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HP++: A Hazard Pointers Extension for Better Applicability

This is an implementation of HP++, a safe memory reclamation scheme proposed in

Jaehwang Jung, Janggun Lee, Jeonghyeon Kim and Jeehoon Kang, Applying Hazard Pointers to More Concurrent Data Structures, SPAA 2023.

The benchmark suite which evaluates the performance of HP++ can be found at smr-benchmark repository.

Introduction

HP++ is a backward-compatible extension for hazard pointers (HP) that enables optimistically traversing possibly detached nodes. The key idea is under-approximating the unreachability in validation to allow optimistic traversal by letting the deleter mark the node after detaching, and patching up the potentially unsafe accesses arising from false-negatives by letting the deleter protect such pointers. Thanks to optimistic traversal, data structures with HP++ may outperform same-purpose data structures with HP while consuming a similar amount of memory.

API Design

You can check the actual implementation of Harris's list in tests/harris_list.rs.

This crate provides two major APIs: try_protect_pp and try_unlink, corresponding to TryProtect and TryUnlink in the original paper, respectively.

(.._pp, which stands for plus-plus, is used in try_protect_pp to distinguish it from try_protect function that provides HP version of protecting.)

try_protect_pp

pub fn try_protect_pp<T, S, F>(
    &mut self,
    ptr: *mut T,
    src: &S,
    src_link: &AtomicPtr<T>,
    is_invalid: &F,
) -> Result<(), ProtectError<T>>
where
    F: Fn(&S) -> bool,

Traversing threads use try_protect_pp to protect a pointer loaded from a source object.

It takes 4 arguments (except the HazardPointer to protect with):

  1. ptr: the pointer to protect.
  2. src: the reference of the source object.
  3. src_link: the field of src from which ptr was loaded.
  4. is_invalid: the predicate to check whether src is invalidated.

If src is invalidated, try_protect_pp returns false meaning that it is unsafe to create new protection from src. Otherwise, it returns true, but if src_link has changed from ptr, then the new value is written to ptr.

try_unlink

pub unsafe fn try_unlink<T>(
    unlink: impl Unlink<T>,
    frontier: &[*mut T]
) -> bool
where
    T: Invalidate,

Unlinking threads use try_unlink to physically delete and retire node(s) while protecting the traversing threads. The protection will persist until the retired nodes are invalidated by reclaimers.

It takes 2 arguments:

  1. unlink: the Trait object which implements do_unlink function that performs unlinking and returns the unlinked node(s).
  2. frontier: the pointers that the unlinker has to protect for the traversing threads.

The unlinking frontier is the set of pointers that are reachable by following a single link from the to-be-unlinked objects but are not themselves to-be-unlinked. The frontier argument to try_unlink must be decided ahead of the actual unlinking, and the data structure must guarantee that the frontier does not change once it is decided: otherwise, the traversing thread’s access to a frontier node may not be protected.

Note that the node type T implements the Invalidate trait, so that unlinked nodes can be later invalidated by the reclaimers.

Invalidation can be implemented by adding a flag to the node. But in most cases, this can be done without extra space overhead using tagged pointers, similar to logical deletion.

try_unlink returns whether the unlink was successful.