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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413
use crate::primitive::sync::atomic::AtomicUsize;
use crate::primitive::sync::{Arc, Condvar, Mutex};
use core::sync::atomic::Ordering::SeqCst;
use std::fmt;
use std::marker::PhantomData;
use std::time::{Duration, Instant};
/// A thread parking primitive.
///
/// Conceptually, each `Parker` has an associated token which is initially not present:
///
/// * The [`park`] method blocks the current thread unless or until the token is available, at
/// which point it automatically consumes the token.
///
/// * The [`park_timeout`] and [`park_deadline`] methods work the same as [`park`], but block for
/// a specified maximum time.
///
/// * The [`unpark`] method atomically makes the token available if it wasn't already. Because the
/// token is initially absent, [`unpark`] followed by [`park`] will result in the second call
/// returning immediately.
///
/// In other words, each `Parker` acts a bit like a spinlock that can be locked and unlocked using
/// [`park`] and [`unpark`].
///
/// # Examples
///
/// ```
/// use std::thread;
/// use std::time::Duration;
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
/// let u = p.unparker().clone();
///
/// // Make the token available.
/// u.unpark();
/// // Wakes up immediately and consumes the token.
/// p.park();
///
/// thread::spawn(move || {
/// thread::sleep(Duration::from_millis(500));
/// u.unpark();
/// });
///
/// // Wakes up when `u.unpark()` provides the token.
/// p.park();
/// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
/// ```
///
/// [`park`]: Parker::park
/// [`park_timeout`]: Parker::park_timeout
/// [`park_deadline`]: Parker::park_deadline
/// [`unpark`]: Unparker::unpark
pub struct Parker {
unparker: Unparker,
_marker: PhantomData<*const ()>,
}
unsafe impl Send for Parker {}
impl Default for Parker {
fn default() -> Self {
Self {
unparker: Unparker {
inner: Arc::new(Inner {
state: AtomicUsize::new(EMPTY),
lock: Mutex::new(()),
cvar: Condvar::new(),
}),
},
_marker: PhantomData,
}
}
}
impl Parker {
/// Creates a new `Parker`.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
/// ```
///
pub fn new() -> Parker {
Self::default()
}
/// Blocks the current thread until the token is made available.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
/// let u = p.unparker().clone();
///
/// // Make the token available.
/// u.unpark();
///
/// // Wakes up immediately and consumes the token.
/// p.park();
/// ```
pub fn park(&self) {
self.unparker.inner.park(None);
}
/// Blocks the current thread until the token is made available, but only for a limited time.
///
/// # Examples
///
/// ```
/// use std::time::Duration;
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
///
/// // Waits for the token to become available, but will not wait longer than 500 ms.
/// p.park_timeout(Duration::from_millis(500));
/// ```
pub fn park_timeout(&self, timeout: Duration) {
self.park_deadline(Instant::now() + timeout)
}
/// Blocks the current thread until the token is made available, or until a certain deadline.
///
/// # Examples
///
/// ```
/// use std::time::{Duration, Instant};
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
/// let deadline = Instant::now() + Duration::from_millis(500);
///
/// // Waits for the token to become available, but will not wait longer than 500 ms.
/// p.park_deadline(deadline);
/// ```
pub fn park_deadline(&self, deadline: Instant) {
self.unparker.inner.park(Some(deadline))
}
/// Returns a reference to an associated [`Unparker`].
///
/// The returned [`Unparker`] doesn't have to be used by reference - it can also be cloned.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
/// let u = p.unparker().clone();
///
/// // Make the token available.
/// u.unpark();
/// // Wakes up immediately and consumes the token.
/// p.park();
/// ```
///
/// [`park`]: Parker::park
/// [`park_timeout`]: Parker::park_timeout
pub fn unparker(&self) -> &Unparker {
&self.unparker
}
/// Converts a `Parker` into a raw pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
/// let raw = Parker::into_raw(p);
/// # let _ = unsafe { Parker::from_raw(raw) };
/// ```
pub fn into_raw(this: Parker) -> *const () {
Unparker::into_raw(this.unparker)
}
/// Converts a raw pointer into a `Parker`.
///
/// # Safety
///
/// This method is safe to use only with pointers returned by [`Parker::into_raw`].
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
/// let raw = Parker::into_raw(p);
/// let p = unsafe { Parker::from_raw(raw) };
/// ```
pub unsafe fn from_raw(ptr: *const ()) -> Parker {
Parker {
unparker: Unparker::from_raw(ptr),
_marker: PhantomData,
}
}
}
impl fmt::Debug for Parker {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("Parker { .. }")
}
}
/// Unparks a thread parked by the associated [`Parker`].
pub struct Unparker {
inner: Arc<Inner>,
}
unsafe impl Send for Unparker {}
unsafe impl Sync for Unparker {}
impl Unparker {
/// Atomically makes the token available if it is not already.
///
/// This method will wake up the thread blocked on [`park`] or [`park_timeout`], if there is
/// any.
///
/// # Examples
///
/// ```
/// use std::thread;
/// use std::time::Duration;
/// use crossbeam_utils::sync::Parker;
///
/// let p = Parker::new();
/// let u = p.unparker().clone();
///
/// thread::spawn(move || {
/// thread::sleep(Duration::from_millis(500));
/// u.unpark();
/// });
///
/// // Wakes up when `u.unpark()` provides the token.
/// p.park();
/// # std::thread::sleep(std::time::Duration::from_millis(500)); // wait for background threads closed: https://github.com/rust-lang/miri/issues/1371
/// ```
///
/// [`park`]: Parker::park
/// [`park_timeout`]: Parker::park_timeout
pub fn unpark(&self) {
self.inner.unpark()
}
/// Converts an `Unparker` into a raw pointer.
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::{Parker, Unparker};
///
/// let p = Parker::new();
/// let u = p.unparker().clone();
/// let raw = Unparker::into_raw(u);
/// # let _ = unsafe { Unparker::from_raw(raw) };
/// ```
pub fn into_raw(this: Unparker) -> *const () {
Arc::into_raw(this.inner).cast::<()>()
}
/// Converts a raw pointer into an `Unparker`.
///
/// # Safety
///
/// This method is safe to use only with pointers returned by [`Unparker::into_raw`].
///
/// # Examples
///
/// ```
/// use crossbeam_utils::sync::{Parker, Unparker};
///
/// let p = Parker::new();
/// let u = p.unparker().clone();
///
/// let raw = Unparker::into_raw(u);
/// let u = unsafe { Unparker::from_raw(raw) };
/// ```
pub unsafe fn from_raw(ptr: *const ()) -> Unparker {
Unparker {
inner: Arc::from_raw(ptr.cast::<Inner>()),
}
}
}
impl fmt::Debug for Unparker {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("Unparker { .. }")
}
}
impl Clone for Unparker {
fn clone(&self) -> Unparker {
Unparker {
inner: self.inner.clone(),
}
}
}
const EMPTY: usize = 0;
const PARKED: usize = 1;
const NOTIFIED: usize = 2;
struct Inner {
state: AtomicUsize,
lock: Mutex<()>,
cvar: Condvar,
}
impl Inner {
fn park(&self, deadline: Option<Instant>) {
// 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;
}
// If the timeout is zero, then there is no need to actually block.
if let Some(deadline) = deadline {
if deadline <= Instant::now() {
return;
}
}
// Otherwise we need to coordinate going to sleep.
let mut m = self.lock.lock().unwrap();
match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
// Consume this notification to avoid spurious wakeups in the next park.
Err(NOTIFIED) => {
// We must read `state` 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);
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
}
Err(n) => panic!("inconsistent park_timeout state: {}", n),
}
loop {
// Block the current thread on the conditional variable.
m = match deadline {
None => self.cvar.wait(m).unwrap(),
Some(deadline) => {
let now = Instant::now();
if now < deadline {
// We could check for a timeout here, in the return value of wait_timeout,
// but in the case that a timeout and an unpark arrive simultaneously, we
// prefer to report the former.
self.cvar.wait_timeout(m, deadline - now).unwrap().0
} else {
// We've timed out; swap out the state back to empty on our way out
match self.state.swap(EMPTY, SeqCst) {
NOTIFIED | PARKED => return,
n => panic!("inconsistent park_timeout state: {}", n),
};
}
}
};
if self
.state
.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
.is_ok()
{
// got a notification
return;
}
// Spurious wakeup, go back to sleep. Alternatively, if we timed out, it will be caught
// in the branch above, when we discover the deadline is in the past
}
}
pub(crate) 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 wakeup) 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.lock.lock().unwrap());
self.cvar.notify_one();
}
}