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Add Loom model checking for synchronization primitives

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
30/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
rust
Domain
backend, testing

Research direction

Start with the working Loom model in the RwLock changes linked from PR 184, then review the existing unit, integration, stress, and multithreaded test infrastructure. Determine whether Loom should be adopted and which synchronization internals are in scope. Done means bounded models cover the selected synchronization cases while keeping CI execution practical.

Written by the indexing model from the issue text.

Description

As a library providing several synchronization primitives, Asyncband relies on atomic state transitions, waiter queues, and wake-up handling. Conventional unit and stress tests exercise many concurrent schedules, but they cannot systematically explore every relevant execution ordering. Rare interleavings may therefore hide missed wake-ups, leaked ownership, deadlocks, or invalid state transitions.

Loom provides deterministic model checking for concurrent Rust code. Small, focused models can systematically explore relevant interleavings and validate invariants such as:

  • Readers and writers never holding incompatible ownership
  • Waiter publication racing with lock release without missing a wake-up
  • Cancellation before or after ownership is granted
  • Queue-head cancellation preserving follower progress
  • Atomic upgrade and downgrade transitions
  • Correct memory ordering between lock holders

Loom can only explore concurrency performed through its own instrumented primitives. Every atomic, mutex, thread, cell, or other synchronization operation relevant to a model must therefore use the corresponding Loom type. Any synchronization left using an uninstrumented std type falls outside Loom’s scheduler and reduces the model’s coverage, however, this is for the most part covered by the existing test infrastructure. Asyncband would use conditional compilation to select Loom primitives during model checking and the existing std implementations in normal production builds.

A working draft has been added to the RwLock changes (here), where the previous semaphore-based implementation was replaced with a purpose-built atomic scheduler. The draft model-checks ownership exclusion, waiter publication, cancellation around grant, queue progress, upgrade priority, and downgrade handoff.

If the project decides not to adopt Loom, the draft can be removed from the RwLock PR before merging. If adopted, support can be expanded through separate, individual PRs for other synchronization internals, such as:

  • Semaphore
  • AtomicOptionBox
  • Mutex scheduling
  • Countdown and wait-set primitives
  • Channel state machines

The models should remain small and bounded to keep CI execution practical. Loom would exist only to complement the existing unit, integration, stress, and multithreaded tests.

Dominant language
Rust
Stars
274
Forks
42
Avg merge
20h 18m
Merged PRs (30d)
55

Getting set up

This project ships no dev container, Dockerfile or contributing guide, so setting up is up to you: start from its README, and see our first-contribution guide for the general steps.

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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