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d8168a2021
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Fix C7 TLS SLL header restoration regression + Document Larson MT race condition
## Bug Fix: Restore C7 Exception in TLS SLL Push
**File**: `core/box/tls_sll_box.h:309`
**Problem**: Commit 25d963a4a (Code Cleanup) accidentally reverted the C7 fix by changing:
```c
if (class_idx != 0 && class_idx != 7) { // CORRECT (commit 8b67718bf)
if (class_idx != 0) { // BROKEN (commit 25d963a4a)
```
**Impact**: C7 (1024B class) header restoration in TLS SLL push overwrote next pointer at base[0], causing corruption.
**Fix**: Restored `&& class_idx != 7` check to prevent header restoration for C7.
**Why C7 Needs Exception**:
- C7 uses offset=0 (stores next pointer at base[0])
- User pointer is at base+1
- Next pointer MUST NOT be overwritten by header restoration
- C1-C6 use offset=1 (next at base[1]), so base[0] header restoration is safe
## Investigation: Larson MT Race Condition (SEPARATE ISSUE)
**Finding**: Larson still crashes with 3+ threads due to UNRELATED multi-threading race condition in unified cache freelist management.
**Root Cause**: Non-atomic freelist operations in `TinySlabMeta`:
```c
typedef struct TinySlabMeta {
void* freelist; // ❌ NOT ATOMIC
uint16_t used; // ❌ NOT ATOMIC
} TinySlabMeta;
```
**Evidence**:
```
1 thread: ✅ PASS (1.88M - 41.8M ops/s)
2 threads: ✅ PASS (24.6M ops/s)
3 threads: ❌ SEGV (race condition)
4+ threads: ❌ SEGV (race condition)
```
**Status**: C7 fix is CORRECT. Larson crash is separate MT issue requiring atomic freelist implementation.
## Documentation Added
Created comprehensive investigation reports:
- `LARSON_CRASH_ROOT_CAUSE_REPORT.md` - Full technical analysis
- `LARSON_DIAGNOSTIC_PATCH.md` - Implementation guide
- `LARSON_INVESTIGATION_SUMMARY.md` - Executive summary
- `LARSON_QUICK_REF.md` - Quick reference
- `verify_race_condition.sh` - Automated verification script
## Next Steps
Implement atomic freelist operations for full MT safety (7-9 hour effort):
1. Make `TinySlabMeta.freelist` atomic with CAS loop
2. Audit 87 freelist access sites
3. Test with Larson 8+ threads
🔧 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
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2025-11-22 02:15:34 +09:00 |
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