Serialize stop-the-world transactions across interpreters by youknowone · Pull Request #9094 · RustPython/RustPython

youknowone · 2026-10-13T11:21:47Z

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Summary

Serialize complete stop-the-world transactions across interpreters, including process-wide GC, using one process-wide permit.

A requester attached to interpreter A may stop interpreter B while a B thread starts GC that stops A before B. Per-interpreter exclusions allow the requester and collector to wait for each other. The former contention loop also used the target interpreter's state to service the current thread's stop request.

Changes

Wait for the global transaction while detached from the actual current VM, then reattach without ownership before retrying acquisition.
Transfer successful public stop ownership to the existing state flag until the matching restart, preserving cross-thread handoff.
Hold one outer transaction for GC's entire interpreter snapshot, with admission acquired afterward and released before the transaction.
Roll back partial stops before releasing ownership. Retain all snapshot references until restart and transaction release so unwinding cannot drop Python-owned state while another interpreter remains stopped.
Release the inherited fork transaction only through the forking state's ownership flag. No new mutex is needed. Non-reused logical requester IDs reject same-thread nesting before state changes.

Only two production Rust files change. Tests, standard-library fixtures, timeouts, and signal/watchdog dump paths are unchanged.

Validation

Normal commit hooks passed.
Independent source review covered ownership transfer, partial-stop unwind, restart ordering, fork reset, and production stop callers.
Linux threading VM compile-only check and strict Clippy (-D warnings): passed, jobs1.
No new local STW stress or hang reproducer was executed.

The change addresses source-supported cross-interpreter deadlock hazards found while investigating the four interpreter/STW tests left running in compiler CI. Their exact hosted interleaving was not captured, so this does not claim that those timeouts have already been reproduced or resolved. The initial 43eae7b revision passed all 25 hosted CI jobs. The subsequent same-thread reentry hardening requires fresh hosted validation.

Recent main-based CI passed these tests; that passing run does not rule out the source-supported intermittent interleaving described above.

Summary by CodeRabbit

Bug Fixes
- Stop-and-resume operations are coordinated across the process to prevent overlapping operations from interfering with one another.
- If a stop operation cannot complete, affected threads are resumed rather than left paused.
- Restart and recovery after creating a child process now release only the coordination state they own.

Coordinate individual stop requests with process-wide collection, retain snapshot owners through rollback, and release transferred ownership after restart. Assisted-by: OpenAI Codex: model identifier unavailable

coderabbitai · 2026-10-13T11:21:57Z

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crates/vm/src/vm/mod.rs

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📝 Walkthrough

Stop-the-world operations now use a process-wide transaction that remains held across stopping and resuming interpreter states. Garbage collection retains the transaction and state snapshot, then restarts successfully stopped states in reverse order.

Changes: Stop-the-world coordination

Layer / File(s)	Summary
Transaction and stop/resume lifecycle `crates/vm/src/vm/mod.rs`	Stop operations acquire and retain a process-wide transaction. Incomplete stops roll back, resume no longer releases ownership itself, and transaction release follows state ownership during restart or fork cleanup.
Garbage-collection stop and restart `crates/vm/src/gc_state.rs`	Garbage collection retains the transaction and interpreter snapshot, counts successful stops, and restarts only those states in reverse order.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant GarbageCollection
  participant StopTheWorldTransaction
  participant InterpreterStates
  GarbageCollection->>StopTheWorldTransaction: acquire transaction
  GarbageCollection->>InterpreterStates: snapshot and stop states
  GarbageCollection->>InterpreterStates: restart successfully stopped states in reverse order
  GarbageCollection->>StopTheWorldTransaction: release transaction

Suggested reviewers: 1ndahous3

Merge Risk: ⚪ Minimal · up to f2c9b

No actionable merge-blocking issue was established. Normal validation can proceed, though the reported intermittent interleaving has not been reproduced.

Pre-merge checks · 4 · 1

Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 76.92% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 13 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: serializing stop-the-world transactions across interpreters.

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chatgpt-codex-connector · 2026-10-13T12:04:55Z

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-13T13:15:05.554871Z	`0ba1076`	New commits

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coderabbitai

Actionable comments posted: 1

🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @crates/vm/src/vm/mod.rs:
- Around line 205-225: Update STW_TRANSACTION ownership handling in acquire() to
detect when the current thread already owns the permit and fail fast on
re-entry, including nested stop_the_world() or gc.collect() calls before
start_the_world(). Preserve waiting behavior for other threads.

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crates/vm/src/gc_state.rs
crates/vm/src/vm/mod.rs

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Use non-reused logical requester identities to fail before waiting on the same thread, while preserving cross-thread restart and fork ownership. Assisted-by: OpenAI Codex: model identifier unavailable

Use upstream's merged PyRef initializer repair to restore merge-ref compilation without changing the reviewed STW implementation. Assisted-by: OpenAI Codex: model identifier unavailable

Serialize stop-the-world transactions across interpreters

43eae7b

Coordinate individual stop requests with process-wide collection, retain snapshot owners through rollback, and release transferred ownership after restart. Assisted-by: OpenAI Codex: model identifier unavailable

youknowone marked this pull request as ready for review October 13, 2026 12:04

coderabbitai Bot reviewed Oct 13, 2026

View reviewed changes

Comment thread crates/vm/src/vm/mod.rs

Reject reentrant stop-the-world transactions

f2c9b28

Use non-reused logical requester identities to fail before waiting on the same thread, while preserving cross-thread restart and fork ownership. Assisted-by: OpenAI Codex: model identifier unavailable

youknowone mentioned this pull request Oct 13, 2026

Initialize PyRef marker in native base conversion #9096

Merged

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Merge repaired main into STW transaction branch

0ba1076

Use upstream's merged PyRef initializer repair to restore merge-ref compilation without changing the reviewed STW implementation. Assisted-by: OpenAI Codex: model identifier unavailable

youknowone mentioned this pull request Oct 13, 2026

Align compiler and VM bytecode contracts with CPython 3.15 #9088

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Serialize stop-the-world transactions across interpreters - #9094

Serialize stop-the-world transactions across interpreters#9094
youknowone wants to merge 3 commits into
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youknowdot:fix/stw-cross-interpreter-transaction

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