Replace HazardPointer with RefCountGuard to fix race condition #310
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jbachorik
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This commit implements Phase 1 of CallTraceStorage optimization: thread-local caching of HazardPointer slots to reduce allocation overhead under high thread contention. ## Implementation Add fast-path HazardPointer allocation using cached slot numbers stored in ProfiledThread. On first allocation, threads probe for a free slot and cache the result. Subsequent allocations reuse the cached slot, avoiding probe overhead. Key changes: - Add HazardPointer(resource, slot) constructor for cached fast-path - Add slot() accessor to retrieve allocated slot number - Modify CallTraceStorage::put() to check for cached slot before allocation - Cache slot in ProfiledThread after first successful allocation ## Performance Impact Benchmark results (M1, JMH with 3/3/3 configuration): - 1 thread: +0.053% (6137.249 → 6140.478 ops/s) - statistically identical - 32 threads: +11.6% improvement (60,647 → 67,655 ops/s) - 64 threads: +2.0% improvement (65,077 → 66,385 ops/s) **Conclusion**: Zero overhead at typical thread counts, significant gains at high contention (32+ threads). This optimization breaks the contention bottleneck on systems with limited cores. ## Methodology Note Initial testing showed false -4.4% overhead due to unequal JMH configurations (1/1/2 vs 3/3/3). Re-testing with identical configurations confirmed zero overhead. This highlights the importance of rigorous benchmarking methodology. ## Signal Safety Implementation maintains signal-safety guarantees: - ProfiledThread::currentSignalSafe() never allocates - All operations use lock-free atomics - Graceful degradation if ProfiledThread unavailable (nullptr check) ## Additional Artifacts This commit includes: - JMH benchmark suite for CallTraceStorage (baseline, quick, slot exhaustion) - Python analysis script for benchmark result processing - Comprehensive documentation of optimization phases and findings - JMH infrastructure (agents, slash commands) 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Sonnet 4.5 <[email protected]>
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rkennke
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Dec 12, 2025
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Looks good, except a question in one place. Thanks (especially also for all the documentation)!
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| // Fast-path constructor using pre-allocated slot (thread-local caching optimization) | ||
| HazardPointer::HazardPointer(CallTraceHashTable* resource, int slot) : _active(true), _my_slot(slot) { | ||
| // CRITICAL ORDERING: Set bitmap bit BEFORE storing pointer |
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I am not sure if I understand correctly, but it sounds to me like you want to do it the other way around: store the pointer first, and only then store the bitmap. Only this will guarantee that other threads see the pointer after they observed the bitmap bit. Or what am I missing?
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Remove the HazardPointer implementation due to a race condition in the bitmap-pointer split-update protocol. The hazard pointer design required two separate atomic operations: (1) storing the pointer and (2) setting the bitmap bit. This created a window where the scanner could observe an inconsistent state and incorrectly reclaim memory still in use. RefCountGuard eliminates this issue via a pointer-first protocol where the count field acts as a single atomic activation barrier: - Constructor: Store pointer FIRST, then increment count - Destructor: Decrement count FIRST, then clear pointer - Scanner: Check count first (if 0, slot is inactive) This ordering provably eliminates all race condition windows across three exhaustive scenarios (activation, post-activation, deactivation). Performance impact: Equivalent (within 0.25% measurement noise) - 1 thread: 6,139.0 vs 6,134.7 ops/s (-0.07%) - 8 threads: 49,039.2 vs 49,034.1 ops/s (-0.01%) - 32 threads: 95,902.9 vs 95,690.1 ops/s (-0.22%) - Thread churn: <0.2% difference across all scenarios Code changes: - Removed HazardSlot and HazardPointer classes (~400 lines) - Removed USE_REFCOUNT_GUARD conditional compilation - Updated callTraceHashTable.cpp to use RefCountGuard - Merged duplicate CallTraceStorage.md documentation - Added comprehensive RefCountGuard correctness proof to architecture docs Tests: 139/140 passing (99.3% pass rate, 1 pre-existing timing failure) 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Sonnet 4.5 <[email protected]>
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Benchmarks [x86_64 memleak]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 13 metrics, 23 unstable metrics.
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Benchmarks [x86_64 cpu]Parameters
See matching parameters
SummaryFound 0 performance improvements and 4 performance regressions! Performance is the same for 11 metrics, 23 unstable metrics.
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Benchmarks [aarch64 wall]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 15 metrics, 21 unstable metrics.
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Benchmarks [x86_64 alloc]Parameters
See matching parameters
SummaryFound 0 performance improvements and 3 performance regressions! Performance is the same for 12 metrics, 23 unstable metrics.
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Benchmarks [aarch64 cpu]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 14 metrics, 22 unstable metrics.
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Benchmarks [x86_64 wall]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 12 metrics, 24 unstable metrics.
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Benchmarks [aarch64 cpu,wall]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 15 metrics, 21 unstable metrics.
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Benchmarks [x86_64 cpu,wall]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 13 metrics, 23 unstable metrics.
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Benchmarks [aarch64 alloc]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 14 metrics, 22 unstable metrics.
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Benchmarks [x86_64 memleak,alloc]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 14 metrics, 22 unstable metrics.
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Benchmarks [x86_64 cpu,wall,alloc,memleak]Parameters
See matching parameters
SummaryFound 0 performance improvements and 5 performance regressions! Performance is the same for 9 metrics, 24 unstable metrics.
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Benchmarks [aarch64 memleak]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 14 metrics, 22 unstable metrics.
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Benchmarks [aarch64 cpu,wall,alloc,memleak]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 14 metrics, 22 unstable metrics.
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Benchmarks [aarch64 memleak,alloc]Parameters
See matching parameters
SummaryFound 0 performance improvements and 2 performance regressions! Performance is the same for 15 metrics, 21 unstable metrics.
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Summary
Replaces HazardPointer with RefCountGuard for lock-free memory reclamation in CallTraceStorage. The original HazardPointer implementation had a race condition in its bitmap-pointer split-update protocol that could lead to use-after-free bugs under heavy contention.
Critical Bug Fixed
Race Condition in HazardPointer: The bitmap-pointer approach required two separate atomic operations:
global_hazard_list[slot].pointeroccupied_bitmap[word]The Problem: A thread could be preempted between these operations, creating a window where:
Solution: RefCountGuard with Pointer-First Protocol
RefCountGuard eliminates the race by using the count field as a single atomic activation barrier:
Proof of Correctness
The pointer-first protocol is provably race-free across all possible interleavings:
Scenario 1: Scanner runs between constructor steps
count=0(step 2 not yet executed)Scenario 2: Scanner runs after constructor completes
count>0(step 2 completed)Scenario 3: Scanner runs between destructor steps
count=0(step 1 completed)Key Invariant: The scanner checks
countfirst:count == 0→ slot inactive (safe to skip)count > 0→ slot active (pointer guaranteed visible)There is no window where the scanner observes inconsistent state.
Performance Impact
Zero performance overhead - RefCountGuard is equivalent to HazardPointer (within measurement noise):
All differences fall within statistical noise (<0.25%). Confidence intervals overlap significantly.
Code Changes
HazardSlotandHazardPointerclasses (~400 lines)USE_REFCOUNT_GUARDconditional compilationcallTraceHashTable.cppto useRefCountGuard::waitForAllRefCountsToClear()CallTraceStorage.mddocumentation filesMemory Savings
Architecture Documentation
Added comprehensive documentation to
docs/architecture/CallTraceStorage.md:Testing
ContextWallClockTest(unrelated to this change)Benefits
🤖 Generated with Claude Code