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use alloc::boxed::Box;
use core::{
cell::UnsafeCell,
mem::MaybeUninit,
ptr,
sync::atomic::{
AtomicPtr, AtomicUsize,
Ordering::{Relaxed, SeqCst},
},
task::Waker,
};
/// Status of wake node
#[repr(usize)]
enum WakeState {
/// Waiting to be re-allocated
Garbage = 0,
/// Waiting for registration
Empty = 1,
/// Ready to be awoken (contains waker)
Ready = 2,
/// 1 task can register at a time (in the process of getting a waker)
Registering = 3,
/// Multiple tasks could be waking (in the process of waking, losing waker)
Waking = 4,
/// Registration is being canceled by waking
Canceling = 5,
/// Waiting to become garbage
Freeing = 6,
}
struct WakeNode {
/// Atomic `WakeState` for waker
state: AtomicUsize,
/// Waker and a fallback waker
waker: UnsafeCell<MaybeUninit<Waker>>,
}
impl WakeNode {
/// Try to allocate wake node
fn allocate(&self) -> Result<*const WakeNode, ()> {
self.state
.compare_exchange(
WakeState::Garbage as usize,
WakeState::Empty as usize,
SeqCst,
SeqCst,
)
.map_err(|_| ())
.map(|_| -> *const WakeNode { self })
}
/// Register a new waker
///
/// Slots can be Empty, Ready or Waking (If Waking, wakes immediately)
fn register(&self, waker: Waker) {
// Attempt to clear first slot and begin registering
let r = self
.state
.fetch_update(SeqCst, SeqCst, |state| match state {
// Switch to registering state
x if x == WakeState::Empty as usize => {
Some(WakeState::Registering as usize)
}
// Switch to registering state, dropping previous waker
x if x == WakeState::Ready as usize => {
Some(WakeState::Registering as usize)
}
// Contention with waking, re-wake immediately
x if x == WakeState::Waking as usize => None,
_ => unreachable!(),
});
// Set waker and mark ready
match r {
Ok(prev) => {
// Drop before overwriting
if prev == WakeState::Ready as usize {
unsafe { (*self.waker.get()).assume_init_drop() }
}
// Use first waker slot and set to ready for waking
unsafe { *self.waker.get() = MaybeUninit::new(waker) };
// Finish, checking if canceled
let r =
self.state.fetch_update(
SeqCst,
SeqCst,
|state| match state {
// Switch to registering state
x if x == WakeState::Registering as usize => {
Some(WakeState::Ready as usize)
}
// Switch to registering state
x if x == WakeState::Canceling as usize => None,
_ => unreachable!(),
},
);
if r.is_err() {
unsafe { (*self.waker.get()).assume_init_read().wake() }
self.state.store(WakeState::Empty as usize, SeqCst);
}
}
Err(_) => waker.wake(),
}
}
/// Try to wake this node
///
/// If already waking, won't wake again
fn wake(&self) -> Result<(), ()> {
let r = self
.state
.fetch_update(SeqCst, SeqCst, |state| match state {
// Ready to be awoken
x if x == WakeState::Ready as usize => {
Some(WakeState::Waking as usize)
}
// Currently registering, wake now
x if x == WakeState::Registering as usize => {
Some(WakeState::Canceling as usize)
}
// Not wakeable
_ => None,
})
.map_err(|_| ())?;
if r == WakeState::Registering as usize {
return Ok(());
}
// Take and wake the waker
unsafe { (*self.waker.get()).assume_init_read().wake() };
// Update state
self.state
.fetch_update(SeqCst, SeqCst, |state| match state {
x if x == WakeState::Waking as usize => {
Some(WakeState::Empty as usize)
}
x if x == WakeState::Freeing as usize => {
Some(WakeState::Garbage as usize)
}
_ => unreachable!(),
})
.map(|_| ())
.map_err(|_| ())
}
/// Free this wake node (must be done on registration task)
fn free(&self) {
match self.state.swap(WakeState::Freeing as usize, SeqCst) {
x if x == WakeState::Empty as usize => {}
x if x == WakeState::Ready as usize => {
unsafe { (*self.waker.get()).assume_init_drop() };
}
x if x == WakeState::Waking as usize => return,
_ => unreachable!(),
}
self.state.store(WakeState::Garbage as usize, SeqCst);
}
}
/// Handle to an optional waker node
pub(crate) struct WakeHandle(*const WakeNode);
unsafe impl Send for WakeHandle {}
unsafe impl Sync for WakeHandle {}
impl WakeHandle {
/// Create a new mock handle
pub(crate) fn new() -> Self {
Self(ptr::null())
}
/// Register a waker
pub(crate) fn register(&mut self, wl: &WakeList, waker: Waker) {
// Allocate a waker if needed
if self.0.is_null() {
self.0 = wl.allocate();
}
// Register the waker
unsafe { (*self.0).register(waker) }
}
}
impl Drop for WakeHandle {
fn drop(&mut self) {
// Unregister the waker if set
if !self.0.is_null() {
unsafe { (*self.0).free() }
}
}
}
struct Node<T> {
next: AtomicPtr<Node<T>>,
data: T,
}
/// A `WakeList` stores an append-only atomic linked list of wakers
pub(crate) struct WakeList {
// Root node of list of wakers
root: AtomicPtr<Node<WakeNode>>,
// Next one to try waking ("fairness" mechanism)
next: AtomicPtr<Node<WakeNode>>,
}
impl Drop for WakeList {
fn drop(&mut self) {
let mut tmp = self.root.load(Relaxed);
while !tmp.is_null() {
let node = unsafe { Box::from_raw(tmp) };
tmp = node.next.load(Relaxed);
}
}
}
impl WakeList {
/// Create a new empty wake list
pub(crate) const fn new() -> Self {
Self {
root: AtomicPtr::new(ptr::null_mut()),
next: AtomicPtr::new(ptr::null_mut()),
}
}
/// Attempt to wake one waker.
///
/// If no wakers are registered, doesn't do anything.
pub(crate) fn wake_one(&self) {
// Start from next pointer into list
let next = self.next.load(SeqCst);
let mut tmp = next;
while !tmp.is_null() {
let next = unsafe { (*tmp).next.load(Relaxed) };
if unsafe { (*tmp).data.wake().is_ok() } {
self.next.store(next, SeqCst);
return;
}
tmp = next;
}
// Start back at beginning of list
tmp = self.root.load(SeqCst);
while tmp != next {
let next = unsafe { (*tmp).next.load(Relaxed) };
if unsafe { (*tmp).data.wake().is_ok() } {
self.next.store(next, SeqCst);
return;
}
tmp = next;
}
}
/// Allocate a new `WakeNode`
fn allocate(&self) -> *const WakeNode {
// Go through list to see if unused existing allocation to use
let mut tmp = self.root.load(SeqCst);
while !tmp.is_null() {
if let Ok(wn) = unsafe { (*tmp).data.allocate() } {
return wn;
}
tmp = unsafe { (*tmp).next.load(Relaxed) };
}
// Push to front
let data = WakeNode {
state: AtomicUsize::new(WakeState::Empty as usize),
waker: UnsafeCell::new(MaybeUninit::uninit()),
};
let mut root = self.root.load(SeqCst);
let next = AtomicPtr::new(self.root.load(SeqCst));
let node = Box::into_raw(Box::new(Node { next, data }));
while let Err(r) =
self.root.compare_exchange(root, node, SeqCst, Relaxed)
{
root = r;
unsafe { (*node).next = AtomicPtr::new(root) };
}
unsafe { &(*node).data }
}
}