2nd/10_Wiki/Topics/Coding/CS_Lock_Free_Patterns.md

id, title, category, status, source_trust_level, verification_status, created_at, updated_at, tags, tech_stack, applied_in, aliases

id	title	category	status	source_trust_level	verification_status	created_at	updated_at	tags	tech_stack	applied_in	aliases
cs-lock-free-patterns	Lock-Free Patterns — atomic / CAS / queue	Coding	draft	B	conceptual	2026-05-09	2026-05-09	cs concurrency vibe-coding	language applicable_to C++ / Rust CS Backend		lock-free wait-free CAS compare and swap atomic MPSC MPMC ABA problem

Lock-Free Patterns

Lock = 느린, deadlock 위험. Atomic + CAS = lock-free. Wait-free 가 강함.

📖 핵심 개념

• Atomic: 원자 operation.
• CAS: Compare-And-Swap.
• Lock-free: 1+ thread 가 progress.
• Wait-free: 매 thread 가 progress.

💻 코드 패턴

Atomic counter

use std::sync::atomic::{AtomicU64, Ordering};

let counter = AtomicU64::new(0);

// Thread-safe
counter.fetch_add(1, Ordering::SeqCst);

let val = counter.load(Ordering::SeqCst);

CAS (Compare-And-Swap)

let atomic = AtomicU64::new(0);

loop {
    let old = atomic.load(Ordering::SeqCst);
    let new = old + 1;
    if atomic.compare_exchange(old, new, Ordering::SeqCst, Ordering::SeqCst).is_ok() {
        break;
    }
    // 다른 thread 가 변경. Retry.
}

→ "if 같으면 set". Atomic.

Memory order

Relaxed: 매 thread 의 자체 ordering 만.
Acquire / Release: 인접 op 의 ordering.
SeqCst: total ordering (제일 안전, 느린).

TS / JavaScript SharedArrayBuffer

const sab = new SharedArrayBuffer(8);
const view = new Int32Array(sab);

// Atomic
Atomics.add(view, 0, 1);
Atomics.compareExchange(view, 0, oldVal, newVal);
Atomics.wait(view, 0, 0);
Atomics.notify(view, 0);

→ Worker 간 shared.

Lock-free queue (Michael-Scott)

struct Node<T> {
    value: T,
    next: AtomicPtr<Node<T>>,
}

struct Queue<T> {
    head: AtomicPtr<Node<T>>,
    tail: AtomicPtr<Node<T>>,
}

impl<T> Queue<T> {
    fn enqueue(&self, value: T) {
        let new_node = Box::into_raw(Box::new(Node { value, next: AtomicPtr::new(null_mut()) }));
        
        loop {
            let tail = self.tail.load(SeqCst);
            let next = unsafe { (*tail).next.load(SeqCst) };
            
            if next.is_null() {
                if unsafe { (*tail).next.compare_exchange(null_mut(), new_node, SeqCst, SeqCst) }.is_ok() {
                    self.tail.compare_exchange(tail, new_node, SeqCst, SeqCst).ok();
                    return;
                }
            } else {
                self.tail.compare_exchange(tail, next, SeqCst, SeqCst).ok();
            }
        }
    }
}

→ Multi-producer multi-consumer.

MPSC (single consumer)

// Crossbeam (Rust)
use crossbeam::channel::unbounded;

let (tx, rx) = unbounded();

// Multiple producer
thread::spawn(move || tx.send(1).unwrap());

// Single consumer
let v = rx.recv().unwrap();

→ Lock-free queue.

Crossbeam (Rust)

- Unbounded / bounded channel.
- ArrayQueue / SegQueue.
- AtomicCell.
- Epoch-based GC.

ABA problem

Thread A:
- Load X = 1.
Thread B:
- Set X = 2.
- Set X = 1.
Thread A:
- CAS X (expect 1) = success.
- → wrong (X 가 변경 됐는데 안 알아).

// Solution: tagged pointer (counter + value).
let old: (u64, T) = atomic.load();   // counter + value
let new = (old.0 + 1, new_value);
atomic.compare_exchange(old, new, ...);

→ Counter 가 increment. 같은 value 가 다른 counter.

Memory reclamation

Lock-free 의 함정:
- Free 한 node 의 다른 thread 가 still 참조.

Solution:
- Hazard pointer.
- Epoch-based GC (crossbeam).
- RCU (Read-Copy-Update).

Lock-free hash map

DashMap (Rust):
- Sharded (16 partition).
- 매 partition 의 Lock-free or fine lock.
- 큰 throughput.

use dashmap::DashMap;
let map: DashMap<String, u32> = DashMap::new();

map.insert("key".into(), 42);
let v = map.get("key");

Atomic reference counting (Rust Arc)

use std::sync::Arc;
use std::sync::atomic::AtomicUsize;

let arc = Arc::new(MyData);
let arc2 = arc.clone();   // count++

// Drop = count--. 0 = free.

Java AtomicInteger / AtomicReference

AtomicInteger counter = new AtomicInteger(0);
counter.incrementAndGet();

AtomicReference<Node> head = new AtomicReference<>();
head.compareAndSet(expected, newValue);

Go (sync/atomic)

import "sync/atomic"

var counter int64
atomic.AddInt64(&counter, 1)

old := atomic.LoadInt64(&counter)
atomic.CompareAndSwapInt64(&counter, old, old + 1)

Wait-free

Lock-free: 1+ thread 가 progress.
Wait-free: 매 thread 가 progress.

Wait-free = stronger. 보통 lock-free 가 충분.

Software Transactional Memory (STM)

-- Haskell
atomically $ do
  x <- readTVar var
  writeTVar var (x + 1)

;; Clojure
(dosync
  (alter counter inc))

→ Optimistic concurrency. Conflict 시 retry.

Use case

- Counter (statistics).
- Single producer queue (logging).
- Lock-free hash (cache).
- Reference count (shared ownership).
- LSM tree memtable.
- WAL (write-ahead log).

vs Mutex

Mutex:
- Simple.
- Block 가능 (sleep).
- Deadlock 위험.

Lock-free:
- Complex.
- Non-blocking.
- 더 빠름 (high contention).
- 더 어려움.

→ 작은 critical section = mutex.
큰 contention = lock-free.

Lock contention 측정

perf record / perf report.
- Hot mutex 발견.
- Lock-free 의 candidate.

→ Profile 가 답.

Lock-free의 함정

- ABA problem: tagged pointer.
- Memory reclamation: hazard pointer / epoch.
- Memory ordering: SeqCst 가 안전 default.
- Algorithm 가 매우 어려움.
- Test 어려움 (race rare).

→ Library (Crossbeam / Folly) 사용. 자체 X.

Folly (Facebook C++)

#include <folly/MPMCQueue.h>
folly::MPMCQueue<int> queue(1024);
queue.write(42);
int x;
queue.read(x);

LMAX Disruptor (Java)

RingBuffer<Event> ringBuffer = ...;

long sequence = ringBuffer.next();
Event event = ringBuffer.get(sequence);
event.setValue(42);
ringBuffer.publish(sequence);

→ 매우 빠른 (millions / sec).

Database 의 lock-free

MVCC: lock-free read.
LSM tree: lock-free memtable insert.
B-tree (some): lock-free split.

→ Modern DB 의 핵심.

When use?

✓ High contention.
✓ Real-time (no block).
✓ Embedded / kernel.

✗ Low contention (mutex 충분).
✗ Logic 가 복잡.
✗ Team 가 expert X.

→ Profile 후. Library 사용.

Real-world

• Crossbeam (Rust).
• Folly (C++ Facebook).
• DashMap (Rust hash).
• LMAX Disruptor (Java).
• Java concurrent.atomic.

🤔 의사결정 기준

작업	추천
Counter	Atomic
Queue	Lock-free queue (Crossbeam)
Hash map	DashMap / ConcurrentHashMap
Ref count	Arc / shared_ptr
큰 throughput	LMAX Disruptor
Database	MVCC + LSM
Complex sync	STM (Haskell / Clojure)

❌ 안티패턴

• 자체 lock-free queue: bug.
• No memory order: race.
• No ABA fix: corruption.
• No memory reclamation: leak / use-after-free.
• Mutex on hot path: contention.

🤖 LLM 활용 힌트

• Library 사용 (자체 X).
• ABA + memory reclamation 가 큰 함정.
• Profile + measure.
• MVCC + LSM 가 DB 의 lock-free.

🔗 관련 문서

• CS_LockFree_Atomic
• CS_MVCC_Concurrency
• CS_Skip_List_Persistent_Data