Implementation:LMCache LMCache TTL Lock

Knowledge Sources	LMCache
Domains	Concurrency, Storage Management
Last Updated	2026-02-09 00:00 GMT

Overview

Header file declaring the TTLLock class, a thread-safe counting lock with automatic time-to-live expiration.

Description

The TTLLock class provides a reentrant-style counting lock where each lock() call increments a counter and each unlock() call decrements it. The lock is considered held only when the counter is positive AND the TTL has not expired. All operations are implemented with lock-free atomic operations using std::atomic with sequential consistency ordering for mutations and acquire ordering for reads. The class uses std::chrono::steady_clock for monotonic time measurement.

Usage

Include this header in C++ storage manager code to protect cache entries with time-bounded locks. The TTL mechanism prevents deadlocks from clients that crash or disconnect without releasing the lock, making it suitable for distributed cache coordination scenarios.

Code Reference

Source Location

• Repository: LMCache
• File: csrc/storage_manager/ttl_lock.h
• Lines: 1-90

Signature

namespace lmcache {
namespace storage_manager {

class TTLLock {
 public:
  explicit TTLLock(uint32_t ttl_second = 300);
  void lock();
  void unlock();
  bool is_locked() const;
  void reset();

 private:
  static int64_t now_ms();
  static int64_t to_ms(const TimePoint& tp);

  std::atomic<int64_t> counter_;
  std::atomic<int64_t> expiration_ms_;
  const int64_t ttl_ms_;
};

}  // namespace storage_manager
}  // namespace lmcache

Import

#include "ttl_lock.h"

I/O Contract

Inputs

Name	Type	Required	Description
ttl_second	uint32_t	No (default 300)	TTL duration in seconds; the lock auto-expires after this period of inactivity

Outputs

Name	Type	Description
counter_	std::atomic<int64_t>	Internal atomic counter tracking the number of active lock acquisitions
expiration_ms_	std::atomic<int64_t>	Internal atomic timestamp (milliseconds since steady_clock epoch) of lock expiration
ttl_ms_	const int64_t	Immutable TTL duration in milliseconds, computed from ttl_second at construction

Usage Examples

#include "ttl_lock.h"

using namespace lmcache::storage_manager;

// Default TTL of 300 seconds
TTLLock lock;

// Custom TTL of 10 seconds
TTLLock short_lock(10);