Implementing Hazard Pointers for Lock-Free Memory Reclamation in Rust

Hazard pointers are a technique used in lock-free data structures to safely reclaim memory without requiring locks. They provide a way for threads to signal that they are currently accessing a shared memory location, preventing that location from being deallocated until all threads have finished with it. This challenge asks you to implement a hazard pointer system in Rust, which can then be used to manage memory reclamation in concurrent, lock-free data structures.

Problem Description

Your task is to implement a safe and efficient hazard pointer manager in Rust. This manager will be responsible for:

Registering Hazards: Allowing threads to atomically set a pointer to a memory location they are currently accessing. This pointer acts as a "hazard" to its deallocation.
Scanning for Hazards: Providing a mechanism for another thread to scan all registered hazards and determine if a particular memory location is still in use.
Reclaiming Memory: Safely deallocating memory that is no longer pointed to by any registered hazards.

You need to ensure that this implementation is thread-safe and handles concurrent access correctly. The core of the problem lies in atomically updating hazard pointers and ensuring that memory is only freed when it's safe to do so.

Key Requirements:

Thread-Safety: All operations on the hazard pointer manager must be atomic and safe to call from multiple threads concurrently.
Hazard Registration: A thread must be able to register a pointer as a hazard. This should be an atomic operation.
Hazard Clearing: A thread must be able to atomically clear its registered hazard when it's done accessing the memory.
Memory Reclamation: A mechanism to identify and reclaim memory that is no longer protected by any hazards. This should be efficient.
Integration with a simple list (optional but recommended for testing): Design your hazard pointer system such that it can be integrated with a basic lock-free linked list to demonstrate its functionality.

Expected Behavior:

When a thread accesses a node in a lock-free data structure, it should register a hazard pointer to that node.
Before deallocating a node, a "collector" thread (or the node's thread itself) should scan all registered hazard pointers. If the node is pointed to by any hazard, it should not be deallocated.
Once a node is determined to be safe for deallocation (i.e., not pointed to by any hazard), it should be added to a "deferred reclamation" list.
The actual deallocation (e.g., dropping the memory) should happen when the hazard pointer manager deems it appropriate (e.g., when the deferred reclamation list reaches a certain size or after a period of inactivity).

Edge Cases to Consider:

Concurrent Registration and Deletion: What happens if a thread registers a hazard exactly when another thread is attempting to deallocate the same memory?
Multiple Threads Accessing the Same Node: How does the system handle multiple threads registering hazards to the same memory location?
Starvation: Ensure that threads waiting to reclaim memory are not indefinitely starved.

Examples

Since the output of a hazard pointer system is the absence of memory leaks and correct concurrent behavior, a direct input/output example is less illustrative than a conceptual one.

Conceptual Example:

Consider a lock-free linked list with nodes A, B, and C in that order.

Thread 1 is reading node B. It registers a hazard pointer to B.
Thread 2 is attempting to remove node B from the list.
Thread 2 tries to deallocate B. Before doing so, it calls the hazard pointer manager's scan_hazards() function.
The scan_hazards() function iterates through all registered hazards. It finds that Thread 1 has a hazard pointer to B.
Therefore, Thread 2 knows that B is still in use and cannot be deallocated immediately. It places B in a temporary "to be reclaimed" list.
Thread 1 finishes its operation and clears its hazard pointer to B.
Later, when the hazard pointer manager determines it's safe (e.g., no more hazards to B), it will deallocate B from the "to be reclaimed" list.

Constraints

No Global Allocator Lock: Your hazard pointer implementation should not rely on a global mutex or lock to protect its internal state. All operations must be lock-free or use fine-grained atomic operations.
Memory Safety: You must use Rust's memory safety guarantees. Avoid unsafe code where possible, and if necessary, ensure it is rigorously justified and correct. std::sync::atomic and potentially std::ptr will be your allies.
Performance: While correctness is paramount, consider the performance implications of your design. Frequent and expensive scans can degrade overall application performance.
Number of Hazards: Assume a reasonable, but potentially large, number of concurrent threads that might register hazards (e.g., up to a few hundred). The system should scale accordingly.

Notes

std::sync::atomic: This module will be crucial for implementing atomic operations on pointers and counters. Consider using AtomicPtr or AtomicUsize.
std::mem::forget and std::mem::transmute: You might need these for careful handling of raw pointers and memory management, but use them with extreme caution and only when absolutely necessary, ensuring complete understanding of their implications.
Deferred Reclamation: A common strategy is to collect nodes marked for deletion in a list and only perform the actual deallocation when the list grows or some other condition is met. This amortizes the cost of scanning.
Epoch-Based Reclamation (Alternative): While this challenge focuses on hazard pointers, it's worth noting that other techniques like epoch-based memory reclamation exist. Hazard pointers are generally simpler to grasp initially.
Testing: Thoroughly test your implementation with multiple threads, race conditions, and scenarios where nodes are frequently added and removed. You will likely need to build a small concurrent data structure (like a linked list) to effectively test your hazard pointer manager.
Send and Sync: Ensure your hazard pointer manager and any associated data structures correctly implement Send and Sync for inter-thread communication.