Designing Lock-Free Linked Lists

In this lesson, you will learn how to navigate the "ABA problem" and memory reclamation challenges when building lock-free linked lists in Rust.

The Memory Ownership Dilemma

In a standard linked list, ownership is clear: the List owns the Head, and each Node owns the Next pointer. In a lock-free list, however, multiple threads attempt to traverse or modify these pointers simultaneously using atomic operations.

If Thread A is traversing the list and holds a reference to a Node, but Thread B deletes that node and frees its memory, Thread A is left with a dangling pointer—a classic segmentation fault. Because Rust’s ownership model typically requires unique ownership, we cannot simply rely on Box or &mut to manage these shared, volatile nodes.

Atomic Pointers and Shared Ownership

To solve this, we use AtomicPtr<Node>. Instead of moving nodes, we treat them as shared resources. When we need to modify the list, we use compare_exchange to swap pointers. However, the biggest hurdle is knowing when to drop a node. If we simply drop a node as soon as it is unlinked, we risk the "use-after-free" scenario mentioned above.

Consider this structural skeleton:

use std::sync::atomic::{AtomicPtr, Ordering};

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

pub struct LockFreeList<T> {
    head: AtomicPtr<Node<T>>,
}

In a production-grade lock-free list, you cannot rely on standard Drop because the node might still be referenced by another thread’s local pointer. You must implement a reclamation strategy, such as Hazard Pointers or Read-Copy-Update (RCU). These patterns effectively track which nodes are currently "in-use" by threads, delaying the deallocation until it is mathematically proven that no thread can possibly access that memory.

By combining AtomicPtr with these reclamation strategies, you satisfy the Sync requirements of your data structure, allowing multiple threads to safely traverse the list without ever acquiring a traditional mutex.

Key Takeaway: In lock-free linked lists, memory safety is achieved by decoupling the unlinking of a node from its actual deallocation, ensuring that no thread is left holding a dangling pointer.