tokio/runtime/park.rs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339
#![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))
}