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//! Epoch-based memory reclamation.
//!
//! An interesting problem concurrent collections deal with comes from the remove operation.
//! Suppose that a thread removes an element from a lock-free map, while another thread is reading
//! that same element at the same time. The first thread must wait until the second thread stops
//! reading the element. Only then it is safe to destruct it.
//!
//! Programming languages that come with garbage collectors solve this problem trivially. The
//! garbage collector will destruct the removed element when no thread can hold a reference to it
//! anymore.
//!
//! This crate implements a basic memory reclamation mechanism, which is based on epochs. When an
//! element gets removed from a concurrent collection, it is inserted into a pile of garbage and
//! marked with the current epoch. Every time a thread accesses a collection, it checks the current
//! epoch, attempts to increment it, and destructs some garbage that became so old that no thread
//! can be referencing it anymore.
//!
//! That is the general mechanism behind epoch-based memory reclamation, but the details are a bit
//! more complicated. Anyhow, memory reclamation is designed to be fully automatic and something
//! users of concurrent collections don't have to worry much about.
//!
//! # Pointers
//!
//! Concurrent collections are built using atomic pointers. This module provides [`Atomic`], which
//! is just a shared atomic pointer to a heap-allocated object. Loading an [`Atomic`] yields a
//! [`Shared`], which is an epoch-protected pointer through which the loaded object can be safely
//! read.
//!
//! # Pinning
//!
//! Before an [`Atomic`] can be loaded, a participant must be [`pin`]ned. By pinning a participant
//! we declare that any object that gets removed from now on must not be destructed just
//! yet. Garbage collection of newly removed objects is suspended until the participant gets
//! unpinned.
//!
//! # Garbage
//!
//! Objects that get removed from concurrent collections must be stashed away until all currently
//! pinned participants get unpinned. Such objects can be stored into a thread-local or global
//! storage, where they are kept until the right time for their destruction comes.
//!
//! There is a global shared instance of garbage queue. You can [`defer`](Guard::defer) the execution of an
//! arbitrary function until the global epoch is advanced enough. Most notably, concurrent data
//! structures may defer the deallocation of an object.
//!
//! # APIs
//!
//! For majority of use cases, just use the default garbage collector by invoking [`pin`]. If you
//! want to create your own garbage collector, use the [`Collector`] API.
#![doc(test(
no_crate_inject,
attr(
deny(warnings, rust_2018_idioms),
allow(dead_code, unused_assignments, unused_variables)
)
))]
#![warn(
missing_docs,
missing_debug_implementations,
rust_2018_idioms,
unreachable_pub
)]
#![cfg_attr(not(feature = "std"), no_std)]
#[cfg(crossbeam_loom)]
extern crate loom_crate as loom;
use cfg_if::cfg_if;
#[cfg(crossbeam_loom)]
#[allow(unused_imports, dead_code)]
mod primitive {
pub(crate) mod cell {
pub(crate) use loom::cell::UnsafeCell;
}
pub(crate) mod sync {
pub(crate) mod atomic {
use core::sync::atomic::Ordering;
pub(crate) use loom::sync::atomic::{fence, AtomicUsize};
// FIXME: loom does not support compiler_fence at the moment.
// we use fence as a stand-in for compiler_fence for the time being.
// this may miss some races since fence is stronger than compiler_fence,
// but it's the best we can do for the time being.
pub(crate) use self::fence as compiler_fence;
}
pub(crate) use loom::sync::Arc;
}
pub(crate) use loom::thread_local;
}
#[cfg(not(crossbeam_no_atomic_cas))]
#[cfg(not(crossbeam_loom))]
#[allow(unused_imports, dead_code)]
mod primitive {
#[cfg(feature = "alloc")]
pub(crate) mod cell {
#[derive(Debug)]
#[repr(transparent)]
pub(crate) struct UnsafeCell<T>(::core::cell::UnsafeCell<T>);
// loom's UnsafeCell has a slightly different API than the standard library UnsafeCell.
// Since we want the rest of the code to be agnostic to whether it's running under loom or
// not, we write this small wrapper that provides the loom-supported API for the standard
// library UnsafeCell. This is also what the loom documentation recommends:
impl<T> UnsafeCell<T> {
#[inline]
pub(crate) const fn new(data: T) -> UnsafeCell<T> {
UnsafeCell(::core::cell::UnsafeCell::new(data))
}
#[inline]
pub(crate) fn with<R>(&self, f: impl FnOnce(*const T) -> R) -> R {
f(self.0.get())
}
#[inline]
pub(crate) fn with_mut<R>(&self, f: impl FnOnce(*mut T) -> R) -> R {
f(self.0.get())
}
}
}
#[cfg(feature = "alloc")]
pub(crate) mod sync {
pub(crate) mod atomic {
pub(crate) use core::sync::atomic::compiler_fence;
pub(crate) use core::sync::atomic::fence;
pub(crate) use core::sync::atomic::AtomicUsize;
}
pub(crate) use alloc::sync::Arc;
}
#[cfg(feature = "std")]
pub(crate) use std::thread_local;
}
#[cfg(not(crossbeam_no_atomic_cas))]
cfg_if! {
if #[cfg(feature = "alloc")] {
extern crate alloc;
mod atomic;
mod collector;
mod deferred;
mod epoch;
mod guard;
mod internal;
mod sync;
pub use self::atomic::{
Pointable, Atomic, CompareExchangeError,
Owned, Pointer, Shared,
};
pub use self::collector::{Collector, LocalHandle};
pub use self::guard::{unprotected, Guard};
#[allow(deprecated)]
pub use self::atomic::{CompareAndSetError, CompareAndSetOrdering};
}
}
cfg_if! {
if #[cfg(feature = "std")] {
mod default;
pub use self::default::{default_collector, is_pinned, pin};
}
}