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#[cfg(feature = "alloc")]
use alloc::{boxed::Box, vec::Vec};
use core::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
use crossbeam_utils::Backoff;
use crate::buffer::{Buffer, BufferPtr};
// Utility MPMC concurrent stack for BufferPool to store freed batches of buffers.
pub struct FreeStack {
slots: Box<[AtomicPtr<Buffer>]>,
count: AtomicUsize,
}
impl FreeStack {
pub fn new(capacity: usize) -> Self {
Self {
slots: (0..capacity)
.map(|_| AtomicPtr::new(core::ptr::null_mut()))
.collect::<Vec<_>>()
.into_boxed_slice(),
count: AtomicUsize::new(0),
}
}
pub fn pop(&self) -> Option<BufferPtr> {
let backoff = Backoff::new();
let mut count = self.count.load(Ordering::Relaxed);
while count > 0 {
match self.count.compare_exchange(
count,
count.saturating_sub(1),
Ordering::Relaxed,
Ordering::Relaxed,
) {
// There are no available slots.
Ok(_) => {
// Successfully acquired a slot.
backoff.reset();
loop {
if let Some(taken) = BufferPtr::from_ptr(
self.slots[count - 1].swap(core::ptr::null_mut(), Ordering::AcqRel),
) {
return Some(taken);
}
backoff.snooze();
}
}
Err(0) => break,
Err(new_count) => {
count = new_count;
}
}
backoff.spin();
}
None
}
pub fn push_if(&self, buffer: BufferPtr, mut condition: impl FnMut(usize) -> bool) -> bool {
let backoff = Backoff::new();
let mut count = self.count.load(Ordering::Relaxed);
loop {
if !condition(count) {
return false;
}
match self.count.compare_exchange(
count,
count + 1,
Ordering::Relaxed,
Ordering::Relaxed,
) {
Ok(_) => {
// Successfully reserved a slot.
backoff.reset();
loop {
match self.slots[count].compare_exchange(
core::ptr::null_mut(),
buffer.as_ptr_mut(),
Ordering::AcqRel,
Ordering::Relaxed,
) {
Ok(_) => break,
Err(_) => {}
}
backoff.snooze();
}
return true;
}
Err(new_count) => {
count = new_count;
}
}
backoff.spin();
}
}
}