we just want to unconditionally set the state back to
// empty, either consuming a notification or un-flagging ourselves as
// parked.
let (_m, _result) = self.condvar.wait_timeout(m, dur).unwrap();
match self.state.swap(EMPTY, SeqCst) {
NOTIFIED => {} // got a notification, hurray!
PARKED => {} // no notification, alas
n => panic!("inconsistent park_timeout state: {}", n),
}
}
fn unpark(&self) {
// To ensure the unparked thread will observe any writes we made before
// this call, we must perform a release operation that `park` can
// synchronize with. To do that we must write `NOTIFIED` even if `state`
// is already `NOTIFIED`. That is why this must be a swap rather than a
// compare-and-swap that returns if it reads `NOTIFIED` on failure.
match self.state.swap(NOTIFIED, SeqCst) {
EMPTY => return, // no one was waiting
NOTIFIED => return, // already unparked
PARKED => {} // gotta go wake someone up
_ => panic!("inconsistent state in unpark"),
}
// There is a period between when the parked thread sets `state` to
// `PARKED` (or last checked `state` in the case of a spurious wake
// up) and when it actually waits on `cvar`. If we were to notify
// during this period it would be ignored and then when the parked
// thread went to sleep it would never wake up. Fortunately, it has
// `lock` locked at this stage so we can acquire `lock` to wait until
// it is ready to receive the notification.
//
// Releasing `lock` before the call to `notify_one` means that when the
// parked thread wakes it doesn't get woken only to have to wait for us
// to release `lock`.
drop(self.mutex.lock());
self.condvar.notify_one();
}
fn shutdown(&self) {
self.condvar.notify_all();
}
}
impl Default for ParkThread {
fn default() -> Self {
Self::new()
}
}
// ===== impl UnparkThread =====
impl UnparkThread {
pub(crate) fn unpark(&self) {
self.inner.unpark();
}
}
use crate::loom::thread::AccessError;
use std::future::Future;
use std::marker::PhantomData;
use std::rc::Rc;
use std::task::{RawWaker, RawWakerVTable, Waker};
/// Blocks the current thread using a condition variable.
#[derive(Debug)]
pub(crate) struct CachedParkThread {
_anchor: PhantomData<Rc<()>>,
}
impl CachedParkThread {
/// Creates a new `ParkThread` handle for the current thread.
///
/// This type cannot be moved to other threads, so it should be created on
/// the thread that the caller intends to park.
pub(crate) fn new() -> CachedParkThread {
CachedParkThread {
_anchor: PhantomData,
}
}
pub(crate) fn waker(&self) -> Result<Waker, AccessError> {
self.unpark().map(UnparkThread::into_waker)
}
fn unpark(&self) -> Result<UnparkThread, AccessError> {
self.with_current(ParkThread::unpark)
}
pub(crate) fn park(&mut self) {
self.with_current(|park_thread| park_thread.inner.park())
.unwrap();
}
pub(crate) fn park_timeout(&mut self, duration: Duration) {
self.with_current(|park_thread| park_thread.inner.park_timeout(duration))
.unwrap();
}
/// Gets a reference to the `ParkThread` handle for this thread.
fn with_current<F, R>(&self, f: F) -> Result<R, AccessError>
where
F: FnOnce(&ParkThread) -> R,
{
CURRENT_PARKER.try_with(|inner| f(inner))
}
pub(crate) fn block_on<F: Future>(&mut self, f: F) -> Result<F::Output, AccessError> {
use std::task::Context;
use std::task::Poll::Ready;
let waker = self.waker()?;
let mut cx = Context::from_waker(&waker);
pin!(f);
loop {
if let Ready(v) = crate::runtime::coop::budget(|| f.as_mut().poll(&mut cx)) {
return Ok(v);
}
self.park();
}
}
}
impl UnparkThread {
pub(crate) fn into_waker(self) -> Waker {
unsafe {
let raw = unparker_to_raw_waker(self.inner);
Waker::from_raw(raw)
}
}
}
impl Inner {
#[allow(clippy::wrong_self_convention)]
fn into_raw(this: Arc<Inner>) -> *const () {
Arc::into_raw(this) as *const ()
}
unsafe fn from_raw(ptr: *const ()) -> Arc<Inner> {
Arc::from_raw(ptr as *const Inner)
}
}
unsafe fn unparker_to_raw_waker(unparker: Arc<Inner>) -> RawWaker {
RawWaker::new(
Inner::into_raw(unparker),
&RawWakerVTable::new(clone, wake, wake_by_ref, drop_waker),
)
}
unsafe fn clone(raw: *const ()) -> RawWaker {
Arc::increment_strong_count(raw as *const Inner);
unparker_to_raw_waker(Inner::from_raw(raw))
}
unsafe fn drop_waker(raw: *const ()) {
drop(Inner::from_raw(raw));
}
unsafe fn wake(raw: *const ()) {
let unparker = Inner::from_raw(raw);
unparker.unpark();
}
unsafe fn wake_by_ref(raw: *const ()) {
let raw = raw as *const Inner;
(*raw).unpark();
}
#[cfg(loom)]
pub(crate) fn current_thread_park_count() -> usize {
CURRENT_THREAD_PARK_COUNT.with(|count| count.load(SeqCst))
}