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//! Stack based on LL/SC atomics
pub use core::ptr::NonNull as Ptr;
use core::{cell::UnsafeCell, ptr};
#[cfg(cas_atomic_polyfill)]
use atomic_polyfill::{AtomicPtr, Ordering};
#[cfg(not(cas_atomic_polyfill))]
use core::sync::atomic::{AtomicPtr, Ordering};
/// Unfortunate implementation detail required to use the
/// [`Pool.grow_exact`](struct.Pool.html#method.grow_exact) method
pub struct Node<T> {
next: AtomicPtr<Node<T>>,
pub(crate) data: UnsafeCell<T>,
}
impl<T> Node<T> {
fn next(&self) -> &AtomicPtr<Node<T>> {
&self.next
}
}
pub struct Stack<T> {
head: AtomicPtr<Node<T>>,
}
impl<T> Stack<T> {
pub const fn new() -> Self {
Self {
head: AtomicPtr::new(ptr::null_mut()),
}
}
pub fn push(&self, new_head: Ptr<Node<T>>) {
// NOTE `Ordering`s come from crossbeam's (v0.6.0) `TreiberStack`
let mut head = self.head.load(Ordering::Relaxed);
loop {
unsafe { new_head.as_ref().next().store(head, Ordering::Relaxed) }
match self.head.compare_exchange_weak(
head,
new_head.as_ptr(),
Ordering::Release, // success
Ordering::Relaxed, // failure
) {
Ok(_) => return,
// interrupt occurred or other core made a successful STREX op on the head
Err(p) => head = p,
}
}
}
pub fn try_pop(&self) -> Option<Ptr<Node<T>>> {
// NOTE `Ordering`s come from crossbeam's (v0.6.0) `TreiberStack`
loop {
let head = self.head.load(Ordering::Acquire);
if let Some(nn_head) = Ptr::new(head) {
let next = unsafe { nn_head.as_ref().next().load(Ordering::Relaxed) };
match self.head.compare_exchange_weak(
head,
next,
Ordering::Release, // success
Ordering::Relaxed, // failure
) {
Ok(_) => break Some(nn_head),
// interrupt occurred or other core made a successful STREX op on the head
Err(_) => continue,
}
} else {
// stack is observed as empty
break None;
}
}
}
}