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//! Dynamic memory management for lock-free data structures.
//!
//! This library implements the [_hazard pointer memory reclamation mechanism_][hazptr],
//! specifically as proposed for the [C++ Concurrency Technical Specification][cts]. It is adapted
//! from the [implementation][folly-hazptr] found in Facebook's [Folly library][folly]. The initial
//! phases of implementation were all [live streamed].
//!
//! At a high level, hazard pointers provide a mechanism that allows readers of shared pointers to
//! prevent concurrent reclamation of the pointed-to objects by concurrent writers for as long as
//! the read operation is ongoing. When a writer removes an object from a data structure, it
//! instructs the hazard pointer library that said object is no longer reachable (that it is
//! _retired_), and that the library should eventually drop said object (_reclaim_ it) once it is
//! safe to do so. Readers, meanwhile, inform the library any time they wish to read through a
//! pointer shared with writers. Internally, the library notes down the address that was read in
//! such a way that it can ensure that if the pointed-to object is retired while the reader still
//! has access to it, it is not reclaimed. Only once the reader no longer has access to the read
//! pointer does the library allow the object to be reclaimed.
//!
//! TODO: Can also help with the ABA problem (ensure object isn't reused until there are no
//! pointers to it, so cannot "see" A again until there are no As left).
//!
//! For an overview of concurrent garbage collection with hazard pointers, see "_[Fearless
//! concurrency with hazard pointers]_". Aaron Turon post on "_[Lock-freedom without garbage
//! collection]_" which discusses the alternate approach of using epoch-based reclamation (see
//! below) is also a good reference.
//!
//! # High-level API structure
//!
//! TODO: Ref section 3 of [the proposal][cts] and [folly's docs][folly-hazptr].
//!
//! # Hazard pointers vs. other deferred reclamation mechanisms
//!
//! TODO: Ref sections 3.4 and 4 of [the proposal][cts] and [section from folly's
//! docs](https://github.com/facebook/folly/blob/594b7e770176003d0f6b4cf725dd02a09cba533c/folly/synchronization/Hazptr.h#L139).
//!
//! Note esp. [memory usage](https://github.com/facebook/folly/blob/594b7e770176003d0f6b4cf725dd02a09cba533c/folly/synchronization/Hazptr.h#L120).
//!
//! # Examples
//!
//! TODO: Ref section 5 of [the proposal][cts] and [example from folly's
//! docs](https://github.com/facebook/folly/blob/594b7e770176003d0f6b4cf725dd02a09cba533c/folly/synchronization/Hazptr.h#L76).
//!
//! ```
//! use haphazard::{AtomicPtr, Domain, HazardPointer};
//!
//! // First, create something that's intended to be concurrently accessed.
//! let x = AtomicPtr::from(Box::new(42));
//!
//! // All reads must happen through a hazard pointer, so make one of those:
//! let mut h = HazardPointer::new();
//!
//! // We can now use the hazard pointer to read from the pointer without
//! // worrying about it being deallocated while we read.
//! let my_x = x.safe_load(&mut h).expect("not null");
//! assert_eq!(*my_x, 42);
//!
//! // We can willingly give up the guard to allow writers to reclaim the Box.
//! h.reset_protection();
//! // Doing so invalidates the reference we got from .load:
//! // let _ = *my_x; // won't compile
//!
//! // Hazard pointers can be re-used across multiple reads.
//! let my_x = x.safe_load(&mut h).expect("not null");
//! assert_eq!(*my_x, 42);
//!
//! // Dropping the hazard pointer releases our guard on the Box.
//! drop(h);
//! // And it also invalidates the reference we got from .load:
//! // let _ = *my_x; // won't compile
//!
//! // Multiple readers can access a value at once:
//!
//! let mut h = HazardPointer::new();
//! let my_x = x.safe_load(&mut h).expect("not null");
//!
//! let mut h_tmp = HazardPointer::new();
//! let _ = x.safe_load(&mut h_tmp).expect("not null");
//! drop(h_tmp);
//!
//! // Writers can replace the value, but doing so won't reclaim the old Box.
//! let old = x.swap(Box::new(9001)).expect("not null");
//!
//! // New readers will see the new value:
//! let mut h2 = HazardPointer::new();
//! let my_x2 = x.safe_load(&mut h2).expect("not null");
//! assert_eq!(*my_x2, 9001);
//!
//! // And old readers can still access the old value:
//! assert_eq!(*my_x, 42);
//!
//! // The writer can retire the value old readers are seeing.
//! //
//! // Safety: this value has not been retired before.
//! unsafe { old.retire() };
//!
//! // Reads will continue to work fine, as they're guarded by the hazard.
//! assert_eq!(*my_x, 42);
//!
//! // Even if the writer actively tries to reclaim retired objects, the hazard makes readers safe.
//! let n = Domain::global().eager_reclaim();
//! assert_eq!(n, 0);
//! assert_eq!(*my_x, 42);
//!
//! // Only once the last hazard guarding the old value goes away will the value be reclaimed.
//! drop(h);
//! let n = Domain::global().eager_reclaim();
//! assert_eq!(n, 1);
//! ```
//!
//! # Differences from the specification
//!
//! # Differences from the folly
//!
//! TODO: Note differences from spec and from folly. Among other things, see [this note from
//! folly](https://github.com/facebook/folly/blob/594b7e770176003d0f6b4cf725dd02a09cba533c/folly/synchronization/Hazptr.h#L193).
//!
//!
//! [hazptr]: https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.395.378&rep=rep1&type=pdf
//! [cts]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1121r3.pdf
//! [folly-hazptr]: https://github.com/facebook/folly/blob/main/folly/synchronization/Hazptr.h
//! [folly]: https://github.com/facebook/folly
//! [live streamed]: https://www.youtube.com/watch?v=fvcbyCYdR10&list=PLqbS7AVVErFgO7RUIC6lhd0UekFMbjJzb
//! [Fearless concurrency with hazard pointers]: https://web.archive.org/web/20210306120313/https://ticki.github.io/blog/fearless-concurrency-with-hazard-pointers/
//! [Lock-freedom without garbage collection]: https://aturon.github.io/blog/2015/08/27/epoch/
// TODO: Incorporate doc strings around expectations from section 6 of the hazptr TS2 proposal.
#![deny(unsafe_op_in_unsafe_fn)]
#![warn(missing_docs)]
#![warn(rustdoc::broken_intra_doc_links, rust_2018_idioms)]
#![allow(dead_code)]
#![cfg_attr(not(feature = "std"), no_std)]
extern crate alloc;
mod domain;
mod hazard;
mod pointer;
mod record;
mod sync;
fn asymmetric_light_barrier() {
// TODO: if cfg!(linux) {
// https://github.com/facebook/folly/blob/bd600cd4e88f664f285489c76b6ad835d8367cd2/folly/portability/Asm.h#L28
crate::sync::atomic::fence(core::sync::atomic::Ordering::SeqCst);
}
enum HeavyBarrierKind {
Normal,
Expedited,
}
fn asymmetric_heavy_barrier(_: HeavyBarrierKind) {
// TODO: if cfg!(linux) {
// https://github.com/facebook/folly/blob/bd600cd4e88f664f285489c76b6ad835d8367cd2/folly/synchronization/AsymmetricMemoryBarrier.cpp#L84
crate::sync::atomic::fence(core::sync::atomic::Ordering::SeqCst);
}
/// Raw building blocks for managing hazard pointers.
pub mod raw {
pub use crate::domain::Domain;
/// Well-known domain families.
pub mod families {
pub use crate::domain::Global;
}
pub use crate::hazard::HazardPointer;
pub use crate::pointer::{Pointer, Reclaim};
}
use core::marker::PhantomData;
use core::ops::Deref;
use core::ops::DerefMut;
use core::ptr::NonNull;
use core::sync::atomic::Ordering;
pub use domain::Domain;
pub use domain::Global;
pub use hazard::{HazardPointer, HazardPointerArray};
/// A managed pointer type which can be safely shared between threads.
///
/// Unlike [`std::sync::AtomicPtr`](core::sync::AtomicPtr), `haphazard::AtomicPtr` can safely load
/// `&T` directly, and ensure that the referenced `T` is not deallocated until the `&T` is dropped,
/// even in the presence of concurrent writers. Also unlike
/// [`std::sync::AtomicPtr`](core::sync::AtomicPtr), **all loads and stores on this type use
/// `Acquire` and `Release` semantics**.
///
/// To construct one, use `AtomicPtr::from`:
///
/// ```rust
/// # use haphazard::AtomicPtr;
/// let _: AtomicPtr<usize> = AtomicPtr::from(Box::new(42));
/// ```
///
/// Note the explicit use of `AtomicPtr<usize>`, which is needed to get the default values for the
/// generic arguments `F` and `P`, the domain family and pointer types of the stored values.
/// Families are discussed in the documentation for [`Domain`]. The pointer type `P`, which must
/// implement [`raw::Pointer`], is the type originaly used to produce the stored pointer. This is
/// used to ensure that when writers drop a value, it is dropped using the appropriate `Drop`
/// implementation.
///
/// This type has the same in-memory representation as a
/// [`std::sync::AtomicPtr`](core::sync::AtomicPtr).
///
/// **Note:** This type is only available on platforms that support atomic loads and stores of
/// pointers. Its size depends on the target pointer’s size.
///
// The unsafe constract enforced throughout this crate is that a given `AtomicPtr<T, F, P>` only
// ever holds the address of a valid `T` allocated through `P` from a domain with family `F`, or
// `null`. This generally means that there are a fair few safety constraints on _writes_ to an
// `AtomicPtr`, but very few safety constraints on _reads_. This is intentional, with the
// assumption being that most consumers will read in more places than they write.
//
// Also, when working with this code, keep in mind that `AtomicPtr<T>` does not _own_ its `T`. It
// is entirely possible for an application to have multiple `AtomicPtr<T>` that all point to the
// _same_ `T`. This is why most of the safety docs refer to "load from any AtomicPtr".
//
// TODO:
// - copy_and_move test.
// - requires double-retire protection?
#[repr(transparent)]
pub struct AtomicPtr<T, F = domain::Global, P = alloc::boxed::Box<T>>(
crate::sync::atomic::AtomicPtr<T>,
PhantomData<(F, *mut P)>,
);
impl<T, F, P> From<P> for AtomicPtr<T, F, P>
where
P: raw::Pointer<T>,
{
fn from(p: P) -> Self {
Self(
crate::sync::atomic::AtomicPtr::new(p.into_raw()),
PhantomData,
)
}
}
impl<T, F, P> AtomicPtr<T, F, P> {
/// Directly construct an `AtomicPtr` from a raw pointer.
///
/// # Safety
///
/// 1. `p` must reference a valid `T`, or be null.
/// 2. `p` must have been allocated using the pointer type `P`.
/// 3. `*p` must only be dropped through [`Domain::retire_ptr`].
pub unsafe fn new(p: *mut T) -> Self {
Self(crate::sync::atomic::AtomicPtr::new(p), PhantomData)
}
/// Directly access the "real" underlying `AtomicPtr`.
///
/// # Safety
///
/// If the stored pointer is modified, the new value must conform to the same safety
/// requirements as the argument to [`AtomicPtr::new`].
pub unsafe fn as_std(&self) -> &crate::sync::atomic::AtomicPtr<T> {
&self.0
}
/// Directly access the "real" underlying `AtomicPtr` mutably.
///
/// # Safety
///
/// If the stored pointer is modified, the new value must conform to the same safety
/// requirements as the argument to [`AtomicPtr::new`].
pub unsafe fn as_std_mut(&mut self) -> &mut crate::sync::atomic::AtomicPtr<T> {
&mut self.0
}
}
/// A `*mut T` that was previously stored in an [`AtomicPtr`].
///
/// This type exists primarily to capture the family and pointer type of the [`AtomicPtr`] the
/// value was previously stored in, so that callers don't need to provide `F` and `P` to
/// [`Replaced::retire`] and [`Replaced::retire_in`].
///
/// This type has the same in-memory representation as a [`std::ptr::NonNull`](core::ptr::NonNull).
#[repr(transparent)]
pub struct Replaced<T, F, P> {
ptr: NonNull<T>,
_family: PhantomData<F>,
_holder: PhantomData<P>,
}
impl<T, F, P> Clone for Replaced<T, F, P> {
fn clone(&self) -> Self {
Self {
ptr: self.ptr,
_family: self._family,
_holder: self._holder,
}
}
}
impl<T, F, P> Copy for Replaced<T, F, P> {}
impl<T, F, P> Deref for Replaced<T, F, P> {
type Target = NonNull<T>;
fn deref(&self) -> &Self::Target {
&self.ptr
}
}
impl<T, F, P> DerefMut for Replaced<T, F, P> {
fn deref_mut(&mut self) -> &mut Self::Target {
&mut self.ptr
}
}
impl<T, F, P> AsRef<NonNull<T>> for Replaced<T, F, P> {
fn as_ref(&self) -> &NonNull<T> {
&self.ptr
}
}
impl<T, F, P> Replaced<T, F, P> {
/// Extract the pointer originally stored in the [`AtomicPtr`].
pub fn into_inner(self) -> NonNull<T> {
self.ptr
}
}
impl<T, P> Replaced<T, raw::families::Global, P>
where
P: raw::Pointer<T>,
{
/// Retire the referenced object, and reclaim it once it is safe to do so.
///
/// # Safety
///
/// 1. The pointed-to object will never again be returned by any [`AtomicPtr::load`].
/// 2. The pointed-to object has not already been retired.
pub unsafe fn retire(self) -> usize {
// Safety:
//
// 1. Same as our caller requirement #1.
// 2. Same as our caller requirement #2.
// 3. Since there is exactly one Domain<Global>, we know that all calls to `load` that have
// returned this object must have been using the same (global) domain as we're retiring
// to.
unsafe { self.retire_in(Domain::global()) }
}
}
impl<T, F, P> Replaced<T, F, P>
where
P: raw::Pointer<T>,
{
/// Retire the referenced object, and reclaim it once it is safe to do so, through the given
/// `domain`.
///
/// # Safety
///
/// 1. The pointed-to object will never again be returned by any [`AtomicPtr::load`].
/// 2. The pointed-to object has not already been retired.
/// 3. All calls to [`load`](AtomicPtr::load) that can have seen the pointed-to object were
/// using hazard pointers from `domain`.
///
/// Note that requirement #3 is _partially_ enforced by the domain family (`F`), but it's on
/// you to ensure that you don't "cross the streams" between multiple `Domain<F>`, if those can
/// arise in your application.
pub unsafe fn retire_in(self, domain: &Domain<F>) -> usize {
// Safety:
//
// 1. implied by our #1 and #3: if load won't return it, there's no other way to guard it
// 2. implied by our #2
// 3. implied by AtomicPtr::new's #1 and #3
let ptr = self.ptr.as_ptr();
unsafe { domain.retire_ptr::<T, P>(ptr) }
}
}
impl<T, P> AtomicPtr<T, domain::Global, P> {
/// Loads the value from the stored pointer and guards it using the given hazard pointer.
///
/// The guard ensures that the loaded `T` will remain valid for as long as you hold a reference
/// to it.
///
/// Note that this method is _only_ available when using [`Domain<Global>`](domain::Global),
/// since it is a singleton, and thus is guaranteed to fulfill the safety requirement of
/// [`AtomicPtr::load`]. For all other domains, use [`AtomicPtr::load`].
pub fn safe_load<'hp>(
&'_ self,
hp: &'hp mut HazardPointer<'static, domain::Global>,
) -> Option<&'hp T>
where
T: 'hp,
{
// Safety: since there is exactly one Domain<Global>, we know that all calls to `load` that
// have returned this object must have been using the same (global) domain as we're
// retiring to.
unsafe { self.load(hp) }
}
}
impl<T, F, P> AtomicPtr<T, F, P> {
/// Loads the value from the stored pointer and guards it using the given hazard pointer.
///
/// The guard ensures that the loaded `T` will remain valid for as long as you hold a reference
/// to it.
///
/// # Safety
///
/// All objects stored in this [`AtomicPtr`] are retired through the same [`Domain`] as the one
/// that produced `hp`.
///
/// This requirement is _partially_ enforced by the domain family (`F`), but it's on you to
/// ensure that you don't "cross the streams" between multiple `Domain<F>`, if those can arise
/// in your application.
pub unsafe fn load<'hp, 'd>(&'_ self, hp: &'hp mut HazardPointer<'d, F>) -> Option<&'hp T>
where
T: 'hp,
F: 'static,
{
unsafe { hp.protect(&self.0) }
}
/// Returns a mutable reference to the underlying pointer.
///
/// # Safety
///
/// If the stored pointer is modified, the new value must conform to the same safety
/// requirements as the argument to [`AtomicPtr::new`].
#[cfg(not(loom))]
pub unsafe fn get_mut(&mut self) -> &mut *mut T {
self.0.get_mut()
}
/// Consumes the atomic and returns the contained value.
///
/// This is safe because passing `self` by value guarantees that no other threads are
/// concurrently accessing the atomic data, and no loads can happen in the future.
pub fn into_inner(self) -> *mut T {
#[cfg(not(loom))]
let ptr = self.0.into_inner();
// Safety: we own self, so the atomic value is visible to no other threads.
#[cfg(loom)]
let ptr = unsafe { self.0.unsync_load() };
ptr
}
}
impl<T, P> AtomicPtr<T, raw::families::Global, P>
where
P: raw::Pointer<T>,
{
/// Retire the currently-referenced object, and reclaim it once it is safe to do so.
///
/// # Safety
///
/// 1. The currently-referenced object will never again be returned by any [`AtomicPtr::load`].
/// 2. The currently-referenced object has not already been retired.
pub unsafe fn retire(self) -> usize {
// Safety:
//
// 1. Same as our caller requirement #1.
// 2. Same as our caller requirement #2.
// 3. Since there is exactly one Domain<Global>, we know that all calls to `load` that have
// returned this object must have been using the same (global) domain as we're retiring
// to.
unsafe { self.retire_in(Domain::global()) }
}
}
impl<T, F, P> AtomicPtr<T, F, P>
where
P: raw::Pointer<T>,
{
/// Retire the currently-referenced object, and reclaim it once it is safe to do so, through
/// the given `domain`.
///
/// # Safety
///
/// 1. The currently-referenced object will never again be returned by any [`AtomicPtr::load`].
/// 2. The currently-referenced object has not already been retired.
/// 3. All calls to [`load`](AtomicPtr::load) that can have seen the currently-referenced
/// object were using hazard pointers from `domain`.
///
/// Note that requirement #3 is _partially_ enforced by the domain family (`F`), but it's on
/// you to ensure that you don't "cross the streams" between multiple `Domain<F>`, if those can
/// arise in your application.
pub unsafe fn retire_in(self, domain: &Domain<F>) -> usize {
let ptr = self.into_inner();
unsafe { domain.retire_ptr::<T, P>(ptr) }
}
/// Store an object into the pointer.
///
/// Note, crucially, that this will _not_ automatically retire the pointer that's _currently_
/// stored, which is why it is safe.
pub fn store(&self, p: P) {
let ptr = p.into_raw();
// Safety (from AtomicPtr::new):
//
// #1 & #2 are both satisfied by virute of `p` being of type `P`, which holds a valid `T`.
// #3 is satisfied because the `P` is moved into `store`, and so can only be dropped
// through the `unsafe` retire methods on `AtomicPtr`, all of which call
// `Domain::retire_ptr`, or by dereferencing a raw pointer which is unsafe anyway.
unsafe { self.store_ptr(ptr) }
}
/// Overwrite the currently stored pointer with the given one, and return the previous pointer.
pub fn swap(&self, p: P) -> Option<Replaced<T, F, P>> {
let ptr = p.into_raw();
// Safety (from AtomicPtr::new):
//
// #1 & #2 are both satisfied by virute of `p` being of type `P`, which holds a valid `T`.
// #3 is satisfied because the `P` is moved into `store`, and so can only be dropped
// through the `unsafe` retire methods on `AtomicPtr` and `Replaced`, all of which call
// `Domain::retire_ptr`, or by dereferencing a raw pointer which is unsafe anyway.
unsafe { self.swap_ptr(ptr) }
}
/// Stores an object into the pointer if the current pointer is `current`.
///
/// The return value is a result indicating whether the new value was written and containing
/// the previous value. On success this value is guaranteed to be equal to `current`.
#[allow(clippy::not_unsafe_ptr_arg_deref)]
pub fn compare_exchange(
&self,
current: *mut T,
new: P,
) -> Result<Option<Replaced<T, F, P>>, P> {
let new = new.into_raw();
// Safety (from AtomicPtr::new):
//
// #1 & #2 are both satisfied by virute of `p` being of type `P`, which holds a valid `T`.
// #3 is satisfied because the `P` is moved into `store`, and so can only be dropped
// through the `unsafe` retire methods on `AtomicPtr` and `Replaced`, all of which call
// `Domain::retire_ptr`, or by dereferencing a raw pointer which is unsafe anyway.
let r = unsafe { self.compare_exchange_ptr(current, new) };
r.map_err(|ptr| {
// Safety: `ptr` is `new`, which was never shared, and was a valid `P`.
unsafe { P::from_raw(ptr) }
})
}
/// Stores an object into the pointer if the current pointer is `current`.
///
/// Unlike [`AtomicPtr::compare_exchange`], this function is allowed to spuriously fail even
/// when the comparison succeeds, which can result in more efficient code on some platforms.
/// The return value is a result indicating whether the new value was written and containing
/// the previous value. On success this value is guaranteed to be equal to `current`.
#[allow(clippy::not_unsafe_ptr_arg_deref)]
pub fn compare_exchange_weak(
&self,
current: *mut T,
new: P,
) -> Result<Option<Replaced<T, F, P>>, P> {
let new = new.into_raw();
// Safety (from AtomicPtr::new):
//
// #1 & #2 are both satisfied by virute of `p` being of type `P`, which holds a valid `T`.
// #3 is satisfied because the `P` is moved into `store`, and so can only be dropped
// through the `unsafe` retire methods on `AtomicPtr` and `Replaced`, all of which call
// `Domain::retire_ptr`, or by dereferencing a raw pointer which is unsafe anyway.
let r = unsafe { self.compare_exchange_weak_ptr(current, new) };
r.map_err(|ptr| {
// Safety: `ptr` is `new`, which was never shared, and was a valid `P`.
unsafe { P::from_raw(ptr) }
})
}
}
impl<T, F, P> AtomicPtr<T, F, P> {
/// Loads the current pointer.
pub fn load_ptr(&self) -> *mut T {
self.0.load(Ordering::Acquire)
}
/// Overwrite the currently stored pointer with the given one.
///
/// Note, crucially, that this will _not_ automatically retire the pointer that's _currently_
/// stored.
///
/// # Safety
///
/// `ptr must conform to the same safety requirements as the argument to [`AtomicPtr::new`].
pub unsafe fn store_ptr(&self, ptr: *mut T) {
self.0.store(ptr, Ordering::Release)
}
/// Overwrite the currently stored pointer with the given one, and return the previous pointer.
///
/// # Safety
///
/// `ptr must conform to the same safety requirements as the argument to [`AtomicPtr::new`].
pub unsafe fn swap_ptr(&self, ptr: *mut T) -> Option<Replaced<T, F, P>> {
let ptr = self.0.swap(ptr, Ordering::Release);
NonNull::new(ptr).map(|ptr| Replaced {
ptr,
_family: PhantomData::<F>,
_holder: PhantomData::<P>,
})
}
/// Stores `new` if the current pointer is `current`.
///
/// The return value is a result indicating whether the new pointer was written and containing
/// the previous pointer. On success this value is guaranteed to be equal to `current`.
///
/// # Safety
///
/// `ptr must conform to the same safety requirements as the argument to [`AtomicPtr::new`].
pub unsafe fn compare_exchange_ptr(
&self,
current: *mut T,
new: *mut T,
) -> Result<Option<Replaced<T, F, P>>, *mut T> {
let ptr = self
.0
.compare_exchange(current, new, Ordering::Release, Ordering::Relaxed)?;
Ok(NonNull::new(ptr).map(|ptr| Replaced {
ptr,
_family: PhantomData::<F>,
_holder: PhantomData::<P>,
}))
}
/// Stores `new` if the current pointer is `current`.
///
/// Unlike [`AtomicPtr::compare_exchange`], this function is allowed to spuriously fail even
/// when the comparison succeeds, which can result in more efficient code on some platforms.
/// The return value is a result indicating whether the new pointer was written and containing
/// the previous pointer. On success this value is guaranteed to be equal to `current`.
///
/// # Safety
///
/// `ptr must conform to the same safety requirements as the argument to [`AtomicPtr::new`].
pub unsafe fn compare_exchange_weak_ptr(
&self,
current: *mut T,
new: *mut T,
) -> Result<Option<Replaced<T, F, P>>, *mut T> {
let ptr =
self.0
.compare_exchange_weak(current, new, Ordering::Release, Ordering::Relaxed)?;
Ok(NonNull::new(ptr).map(|ptr| Replaced {
ptr,
_family: PhantomData::<F>,
_holder: PhantomData::<P>,
}))
}
}