Race Check

Race Condition Analyzer

Detect and diagnose race conditions using ThreadSanitizer (CPU threads) and NVIDIA Compute Sanitizer memcheck (GPU shared memory).

Reference: See .claude/skills/_shared/build-and-query.md for shared infrastructure (build modes, query execution, multi-run consistency check, autonomy mode, change tracking, debug log conventions).

Workflow

1. Gather context:

   • Determine the scope: SQL query or specific unit test (from $ARGUMENTS)
   • Ask about data format if SQL query (DuckDB or Parquet)
   • Determine autonomy mode: interactive (default), autonomous, or semi-autonomous

2. Multi-run consistency check (quick pre-screen): Run the query/test 5 times and compare results:

   for i in $(seq 1 5); do
     export SIRIUS_LOG_DIR=build/release/log/run_${i}_$(date +%s)
     mkdir -p $SIRIUS_LOG_DIR
     build/release/duckdb <db_path> -c "CALL gpu_execution('...');" > /tmp/claude-1000/result_${i}.txt 2>&1
   done
   

   Compare all results pairwise. If any differ, confirm non-deterministic behavior.

3. Phase 1: CPU thread race detection with ThreadSanitizer Ask user before proceeding. Warn about 5-15x overhead.

   • Build with clang-debug + TSan flags (TSan requires clang-debug -- cannot use relwithdebinfo). Important: TSan and ASan cannot be used simultaneously. This skill uses TSan only -- explicitly disable ASan:

     CMAKE_BUILD_PARALLEL_LEVEL=$(nproc) make clang-debug EXTRA_CMAKE_FLAGS="-DENABLE_TSAN=ON -DENABLE_SANITIZER=0"
     

   • Run the reproduction case:

     TSAN_OPTIONS="second_deadlock_stack=1:history_size=7" build/clang-debug/duckdb <db_path> <<'EOF'
     CALL gpu_execution('<QUERY>');
     EOF
     

   • Parse TSan output for:
     • Data race reports: two threads accessing same memory, at least one write
     • Lock order inversions: potential deadlock patterns
     • Thread leak reports
   • For each race found:
     • Read both code locations involved
     • Analyze the shared data structure and synchronization (or lack thereof)
     • Check if existing mutexes/atomics should cover this access

4. Phase 2: GPU memory race detection (ask user before proceeding)

   • Build with debug symbols (relwithdebinfo or clang-debug):
   • Run with Compute Sanitizer:

     compute-sanitizer --tool memcheck build/<preset>/duckdb <<'EOF'
     CALL gpu_execution('<QUERY>');
     EOF
     

   • Parse output for memory access hazards (races manifesting as out-of-bounds, use-after-free)
   • Cross-reference with CUDA kernel source in src/cuda/

5. Suggest fixes: For CPU races:

   • std::mutex / std::lock_guard for critical sections
   • std::atomic for simple shared counters/flags
   • Redesign to eliminate sharing (thread-local storage, message passing)

   For GPU races:

   • __syncthreads() for block-level synchronization
   • __syncwarp() for warp-level synchronization
   • Shared memory access pattern redesign
   • Consider performance implications of each fix

6. Iterative fix loop (behavior depends on autonomy mode):

   • Apply the fix, rebuild, and re-run with TSan/Compute Sanitizer
   • If races still reported (same or new), analyze and fix
   • Run multi-run consistency check to verify the fix eliminates non-determinism
   • Continue until: no more races reported, max iterations reached, or user intervenes
   • Present final summary: which races found, which fixes applied, verification results

Key Considerations

• TSan overhead: 5-15x slowdown. Warn user about expected execution time.
• Stream-per-thread model: Sirius uses one CUDA stream per GPU thread. Races may involve CUDA stream synchronization issues.
• Common race hotspots in Sirius:
  • GPU thread pool and task queue (src/pipeline/)
  • Data Repository concurrent access (src/data/)
  • Memory Reservation Manager (src/memory/)
  • Task Creator polling and state updates
• TSan + CUDA: TSan may produce false positives for GPU memory operations. Focus on CPU-side synchronization issues first.
• Cannot combine TSan + ASan: They use incompatible runtime instrumentation. Run them in separate builds.

Scope

Only analyze code in namespace sirius plus exceptions listed in shared build-and-query.md. Ignore legacy namespace duckdb code.