Module crossbeam::mem::epoch

Epoch-based memory management

This module provides fast, easy to use memory management for lock free data structures. It's inspired by Keir Fraser's epoch-based reclamation.

The basic problem this is solving is the fact that when one thread has removed a node from a data structure, other threads may still have pointers to that node (in the form of snapshots that will be validated through things like compare-and-swap), so the memory cannot be immediately freed. Put differently:

1. There are two sources of reachability at play -- the data structure, and the snapshots in threads accessing it. Before we delete a node, we need to know that it cannot be reached in either of these ways.

2. Once a node has been unliked from the data structure, no new snapshots reaching it will be created.

Using the epoch scheme is fairly straightforward, and does not require understanding any of the implementation details:

• When operating on a shared data structure, a thread must "pin the current epoch", which is done by calling pin(). This function returns a Guard which unpins the epoch when destroyed.

• When the thread subsequently reads from a lock-free data structure, the pointers it extracts act like references with lifetime tied to the Guard. This allows threads to safely read from snapshotted data, being guaranteed that the data will remain allocated until they exit the epoch.

To put the Guard to use, Crossbeam provides a set of three pointer types meant to work together:

• Owned<T>, akin to Box<T>, which points to uniquely-owned data that has not yet been published in a concurrent data structure.

• Shared<'a, T>, akin to &'a T, which points to shared data that may or may not be reachable from a data structure, but it guaranteed not to be freed during lifetime 'a.

• Atomic<T>, akin to std::sync::atomic::AtomicPtr, which provides atomic updates to a pointer using the Owned and Shared types, and connects them to a Guard.

Each of these types provides further documentation on usage.

Example

use std::sync::atomic::Ordering::{Acquire, Release, Relaxed};
use std::ptr;

use crossbeam::mem::epoch::{self, Atomic, Owned};

struct TreiberStack<T> {
    head: Atomic<Node<T>>,
}

struct Node<T> {
    data: T,
    next: Atomic<Node<T>>,
}

impl<T> TreiberStack<T> {
    fn new() -> TreiberStack<T> {
        TreiberStack {
            head: Atomic::null()
        }
    }

    fn push(&self, t: T) {
        // allocate the node via Owned
        let mut n = Owned::new(Node {
            data: t,
            next: Atomic::null(),
        });

        // become active
        let guard = epoch::pin();

        loop {
            // snapshot current head
            let head = self.head.load(Relaxed, &guard);

            // update `next` pointer with snapshot
            n.next.store_shared(head, Relaxed);

            // if snapshot is still good, link in the new node
            match self.head.cas_and_ref(head, n, Release, &guard) {
                Ok(_) => return,
                Err(owned) => n = owned,
            }
        }
    }

    fn pop(&self) -> Option<T> {
        // become active
        let guard = epoch::pin();

        loop {
            // take a snapshot
            match self.head.load(Acquire, &guard) {
                // the stack is non-empty
                Some(head) => {
                    // read through the snapshot, *safely*!
                    let next = head.next.load(Relaxed, &guard);

                    // if snapshot is still good, update from `head` to `next`
                    if self.head.cas_shared(Some(head), next, Release) {
                        unsafe {
                            // mark the node as unlinked
                            guard.unlinked(head);

                            // extract out the data from the now-unlinked node
                            return Some(ptr::read(&(*head).data))
                        }
                    }
                }

                // we observed the stack empty
                None => return None
            }
        }
    }
}

Structs

Atomic	Like std::sync::atomic::AtomicPtr.
Guard	An RAII-style guard for pinning the current epoch.
Owned	Like Box<T>: an owned, heap-allocated data value of type T.
Shared	Like &'a T: a shared reference valid for lifetime 'a.

Functions

pin	Pin the current epoch.