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//! The fast slots for the primary strategy.
//!
//! They are faster, but fallible (in case the slots run out or if there's a collision with a
//! writer thread, this gives up and falls back to secondary strategy).
//!
//! They are based on hazard pointer ideas. To acquire one, the pointer is loaded, stored in the
//! slot and the debt is confirmed by loading it again and checking it is the same.
//!
//! # Orderings
//!
//! We ensure just one thing here. Since we do both the acquisition of the slot and the exchange of
//! the pointer in the writer with SeqCst, we are guaranteed to either see the change in case it
//! hits somewhere in between the two reads of the pointer, or to have successfully acquired it
//! before the change and before any cleanup of the old pointer happened (in which case we know the
//! writer will see our debt).
use core::cell::Cell;
use core::slice::Iter;
use core::sync::atomic::Ordering::*;
use super::Debt;
const DEBT_SLOT_CNT: usize = 8;
/// Thread-local information for the [`Slots`]
#[derive(Default)]
pub(super) struct Local {
// The next slot in round-robin rotation. Heuristically tries to balance the load across them
// instead of having all of them stuffed towards the start of the array which gets
// unsuccessfully iterated through every time.
offset: Cell<usize>,
}
/// Bunch of fast debt slots.
#[derive(Default)]
pub(super) struct Slots([Debt; DEBT_SLOT_CNT]);
impl Slots {
/// Try to allocate one slot and get the pointer in it.
///
/// Fails if there are no free slots.
#[inline]
pub(super) fn get_debt(&self, ptr: usize, local: &Local) -> Option<&Debt> {
// Trick with offsets: we rotate through the slots (save the value from last time)
// so successive leases are likely to succeed on the first attempt (or soon after)
// instead of going through the list of already held ones.
let offset = local.offset.get();
let len = self.0.len();
for i in 0..len {
let i = (i + offset) % len;
// Note: the indexing check is almost certainly optimised out because the len
// is used above. And using .get_unchecked was actually *slower*.
let slot = &self.0[i];
if slot.0.load(Relaxed) == Debt::NONE {
// We are allowed to split into the check and acquiring the debt. That's because we
// are the only ones allowed to change NONE to something else. But we still need a
// read-write operation wit SeqCst on it :-(
let old = slot.0.swap(ptr, SeqCst);
debug_assert_eq!(Debt::NONE, old);
local.offset.set(i + 1);
return Some(&self.0[i]);
}
}
None
}
}
impl<'a> IntoIterator for &'a Slots {
type Item = &'a Debt;
type IntoIter = Iter<'a, Debt>;
fn into_iter(self) -> Self::IntoIter {
self.0.iter()
}
}