UNDF: UNDF-2026-000000524

rocksdb-001: LockInfo.txn_ids linear scan — O(T²) shared-lock churn

Severity: HIGH

File

utilities/transactions/lock/point/point_lock_manager.cc

Defect

LockInfo::txn_ids is an autovector<TransactionID> (default 8 inline slots, grows to heap). Three hot-path functions perform std::find() — O(T) linear scan — over this vector, where T = number of concurrent transactions sharing a read lock on a single key:

Line	Function	Call site
791	`PointLockManager::UnLockKey`	called in a loop over all keys held by the unlocking txn
1513	`PerKeyPointLockManager::AcquireLocked`	shared-lock reentrant check
1706	`PerKeyPointLockManager::UnLockKey`	called in a loop over all keys held by the unlocking txn

Complexity

Per-key unlock: O(T) scan — T concurrent readers per key
Per-transaction unlock: O(K × T) — K keys in the transaction, T readers per key
Under high shared-lock concurrency (OLAP, batch reads, read-committed workloads): T can reach hundreds; K can be thousands per transaction
Overall unlock path becomes O(K × T) = O(N²) in the worst case

Root cause

struct LockInfo {
  autovector<TransactionID> txn_ids;   // ← vector, not set
  ...
};

// In UnLockKey (both lock managers):
auto txn_it = std::find(txns.begin(), txns.end(), txn_id);  // O(T)

// In PerKeyPointLockManager::AcquireLocked:
auto lock_it = std::find(lock_info.txn_ids.begin(),
                         lock_info.txn_ids.end(), my_txn_id); // O(T)

Fix

Replace autovector<TransactionID> txn_ids with std::unordered_set<TransactionID> txn_ids.

find() becomes O(1) average
push_back() → insert()
erase(iterator) → erase(value) (the RemoveTransaction helper disappears)
txn_ids[0] → *txn_ids.begin() (exclusive lock path already asserts size==1)
Iteration in FillWaitIds / deadlock detection remains O(T) — unchanged

Speedup

Benchmark (unit test): T=1000 concurrent readers, K=1 key unlock

Slow path (autovector + std::find): O(T) = 1000 ops per unlock
Fast path (unordered_set): O(1) = 1 op per unlock
Speedup: ~1000x at T=1000

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