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use crate::loom::sync::atomic::AtomicPtr;
use crate::runtime::task::{Header, Task};
use std::marker::PhantomData;
use std::ptr::{self, NonNull};
use std::sync::atomic::Ordering::{Acquire, Relaxed, Release};
/// Concurrent stack of tasks, used to pass ownership of a task from one worker
/// to another.
pub(crate) struct TransferStack<T: 'static> {
head: AtomicPtr<Header>,
_p: PhantomData<T>,
}
impl<T: 'static> TransferStack<T> {
pub(crate) fn new() -> TransferStack<T> {
TransferStack {
head: AtomicPtr::new(ptr::null_mut()),
_p: PhantomData,
}
}
pub(crate) fn push(&self, task: Task<T>) {
let task = task.into_raw();
// We don't care about any memory associated w/ setting the `head`
// field, just the current value.
//
// The compare-exchange creates a release sequence.
let mut curr = self.head.load(Relaxed);
loop {
unsafe {
task.as_ref()
.stack_next
.with_mut(|ptr| *ptr = NonNull::new(curr))
};
let res = self
.head
.compare_exchange(curr, task.as_ptr() as *mut _, Release, Relaxed);
match res {
Ok(_) => return,
Err(actual) => {
curr = actual;
}
}
}
}
pub(crate) fn drain(&self) -> impl Iterator<Item = Task<T>> {
struct Iter<T: 'static>(Option<NonNull<Header>>, PhantomData<T>);
impl<T: 'static> Iterator for Iter<T> {
type Item = Task<T>;
fn next(&mut self) -> Option<Task<T>> {
let task = self.0?;
// Move the cursor forward
self.0 = unsafe { task.as_ref().stack_next.with(|ptr| *ptr) };
// Return the task
unsafe { Some(Task::from_raw(task)) }
}
}
impl<T: 'static> Drop for Iter<T> {
fn drop(&mut self) {
use std::process;
if self.0.is_some() {
// we have bugs
process::abort();
}
}
}
let ptr = self.head.swap(ptr::null_mut(), Acquire);
Iter(NonNull::new(ptr), PhantomData)
}
}
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