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//! Implements thread wait and notify primitives with `std::sync` primitives.
//!
//! This is a simplified version of the `parking_lot_core` crate.
//!
//! There are two main operations that can be performed:
//!
//! - *Parking* refers to suspending the thread while simultaneously enqueuing it
//! on a queue keyed by some address.
//! - *Unparking* refers to dequeuing a thread from a queue keyed by some address
//! and resuming it.
#![deny(clippy::all)]
#![deny(clippy::pedantic)]
#![deny(missing_docs)]
#![deny(unsafe_code)]
use crate::WaitResult;
use std::collections::BTreeMap;
use std::sync::{Arc, Condvar, Mutex};
use std::time::Instant;
#[derive(Default, Debug)]
struct Spot {
/// The number of threads parked on this spot.
num_parked: u32,
/// The number of threads that have been unparked but not yet woken up.
/// This is used to avoid spurious wakeups.
to_unpark: u32,
/// The [`Condvar`] used to notify parked threads.
cvar: Arc<Condvar>,
}
/// The thread global `ParkingSpot`.
#[derive(Default, Debug)]
pub struct ParkingSpot {
inner: Mutex<BTreeMap<u64, Spot>>,
}
impl ParkingSpot {
/// Park the current thread until it is unparked or a timeout is reached.
///
/// The `key` is used to identify the parking spot. If another thread calls
/// `unpark_all` or `unpark` with the same key, the current thread will be unparked.
///
/// The `validate` callback is called before parking.
/// If it returns `false`, the thread is not parked and `WaitResult::Mismatch` is returned.
///
/// The `timeout` argument specifies the maximum amount of time the thread will be parked.
pub fn park(
&self,
key: u64,
validate: impl FnOnce() -> bool,
timeout: impl Into<Option<Instant>>,
) -> WaitResult {
self.park_inner(key, validate, timeout.into())
}
fn park_inner(
&self,
key: u64,
validate: impl FnOnce() -> bool,
timeout: Option<Instant>,
) -> WaitResult {
let mut inner = self
.inner
.lock()
.expect("failed to lock inner parking table");
// check validation with lock held
if !validate() {
return WaitResult::Mismatch;
}
// clone the condvar, so we can move the lock
let cvar = {
let spot = inner.entry(key).or_insert_with(Spot::default);
spot.num_parked = spot
.num_parked
.checked_add(1)
.expect("parking spot number overflow");
spot.cvar.clone()
};
loop {
let timed_out = if let Some(timeout) = timeout {
let now = Instant::now();
if now >= timeout {
true
} else {
let dur = timeout - now;
let (lock, result) = cvar
.wait_timeout(inner, dur)
.expect("failed to wait for condition");
inner = lock;
result.timed_out()
}
} else {
inner = cvar.wait(inner).expect("failed to wait for condition");
false
};
let spot = inner.get_mut(&key).expect("failed to get spot");
if timed_out {
// If waiting on the cvar timed out then due to how system cvars
// are implemented we may need to continue to sleep longer. If
// the deadline has not been reached then turn the crank again
// and go back to sleep.
if Instant::now() < timeout.unwrap() {
continue;
}
// Opportunistically consume `to_unpark` signals even on
// timeout. From the perspective of `unpark` this "agent" raced
// between its own timeout and receiving the unpark signal, but
// from unpark's perspective it's definitely going to wake up N
// agents as returned from the `unpark` return value.
//
// Note that this may actually prevent other threads from
// getting unparked. For example:
//
// * Thread A parks with a timeout
// * Thread B parks with no timeout
// * Thread C decides to unpark 1 thread
// * Thread A's cvar wakes up due to a timeout, blocks on the
// lock
// * Thread C finishes unpark and signals the cvar once
// * Thread B wakes up
// * Thread A and B contend for the lock and A wins
// * A consumes the "to_unpark" value
// * B goes back to sleep since `to_unpark == 0`, thinking that
// a spurious wakeup happened.
//
// It's believed that this is ok, however, since from C's
// perspective one agent was still woken up and is allowed to
// continue, notably A in this case. C doesn't know that A raced
// with B and "stole" its wakeup signal.
if spot.to_unpark > 0 {
spot.to_unpark -= 1;
}
} else {
if spot.to_unpark == 0 {
// If no timeout happen but nothing has unparked this spot (as
// signaled through `to_unpark`) then this is indicative of a
// spurious wakeup. In this situation turn the crank again and
// go back to sleep as this interface doesn't allow for spurious
// wakeups.
continue;
}
// No timeout happened, and some other thread registered to
// unpark this thread, so consume one unpark notification.
spot.to_unpark -= 1;
}
spot.num_parked = spot
.num_parked
.checked_sub(1)
.expect("corrupted parking spot state");
if spot.num_parked == 0 {
assert_eq!(spot.to_unpark, 0);
inner
.remove(&key)
.expect("failed to remove spot from inner parking table");
}
if timed_out {
return WaitResult::TimedOut;
}
return WaitResult::Ok;
}
}
/// Unpark at most `n` threads that are parked with the given key.
///
/// Returns the number of threads that were actually unparked.
pub fn unpark(&self, key: u64, n: u32) -> u32 {
if n == 0 {
return 0;
}
let mut num_unpark = 0;
self.with_lot(key, |spot| {
num_unpark = n.min(spot.num_parked - spot.to_unpark);
spot.to_unpark += num_unpark;
if n >= num_unpark {
spot.cvar.notify_all();
} else {
for _ in 0..num_unpark {
spot.cvar.notify_one();
}
}
});
num_unpark
}
fn with_lot<F: FnMut(&mut Spot)>(&self, key: u64, mut f: F) {
let mut inner = self
.inner
.lock()
.expect("failed to lock inner parking table");
if let Some(spot) = inner.get_mut(&key) {
f(spot);
}
}
}
#[cfg(test)]
mod tests {
use super::ParkingSpot;
use std::ptr::addr_of;
use std::sync::atomic::{AtomicU64, Ordering};
use std::thread;
use std::time::{Duration, Instant};
#[test]
fn atomic_wait_notify() {
let parking_spot = &ParkingSpot::default();
let atomic = &AtomicU64::new(0);
thread::scope(|s| {
let atomic_key = addr_of!(atomic) as u64;
let thread1 = s.spawn(move || {
atomic.store(1, Ordering::SeqCst);
parking_spot.unpark(atomic_key, u32::MAX);
parking_spot.park(atomic_key, || atomic.load(Ordering::SeqCst) == 1, None);
});
let thread2 = s.spawn(move || {
while atomic.load(Ordering::SeqCst) != 1 {
parking_spot.park(atomic_key, || atomic.load(Ordering::SeqCst) != 1, None);
}
atomic.store(2, Ordering::SeqCst);
parking_spot.unpark(atomic_key, u32::MAX);
parking_spot.park(atomic_key, || atomic.load(Ordering::SeqCst) == 2, None);
});
let thread3 = s.spawn(move || {
while atomic.load(Ordering::SeqCst) != 2 {
parking_spot.park(atomic_key, || atomic.load(Ordering::SeqCst) != 2, None);
}
atomic.store(3, Ordering::SeqCst);
parking_spot.unpark(atomic_key, u32::MAX);
parking_spot.park(atomic_key, || atomic.load(Ordering::SeqCst) == 3, None);
});
while atomic.load(Ordering::SeqCst) != 3 {
parking_spot.park(atomic_key, || atomic.load(Ordering::SeqCst) != 3, None);
}
atomic.store(4, Ordering::SeqCst);
parking_spot.unpark(atomic_key, u32::MAX);
thread1.join().unwrap();
thread2.join().unwrap();
thread3.join().unwrap();
});
}
mod parking_lot {
// This is a modified version of the parking_lot_core tests,
// which are licensed under the MIT and Apache 2.0 licenses.
use super::*;
use std::sync::atomic::{AtomicIsize, AtomicU32};
use std::sync::Arc;
use std::time::Duration;
macro_rules! test {
( $( $name:ident(
repeats: $repeats:expr,
latches: $latches:expr,
delay: $delay:expr,
threads: $threads:expr,
single_unparks: $single_unparks:expr);
)* ) => {
$(
#[test]
fn $name() {
if std::env::var("WASMTIME_TEST_NO_HOG_MEMORY").is_ok() {
return;
}
let delay = Duration::from_micros($delay);
for _ in 0..$repeats {
run_parking_test($latches, delay, $threads, $single_unparks);
}
})*
};
}
test! {
unpark_all_one_fast(
repeats: 10000, latches: 1, delay: 0, threads: 1, single_unparks: 0
);
unpark_all_hundred_fast(
repeats: 100, latches: 1, delay: 0, threads: 100, single_unparks: 0
);
unpark_one_one_fast(
repeats: 1000, latches: 1, delay: 0, threads: 1, single_unparks: 1
);
unpark_one_hundred_fast(
repeats: 20, latches: 1, delay: 0, threads: 100, single_unparks: 100
);
unpark_one_fifty_then_fifty_all_fast(
repeats: 50, latches: 1, delay: 0, threads: 100, single_unparks: 50
);
unpark_all_one(
repeats: 100, latches: 1, delay: 10000, threads: 1, single_unparks: 0
);
unpark_all_hundred(
repeats: 100, latches: 1, delay: 10000, threads: 100, single_unparks: 0
);
unpark_one_one(
repeats: 10, latches: 1, delay: 10000, threads: 1, single_unparks: 1
);
unpark_one_fifty(
repeats: 1, latches: 1, delay: 10000, threads: 50, single_unparks: 50
);
unpark_one_fifty_then_fifty_all(
repeats: 2, latches: 1, delay: 10000, threads: 100, single_unparks: 50
);
hundred_unpark_all_one_fast(
repeats: 100, latches: 100, delay: 0, threads: 1, single_unparks: 0
);
hundred_unpark_all_one(
repeats: 1, latches: 100, delay: 10000, threads: 1, single_unparks: 0
);
}
fn run_parking_test(
num_latches: usize,
delay: Duration,
num_threads: u32,
num_single_unparks: u32,
) {
let spot = ParkingSpot::default();
thread::scope(|s| {
let mut tests = Vec::with_capacity(num_latches);
for _ in 0..num_latches {
let test = Arc::new(SingleLatchTest::new(num_threads, &spot));
let mut threads = Vec::with_capacity(num_threads as _);
for _ in 0..num_threads {
let test = test.clone();
threads.push(s.spawn(move || test.run()));
}
tests.push((test, threads));
}
for unpark_index in 0..num_single_unparks {
thread::sleep(delay);
for (test, _) in &tests {
test.unpark_one(unpark_index);
}
}
for (test, threads) in tests {
test.finish(num_single_unparks);
for thread in threads {
thread.join().expect("Test thread panic");
}
}
});
}
struct SingleLatchTest<'a> {
semaphore: AtomicIsize,
num_awake: AtomicU32,
/// Total number of threads participating in this test.
num_threads: u32,
spot: &'a ParkingSpot,
}
impl<'a> SingleLatchTest<'a> {
pub fn new(num_threads: u32, spot: &'a ParkingSpot) -> Self {
Self {
// This implements a fair (FIFO) semaphore, and it starts out unavailable.
semaphore: AtomicIsize::new(0),
num_awake: AtomicU32::new(0),
num_threads,
spot,
}
}
pub fn run(&self) {
// Get one slot from the semaphore
self.down();
self.num_awake.fetch_add(1, Ordering::SeqCst);
}
pub fn unpark_one(&self, _single_unpark_index: u32) {
let num_awake_before_up = self.num_awake.load(Ordering::SeqCst);
self.up();
// Wait for a parked thread to wake up and update num_awake + last_awoken.
while self.num_awake.load(Ordering::SeqCst) != num_awake_before_up + 1 {
thread::yield_now();
}
}
pub fn finish(&self, num_single_unparks: u32) {
// The amount of threads not unparked via unpark_one
let mut num_threads_left =
self.num_threads.checked_sub(num_single_unparks).unwrap();
// Wake remaining threads up with unpark_all. Has to be in a loop, because there might
// still be threads that has not yet parked.
while num_threads_left > 0 {
let mut num_waiting_on_address = 0;
self.spot.with_lot(self.semaphore_addr(), |thread_data| {
num_waiting_on_address = thread_data.num_parked;
});
assert!(num_waiting_on_address <= num_threads_left);
let num_awake_before_unpark = self.num_awake.load(Ordering::SeqCst);
let num_unparked = self.spot.unpark(self.semaphore_addr(), u32::MAX);
assert!(num_unparked >= num_waiting_on_address);
assert!(num_unparked <= num_threads_left);
// Wait for all unparked threads to wake up and update num_awake + last_awoken.
while self.num_awake.load(Ordering::SeqCst)
!= num_awake_before_unpark + num_unparked
{
thread::yield_now();
}
num_threads_left = num_threads_left.checked_sub(num_unparked).unwrap();
}
// By now, all threads should have been woken up
assert_eq!(self.num_awake.load(Ordering::SeqCst), self.num_threads);
// Make sure no thread is parked on our semaphore address
let mut num_waiting_on_address = 0;
self.spot.with_lot(self.semaphore_addr(), |thread_data| {
num_waiting_on_address = thread_data.num_parked;
});
assert_eq!(num_waiting_on_address, 0);
}
pub fn down(&self) {
let old_semaphore_value = self.semaphore.fetch_sub(1, Ordering::SeqCst);
if old_semaphore_value > 0 {
// We acquired the semaphore. Done.
return;
}
// We need to wait.
let validate = || true;
self.spot.park(self.semaphore_addr(), validate, None);
}
pub fn up(&self) {
let old_semaphore_value = self.semaphore.fetch_add(1, Ordering::SeqCst);
// Check if anyone was waiting on the semaphore. If they were, then pass ownership to them.
if old_semaphore_value < 0 {
// We need to continue until we have actually unparked someone. It might be that
// the thread we want to pass ownership to has decremented the semaphore counter,
// but not yet parked.
loop {
match self.spot.unpark(self.semaphore_addr(), 1) {
1 => break,
0 => (),
i => panic!("Should not wake up {i} threads"),
}
}
}
}
fn semaphore_addr(&self) -> u64 {
addr_of!(self.semaphore) as _
}
}
}
#[test]
fn wait_with_timeout() {
let parking_spot = &ParkingSpot::default();
let atomic = &AtomicU64::new(0);
thread::scope(|s| {
let atomic_key = addr_of!(atomic) as u64;
const N: u64 = 5;
const M: u64 = 1000;
let thread = s.spawn(move || {
while atomic.load(Ordering::SeqCst) != N * M {
let timeout = Instant::now() + Duration::from_millis(1);
parking_spot.park(
atomic_key,
|| atomic.load(Ordering::SeqCst) != N * M,
Some(timeout),
);
}
});
let mut threads = vec![thread];
for _ in 0..N {
threads.push(s.spawn(move || {
for _ in 0..M {
atomic.fetch_add(1, Ordering::SeqCst);
parking_spot.unpark(atomic_key, 1);
}
}));
}
for thread in threads {
thread.join().unwrap();
}
});
}
}