[PR #1373] [MERGED] [TE] Fix race condition in receivePeerMetadata and getSegmentDesc #2200

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opened 2026-10-29 21:33:32 +00:00 by bsakaguchi · 0 comments

bsakaguchi commented

2026-10-29 21:33:32 +00:00

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📋 Pull Request Information

Original PR: https://github.com/kvcache-ai/Mooncake/pull/1373
Author: @staryxchen
Created: 1/14/2026
Status: ✅ Merged
Merged: 1/23/2026
Merged by: @alogfans

Base: main ← Head: fix-add-lock-for-local-segment-access

📝 Commits (1)

901085f refactor(transfer_metadata): improve thread safety and reduce lock contention

📊 Changes

1 file changed (+86 additions, -24 deletions)

View changed files

📝 mooncake-transfer-engine/src/transfer_metadata.cpp (+86 -24)

📄 Description

Summary

This PR fixes a race condition in TransferMetadata::receivePeerMetadata and TransferMetadata::getSegmentDesc functions where concurrent access to segment_id_to_desc_map_ without proper locking could cause crashes under high concurrency scenarios.

As reported in Issue #1369, when running with high RDMA concurrency (~2048 concurrent sends, ~1024 concurrent receives), the SocketHandShakePlugin crashes during handshake processing with:

Segmentation fault in Json::Value::Value(std::string const&)
Json::LogicError with message length too big for prefixing

Root Cause

The receivePeerMetadata and getSegmentDesc functions accessed segment_id_to_desc_map_[LOCAL_SEGMENT_ID] without holding segment_lock_, while other functions (addLocalMemoryBuffer, removeLocalMemoryBuffer) modify this map and the SegmentDesc contents under write lock protection. This creates a race condition where:

The daemon thread calls receivePeerMetadata to handle handshake requests
Other threads simultaneously modify segment_id_to_desc_map_[LOCAL_SEGMENT_ID] via addLocalMemoryBuffer/removeLocalMemoryBuffer
The unsynchronized read can access corrupted or partially modified data

Solution

Added RWSpinlock::ReadGuard protection when accessing segment_id_to_desc_map_ in both affected functions receivePeerMetadata and getSegmentDesc(in p2p handshake mode).

Type of Change

Types
- Bug fix
- New feature
  - Transfer Engine
  - Mooncake Store
  - Mooncake EP
  - Integration
  - P2P Store
  - Python Wheel
- Breaking change
- CI/CD
- Documentation update
- Other

How Has This Been Tested?

Reproduction

The race condition can be reproduced by creating a test that:

Writer threads: Continuously call addLocalMemoryBuffer/removeLocalMemoryBuffer to modify segment_id_to_desc_map_[LOCAL_SEGMENT_ID]
Reader threads: Continuously call getSegmentDescByID(LOCAL_SEGMENT_ID) and access the returned SegmentDesc fields (especially buffers vector and string fields)
Since race conditions are probabilistic and may not always cause visible crashes, ThreadSanitizer should be used to reliably detect the issue:

# Build with TSan
cmake .. -DCMAKE_BUILD_TYPE=Debug \
    -DCMAKE_CXX_FLAGS="-fsanitize=thread -g" \
    -DCMAKE_C_FLAGS="-fsanitize=thread -g" \
    -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=thread"

# Run test
TSAN_OPTIONS="halt_on_error=0" ./your_test

Checklist

I have performed a self-review of my own code.
I have updated the documentation.
I have added tests to prove my changes are effective.

_{🔄 This issue represents a GitHub Pull Request. It cannot be merged through Gitea due to API limitations.}

## 📋 Pull Request Information **Original PR:** https://github.com/kvcache-ai/Mooncake/pull/1373 **Author:** [@staryxchen](https://github.com/staryxchen) **Created:** 1/14/2026 **Status:** ✅ Merged **Merged:** 1/23/2026 **Merged by:** [@alogfans](https://github.com/alogfans) **Base:** `main` ← **Head:** `fix-add-lock-for-local-segment-access` --- ### 📝 Commits (1) - [`901085f`](https://github.com/kvcache-ai/Mooncake/commit/901085f96cac1f932e7b1ee15612efbf18aa2177) refactor(transfer_metadata): improve thread safety and reduce lock contention ### 📊 Changes **1 file changed** (+86 additions, -24 deletions) <details> <summary>View changed files</summary> 📝 `mooncake-transfer-engine/src/transfer_metadata.cpp` (+86 -24) </details> ### 📄 Description ## Summary This PR fixes a race condition in `TransferMetadata::receivePeerMetadata` and `TransferMetadata::getSegmentDesc` functions where concurrent access to `segment_id_to_desc_map_` without proper locking could cause crashes under high concurrency scenarios. As reported in Issue #1369, when running with high RDMA concurrency (~2048 concurrent sends, ~1024 concurrent receives), the `SocketHandShakePlugin` crashes during handshake processing with: - **Segmentation fault** in `Json::Value::Value(std::string const&)` - **Json::LogicError** with message `length too big for prefixing` ## Root Cause The `receivePeerMetadata` and `getSegmentDesc` functions accessed `segment_id_to_desc_map_[LOCAL_SEGMENT_ID]` without holding `segment_lock_`, while other functions (`addLocalMemoryBuffer`, `removeLocalMemoryBuffer`) modify this map and the `SegmentDesc` contents under write lock protection. This creates a race condition where: 1. The daemon thread calls `receivePeerMetadata` to handle handshake requests 2. Other threads simultaneously modify `segment_id_to_desc_map_[LOCAL_SEGMENT_ID]` via `addLocalMemoryBuffer`/`removeLocalMemoryBuffer` 3. The unsynchronized read can access corrupted or partially modified data ## Solution Added `RWSpinlock::ReadGuard` protection when accessing `segment_id_to_desc_map_` in both affected functions `receivePeerMetadata` and `getSegmentDesc`(in p2p handshake mode). ## Type of Change * Types - [x] Bug fix - [ ] New feature - [ ] Transfer Engine - [ ] Mooncake Store - [ ] Mooncake EP - [ ] Integration - [ ] P2P Store - [ ] Python Wheel - [ ] Breaking change - [ ] CI/CD - [ ] Documentation update - [ ] Other ## How Has This Been Tested? ### Reproduction The race condition can be reproduced by creating a test that: 1. Writer threads: Continuously call `addLocalMemoryBuffer`/`removeLocalMemoryBuffer` to modify `segment_id_to_desc_map_[LOCAL_SEGMENT_ID]` 2. Reader threads: Continuously call `getSegmentDescByID(LOCAL_SEGMENT_ID)` and access the returned `SegmentDesc` fields (especially `buffers` vector and string fields) 3. Since race conditions are probabilistic and may not always cause visible crashes, ThreadSanitizer should be used to reliably detect the issue: ``` # Build with TSan cmake .. -DCMAKE_BUILD_TYPE=Debug \ -DCMAKE_CXX_FLAGS="-fsanitize=thread -g" \ -DCMAKE_C_FLAGS="-fsanitize=thread -g" \ -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=thread" # Run test TSAN_OPTIONS="halt_on_error=0" ./your_test ``` ## Checklist - [x] I have performed a self-review of my own code. - [x] I have updated the documentation. - [x] I have added tests to prove my changes are effective. --- <sub>🔄 This issue represents a GitHub Pull Request. It cannot be merged through Gitea due to API limitations.</sub>

bsakaguchi

2026-10-29 21:33:32 +00:00