Improve HNSW concurrency: shared locks for read-only paths #904
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…correctness - Upgrade per-node neighborsGuard from std::mutex to std::shared_mutex to allow concurrent read access during search operations. - Add lockNodeLinksShared/unlockNodeLinksShared methods and use them in all read-only paths: processCandidate, processCandidate_RangeSearch, greedySearchLevel, replaceEntryPoint, safeCollectAllNodeIncomingNeighbors, repairNodeConnections (read phases), getHNSWElementNeighbors, and the batch iterator. Write paths (mutuallyConnectNewElement, revisitNeighborConnections, mutuallyUpdateForRepairedNode) retain exclusive locks. - Make getElementIds const-correct across the HNSW class hierarchy. - Remove unused <iostream> include. Co-authored-by: Cursor <cursoragent@cursor.com>
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Codecov Report❌ Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## main #904 +/- ##
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- Coverage 97.12% 97.10% -0.02%
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Files 129 129
Lines 7500 7488 -12
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- Hits 7284 7271 -13
- Misses 216 217 +1 ☔ View full report in Codecov by Sentry. 🚀 New features to boost your workflow:
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JoanFM
reviewed
Feb 9, 2026
src/VecSim/algorithms/hnsw/hnsw.h
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| ElementGraphData *cur_element = getGraphDataByInternalId(curNodeId); | ||
| lockNodeLinks(cur_element); | ||
| lockNodeLinksShared(cur_element); |
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should we actually use lock_guards here?
JoanFM
reviewed
Feb 9, 2026
src/VecSim/algorithms/hnsw/hnsw.h
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| auto *cur_element = getGraphDataByInternalId(curNodeId); | ||
| lockNodeLinks(cur_element); | ||
| lockNodeLinksShared(cur_element); |
JoanFM
requested changes
Feb 9, 2026
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JoanFM
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JoanFM
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In general, can we try to aim for having lock guards for more safety?
- Replace manual lock/unlock pairs with std::shared_lock/std::unique_lock in hnsw.h and hnsw_batch_iterator.h to make lock release scope-safe across early exits. - Deduplicate sorted node lock sets and guard against self-neighbor locking before acquiring paired locks, reducing deadlock/pathological lock risks without changing lock semantics.
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Describe the changes in the pull request
lock audit:
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