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use crate::shared::{SpinWait, Waiter};
use std::{
fmt,
marker::PhantomData,
ptr::NonNull,
sync::atomic::{fence, AtomicUsize, Ordering},
};
const UNINIT: usize = 0;
const CALLING: usize = 1;
const POISONED: usize = 2;
const COMPLETED: usize = 3;
/// A synchronization primitive which can be used to run a one-time
/// initialization. Useful for one-time initialization for globals, FFI or
/// related functionality.
///
/// # Differences from the standard library `Once`
///
/// - Only requires 1 byte of space, instead of 1 word.
/// - Not required to be `'static`.
/// - Relaxed memory barriers in the fast path, which can significantly improve
/// performance on some architectures.
/// - Efficient handling of micro-contention using adaptive spinning.
///
/// # Examples
///
/// ```
/// use usync::Once;
///
/// static START: Once = Once::new();
///
/// START.call_once(|| {
/// // run initialization here
/// });
/// ```
#[derive(Default)]
pub struct Once {
/// This atomic integer holds the current state of the Barrier instance.
/// The QUEUED bit switches between counting the barrier value and recording the waiters.
///
/// # State table:
///
/// state 0b11 | Remaining | Description
/// UNINIT | 0 | The once state is fresh and empty.
/// -----------+-----------+---------------------------------------------------------------------
/// CALLING | ?*Waiter | There is a thread calling a function on the Once state.
/// | | The Remaining bits point to the head of the waiting-thread queue
// | | if there is any.
/// -----------+-----------+--------------------------------------------------------------------
/// POISONED | 0 | The once was once calling and the function panicked, poisoning the state.
/// -----------+-----------+--------------------------------------------------------------------
/// COMPLETED | 0 | The once was once calling and the function completed, resolving the state.
/// -----------+-----------+--------------------------------------------------------------------
state: AtomicUsize,
_p: PhantomData<*const Waiter>,
}
unsafe impl Send for Once {}
unsafe impl Sync for Once {}
impl fmt::Debug for Once {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Once")
.field("state", &self.state())
.finish()
}
}
impl Once {
/// Creates a new `Once` value.
pub const fn new() -> Self {
Self {
state: AtomicUsize::new(UNINIT),
_p: PhantomData,
}
}
/// Returns the current state of this `Once`.
pub fn state(&self) -> OnceState {
let state = self.state.load(Ordering::Relaxed);
match state & !Waiter::MASK {
UNINIT => OnceState::New,
CALLING => OnceState::InProgress,
POISONED => OnceState::Poisoned,
COMPLETED => OnceState::Done,
_ => unreachable!("invalid state"),
}
}
/// Performs an initialization routine once and only once. The given closure
/// will be executed if this is the first time `call_once` has been called,
/// and otherwise the routine will *not* be invoked.
///
/// This method will block the calling thread if another initialization
/// routine is currently running.
///
/// When this function returns, it is guaranteed that some initialization
/// has run and completed (it may not be the closure specified). It is also
/// guaranteed that any memory writes performed by the executed closure can
/// be reliably observed by other threads at this point (there is a
/// happens-before relation between the closure and code executing after the
/// return).
///
/// # Examples
///
/// ```
/// use usync::Once;
///
/// static mut VAL: usize = 0;
/// static INIT: Once = Once::new();
///
/// // Accessing a `static mut` is unsafe much of the time, but if we do so
/// // in a synchronized fashion (e.g. write once or read all) then we're
/// // good to go!
/// //
/// // This function will only call `expensive_computation` once, and will
/// // otherwise always return the value returned from the first invocation.
/// fn get_cached_val() -> usize {
/// unsafe {
/// INIT.call_once(|| {
/// VAL = expensive_computation();
/// });
/// VAL
/// }
/// }
///
/// fn expensive_computation() -> usize {
/// // ...
/// # 2
/// }
/// ```
///
/// # Panics
///
/// The closure `f` will only be executed once if this is called
/// concurrently amongst many threads. If that closure panics, however, then
/// it will *poison* this `Once` instance, causing all future invocations of
/// `call_once` to also panic.
#[inline]
pub fn call_once<F>(&self, f: F)
where
F: FnOnce(),
{
// Fast path to check if the state was completed.
// Acquire barrier to ensure that Once function call and completion happens before we return.
let state = self.state.load(Ordering::Acquire);
if state == COMPLETED {
return;
}
self.call_once_slow(false, |_: OnceState| f());
}
/// Performs the same function as `call_once` except ignores poisoning.
///
/// If this `Once` has been poisoned (some initialization panicked) then
/// this function will continue to attempt to call initialization functions
/// until one of them doesn't panic.
///
/// The closure `f` is yielded a structure which can be used to query the
/// state of this `Once` (whether initialization has previously panicked or
/// not).
#[inline]
pub fn call_once_force<F>(&self, f: F)
where
F: FnOnce(OnceState),
{
// Fast path to check if the state was completed.
// Acquire barrier to ensure that Once function call and completion happens before we return.
let state = self.state.load(Ordering::Acquire);
if state == COMPLETED {
return;
}
self.call_once_slow(true, f);
}
#[cold]
fn call_once_slow<F>(&self, ignore_poison: bool, f: F)
where
F: FnOnce(OnceState),
{
Waiter::with(|waiter| {
let mut spin = SpinWait::default();
let mut state = self.state.load(Ordering::Relaxed);
loop {
// Once the state is completed, we can return.
// Acquire barrier to ensure the Once function call and completion happen before we return.
if state == COMPLETED {
fence(Ordering::Acquire);
return;
}
// Check for poision and panic if the caller can't ignore it.
// Acquire barrier to ensure the Once function call panic happened before we return.
if state == POISONED && !ignore_poison {
fence(Ordering::Acquire);
panic!("Once instance was previously poisoned");
}
// There is a thread in the middle of calling their function on the Once.
// Queue our waiter in order to wait for them to finish calling.
if state & 0b11 == CALLING {
// Try to spin a little bit in hopes that they finish the function call soon.
// Don't spin if there's already threads waiting as we should start waiting too.
let head = NonNull::new((state & Waiter::MASK) as *mut Waiter);
if head.is_none() && spin.try_yield_now() {
state = self.state.load(Ordering::Relaxed);
continue;
}
// Push our waiter to the queue in a stack-like manner.
waiter.next.set(head);
let waiter_ptr = &*waiter as *const Waiter as usize;
let new_state = waiter_ptr | CALLING;
// Release barrier to ensure our waiter's writes happen before the calling thread
// iterates the queue in order to wake us up.
if let Err(e) = self.state.compare_exchange_weak(
state,
new_state,
Ordering::Release,
Ordering::Relaxed,
) {
state = e;
continue;
}
// Sleep and check the Once state again.
assert!(waiter.parker.park(None));
state = self.state.load(Ordering::Relaxed);
continue;
}
match self.state.compare_exchange_weak(
state,
CALLING,
Ordering::Acquire,
Ordering::Relaxed,
) {
Ok(_) => return self.do_call(state, f),
Err(e) => state = e,
}
}
})
}
#[cold]
fn do_call<F>(&self, old_state: usize, f: F)
where
F: FnOnce(OnceState),
{
/// The state guard is used to ensure that waiting threads are woken up
/// regardless of it a panic occurs when calling f() or not.
struct StateGuard<'a> {
once: &'a Once,
reset_to: usize,
}
impl<'a> Drop for StateGuard<'a> {
fn drop(&mut self) {
// Complete the once state using the reset_to.
// AcqRel as Acquire to ensure writes to pushed waiters happen before we iterate and wake them below.
// AcqRel as Release to ensure our function call happens before the waiters return from call_once_*.
let state = self.once.state.swap(self.reset_to, Ordering::AcqRel);
assert_eq!(state & 0b11, CALLING);
let mut waiters = NonNull::new((state & Waiter::MASK) as *mut Waiter);
while let Some(waiter) = waiters {
unsafe {
waiters = waiter.as_ref().next.get();
waiter.as_ref().parker.unpark();
}
}
}
}
// Initialize the StateGuard to complete with POISONED state.
// If the function call below panics, it will poison the Once.
let mut state_guard = StateGuard {
once: self,
reset_to: POISONED,
};
f(match old_state {
UNINIT => OnceState::New,
POISONED => OnceState::Poisoned,
_ => unreachable!("invalid once state on invokation"),
});
// The function call completed safely,
// so resolve the Once with COMPLETED instead of POISONED.
state_guard.reset_to = COMPLETED;
std::mem::drop(state_guard);
}
}
/// Current state of a `Once`.
#[derive(Copy, Clone, Eq, PartialEq, Debug)]
pub enum OnceState {
/// A closure has not been executed yet
New,
/// A closure was executed but panicked.
Poisoned,
/// A thread is currently executing a closure.
InProgress,
/// A closure has completed successfully.
Done,
}
impl OnceState {
/// Returns whether the associated `Once` has been poisoned.
///
/// Once an initialization routine for a `Once` has panicked it will forever
/// indicate to future forced initialization routines that it is poisoned.
#[inline]
pub fn poisoned(self) -> bool {
self == Self::Poisoned
}
/// Returns whether the associated `Once` has successfully executed a
/// closure.
#[inline]
pub fn done(self) -> bool {
self == Self::Done
}
}
#[cfg(test)]
mod tests {
use crate::Once;
use std::{panic, sync::mpsc::channel, thread};
#[test]
fn smoke_once() {
static O: Once = Once::new();
let mut a = 0;
O.call_once(|| a += 1);
assert_eq!(a, 1);
O.call_once(|| a += 1);
assert_eq!(a, 1);
}
#[test]
fn stampede_once() {
static O: Once = Once::new();
static mut RUN: bool = false;
let (tx, rx) = channel();
for _ in 0..10 {
let tx = tx.clone();
thread::spawn(move || {
for _ in 0..4 {
thread::yield_now()
}
unsafe {
O.call_once(|| {
assert!(!RUN);
RUN = true;
});
assert!(RUN);
}
tx.send(()).unwrap();
});
}
unsafe {
O.call_once(|| {
assert!(!RUN);
RUN = true;
});
assert!(RUN);
}
for _ in 0..10 {
rx.recv().unwrap();
}
}
#[test]
fn poison_bad() {
static O: Once = Once::new();
// poison the once
let t = panic::catch_unwind(|| {
O.call_once(|| panic!());
});
assert!(t.is_err());
// poisoning propagates
let t = panic::catch_unwind(|| {
O.call_once(|| {});
});
assert!(t.is_err());
// we can subvert poisoning, however
let mut called = false;
O.call_once_force(|p| {
called = true;
assert!(p.poisoned())
});
assert!(called);
// once any success happens, we stop propagating the poison
O.call_once(|| {});
}
#[test]
fn wait_for_force_to_finish() {
static O: Once = Once::new();
// poison the once
let t = panic::catch_unwind(|| {
O.call_once(|| panic!());
});
assert!(t.is_err());
// make sure someone's waiting inside the once via a force
let (tx1, rx1) = channel();
let (tx2, rx2) = channel();
let t1 = thread::spawn(move || {
O.call_once_force(|p| {
assert!(p.poisoned());
tx1.send(()).unwrap();
rx2.recv().unwrap();
});
});
rx1.recv().unwrap();
// put another waiter on the once
let t2 = thread::spawn(|| {
let mut called = false;
O.call_once(|| {
called = true;
});
assert!(!called);
});
tx2.send(()).unwrap();
assert!(t1.join().is_ok());
assert!(t2.join().is_ok());
}
#[test]
fn test_once_debug() {
static O: Once = Once::new();
assert_eq!(format!("{:?}", O), "Once { state: New }");
}
}