txn: avoid xlock starvation by setting slock shrink-only by zyguan · Pull Request #19313 · tikv/tikv

zyguan · 2026-01-23T03:35:57Z

What is changed and how it works?

Issue Number: ref #19087

What's Changed:

Avoid exclusive lock starvation by setting shared locks shrink-only:

- `acquire_pessimistic_lock` returns `NotInShrinkMode` when an exclusive lock meets non-shrink-only shared locks.
- `AcquirePessimisticLock.process_write` converts shared locks to shrink-only, persists them, and returns `KeyIsLocked` to the client.
- The scheduler skips in-memory pessimistic lock writes when a shared-lock update is being persisted.

Related changes

PR to update pingcap/docs/pingcap/docs-cn:
Need to cherry-pick to the release branch

Check List

Tests

Unit test
Integration test
Manual test (add detailed scripts or steps below)
No code

Side effects

Performance regression: Consumes more CPU
Performance regression: Consumes more Memory
Breaking backward compatibility

Release note

None

Signed-off-by: zyguan <zhongyangguan@gmail.com>

Copilot

Pull request overview

This PR refines the interaction between exclusive (xlock) and shared (slock) pessimistic locks to avoid exclusive-lock starvation when shared locks are present, especially under in-memory pessimistic locking.

Changes:

Introduces a new MVCC error NotInShrinkMode(SharedLocks) to represent xlock encountering shared locks that are not yet marked shrink-only, and wires it through the error and cloning infrastructure.
Updates acquire_pessimistic_lock/AcquirePessimisticLock to (a) surface NotInShrinkMode from the MVCC layer, (b) convert shared locks to shrink-only, persist them, and then return a KeyIsLocked error containing a shared-lock LockInfo.
Extends the scheduler to detect KeyIsLocked responses carrying shared-lock info and, in that case, skip the in-memory pessimistic lock fast path to ensure the shared-lock shrink-only update is actually persisted; adds tests around shared-lock behavior and the in-memory mode skip.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
`src/storage/txn/scheduler.rs`	Adds detection of `KeyIsLocked` on a shared lock (`LockInfoExt::is_shared_lock`) and prevents in-memory pessimistic lock insertion when persisting a shared-lock shrink-only update; adds a unit test ensuring the scheduler writes such updates to storage and not the in-memory table.
`src/storage/txn/mod.rs`	Exposes `ProcessResult::get_key_lock_info` to extract the inner `LockInfo` from `PessimisticLockRes` errors, enabling the scheduler’s new shared-lock detection.
`src/storage/txn/commands/acquire_pessimistic_lock.rs`	On `NotInShrinkMode`, clears prior mutations, marks the shared locks as shrink-only, writes them back, and then turns the result into a `KeyIsLocked` error with shared-lock info; adds an integration-style test for shared-lock semantics and a helper for invoking the command.
`src/storage/txn/commands/acquire_pessimistic_lock_resumed.rs`	Handles `NotInShrinkMode` in the resumed path by converting the shared locks to a `LockInfo` and treating the key as waiting (without modifying the shared locks), so resumed requests queue properly while acknowledging shared-lock conflicts.
`src/storage/txn/actions/acquire_pessimistic_lock.rs`	Extends core lock-acquisition logic to distinguish between shrink-only and non–shrink-only shared locks: xlock vs. slock interactions now return `NotInShrinkMode` or `KeyIsLocked` as appropriate; adds tests covering xlock meeting non-shrink-only slocks and slock meeting shrink-only slocks.
`src/storage/mvcc/mod.rs`	Introduces `ErrorInner::NotInShrinkMode(SharedLocks)`, makes it cloneable via `maybe_clone`, and maps it to the `KEY_IS_LOCKED` error code for external error reporting.
`components/txn_types/src/lock.rs`	Adds `SharedLocks` shrink-only flag infrastructure (already existing) and introduces `LockInfoExt::is_shared_lock()` to check whether a `LockInfo` is a shared lock.
`components/txn_types/src/lib.rs`	Re-exports `LockInfoExt` from the `lock` module so that transaction code (e.g., scheduler) can use it when inspecting `LockInfo` values.

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Signed-off-by: zyguan <zhongyangguan@gmail.com>

cfzjywxk · 2026-01-26T07:16:38Z

src/storage/txn/commands/acquire_pessimistic_lock_resumed.rs

+                    lock_info.lock_wait_token = lock_wait_token;
+                    lock_info.req_states = Some(req_states);
+                    res.push(PessimisticLockKeyResult::Waiting);
+                    encountered_locks.push(lock_info);


Would this lead to panic when resumable requests counter shared locks for the assert at scheduler.rs:2131?

Fixed, PTAL.

Signed-off-by: you06 <you1474600@gmail.com>

Signed-off-by: zyguan <zhongyangguan@gmail.com>

ti-chi-bot · 2026-01-26T15:12:13Z

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cfzjywxk, you06

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ti-chi-bot · 2026-01-26T15:12:15Z

[LGTM Timeline notifier]

Timeline:

2026-01-23 11:12:07.137523534 +0000 UTC m=+751554.751480390: ☑️ agreed by you06.
2026-01-26 15:12:13.501420656 +0000 UTC m=+1025161.115377512: ☑️ agreed by cfzjywxk.

ref tikv#19087 Avoid exclusive lock starvation by setting shared locks shrink-only: - `acquire_pessimistic_lock` returns `NotInShrinkMode` when an exclusive lock meets non-shrink-only shared locks. - `AcquirePessimisticLock.process_write` converts shared locks to shrink-only, persists them, and returns `KeyIsLocked` to the client. - The scheduler skips in-memory pessimistic lock writes when a shared-lock update is being persisted. Signed-off-by: you06 <you1474600@gmail.com> Signed-off-by: zyguan <zhongyangguan@gmail.com> Co-authored-by: you06 <you1474600@gmail.com>

txn: avoid xlock starvation by setting slock shrink-only

321e238

Signed-off-by: zyguan <zhongyangguan@gmail.com>

Copilot AI review requested due to automatic review settings January 23, 2026 03:35

Copilot started reviewing on behalf of zyguan January 23, 2026 03:36 View session

Copilot AI reviewed Jan 23, 2026

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zyguan requested review from cfzjywxk and you06 January 23, 2026 03:44

you06 mentioned this pull request Jan 23, 2026

Support shared lock for foreign key #19087

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13 tasks

zyguan added 2 commits January 23, 2026 07:26

fix test_shared_exclusive_lock_conflict

bd905aa

Signed-off-by: zyguan <zhongyangguan@gmail.com>

format code

35dbbf7

Signed-off-by: zyguan <zhongyangguan@gmail.com>

you06 approved these changes Jan 23, 2026

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ti-chi-bot bot added needs-1-more-lgtm Indicates a PR needs 1 more LGTM. approved labels Jan 23, 2026

cfzjywxk reviewed Jan 26, 2026

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ti-chi-bot bot added release-note-none Denotes a PR that doesn't merit a release note. and removed do-not-merge/release-note-label-needed Indicates that a PR should not merge because it's missing one of the release note labels. labels Jan 26, 2026

zyguan and others added 2 commits January 26, 2026 08:43

Merge remote-tracking branch 'origin/master' into dev/slock-shrink-only

3b1cfd8

shared lock compatible with resumed force lock

d027b06

Signed-off-by: you06 <you1474600@gmail.com>

ti-chi-bot bot added size/XXL Denotes a PR that changes 1000+ lines, ignoring generated files. and removed size/XL Denotes a PR that changes 500-999 lines, ignoring generated files. labels Jan 26, 2026

zyguan added 2 commits January 26, 2026 09:11

fix tests

d23119d

Signed-off-by: zyguan <zhongyangguan@gmail.com>

fix test_resolve_shared_locks

ce1365d

Signed-off-by: zyguan <zhongyangguan@gmail.com>

cfzjywxk approved these changes Jan 26, 2026

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ti-chi-bot bot added lgtm and removed needs-1-more-lgtm Indicates a PR needs 1 more LGTM. labels Jan 26, 2026

ti-chi-bot bot merged commit 6182259 into tikv:master Jan 26, 2026
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txn: avoid xlock starvation by setting slock shrink-only#19313

txn: avoid xlock starvation by setting slock shrink-only#19313
ti-chi-bot[bot] merged 7 commits intotikv:masterfrom
zyguan:dev/slock-shrink-only

zyguan commented Jan 23, 2026 •

edited by cfzjywxk

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What is changed and how it works?

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zyguan commented Jan 23, 2026 •

edited by cfzjywxk

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