Mutex & Synchronization — Thread-Safe Primitives

Low-level synchronization primitives for when actors are too slow or heavyweight.

When to Use Mutex vs Actor

Need	Use	Reason
Microsecond operations	Mutex	No async hop overhead
Protect single property	Mutex	Simpler, faster
Complex async workflows	Actor	Proper suspension handling
Suspension points needed	Actor	Mutex can't suspend
Shared across modules	Mutex	Sendable, no await needed
High-frequency counters	Atomic	Lock-free performance

API Reference

Mutex (iOS 18+ / Swift 6)

swift

import Synchronization

let mutex = Mutex<Int>(0)

// Read
let value = mutex.withLock { $0 }

// Write
mutex.withLock { $0 += 1 }

// Non-blocking attempt
if let value = mutex.withLockIfAvailable({ $0 }) {
    // Got the lock
}

Properties:

•Generic over protected value
•Sendable — safe to share across concurrency boundaries
•Closure-based access only (no lock/unlock methods)

OSAllocatedUnfairLock (iOS 16+)

swift

import os

let lock = OSAllocatedUnfairLock(initialState: 0)

// Closure-based (recommended)
lock.withLock { state in
    state += 1
}

// Traditional (same-thread only)
lock.lock()
defer { lock.unlock() }
// access protected state

Properties:

•Heap-allocated, stable memory address
•Non-recursive (can't re-lock from same thread)
•Sendable

Atomic Types (iOS 18+)

swift

import Synchronization

let counter = Atomic<Int>(0)

// Atomic increment
counter.wrappingAdd(1, ordering: .relaxed)

// Compare-and-swap
let (exchanged, original) = counter.compareExchange(
    expected: 0,
    desired: 42,
    ordering: .acquiringAndReleasing
)

Patterns

Pattern 1: Thread-Safe Counter

swift

final class Counter: Sendable {
    private let mutex = Mutex<Int>(0)

    var value: Int { mutex.withLock { $0 } }
    func increment() { mutex.withLock { $0 += 1 } }
}

Pattern 2: Sendable Wrapper

swift

final class ThreadSafeValue<T: Sendable>: @unchecked Sendable {
    private let mutex: Mutex<T>

    init(_ value: T) { mutex = Mutex(value) }

    var value: T {
        get { mutex.withLock { $0 } }
        set { mutex.withLock { $0 = newValue } }
    }
}

Pattern 3: Fast Sync Access in Actor

swift

actor ImageCache {
    // Mutex for fast sync reads without actor hop
    private let mutex = Mutex<[URL: Data]>([:])

    nonisolated func cachedSync(_ url: URL) -> Data? {
        mutex.withLock { $0[url] }
    }

    func cacheAsync(_ url: URL, data: Data) {
        mutex.withLock { $0[url] = data }
    }
}

Pattern 4: Lock-Free Counter with Atomic

swift

final class FastCounter: Sendable {
    private let _value = Atomic<Int>(0)

    var value: Int { _value.load(ordering: .relaxed) }

    func increment() {
        _value.wrappingAdd(1, ordering: .relaxed)
    }
}

Pattern 5: iOS 16 Fallback

swift

#if compiler(>=6.0)
import Synchronization
typealias Lock<T> = Mutex<T>
#else
import os
// Use OSAllocatedUnfairLock for iOS 16-17
#endif

Danger: Mixing with Swift Concurrency

Never Hold Locks Across Await

swift

// ❌ DEADLOCK RISK
mutex.withLock {
    await someAsyncWork()  // Task suspends while holding lock!
}

// ✅ SAFE: Release before await
let value = mutex.withLock { $0 }
let result = await process(value)
mutex.withLock { $0 = result }

Why Semaphores/RWLocks Are Unsafe

Swift's cooperative thread pool has limited threads. Blocking primitives exhaust the pool:

swift

// ❌ DANGEROUS: Blocks cooperative thread
let semaphore = DispatchSemaphore(value: 0)
Task {
    semaphore.wait()  // Thread blocked, can't run other tasks!
}

// ✅ Use async continuation instead
await withCheckedContinuation { continuation in
    // Non-blocking callback
    callback { continuation.resume() }
}

os_unfair_lock Danger

Never use os_unfair_lock directly in Swift — it can be moved in memory:

swift

// ❌ UNDEFINED BEHAVIOR: Lock may move
var lock = os_unfair_lock()
os_unfair_lock_lock(&lock)  // Address may be invalid

// ✅ Use OSAllocatedUnfairLock (heap-allocated, stable address)
let lock = OSAllocatedUnfairLock()

Decision Tree

code

Need synchronization?
├─ Lock-free operation needed?
│  └─ Simple counter/flag? → Atomic
│  └─ Complex state? → Mutex
├─ iOS 18+ available?
│  └─ Yes → Mutex
│  └─ No, iOS 16+? → OSAllocatedUnfairLock
├─ Need suspension points?
│  └─ Yes → Actor (not lock)
├─ Cross-await access?
│  └─ Yes → Actor (not lock)
└─ Performance-critical hot path?
   └─ Yes → Mutex/Atomic (not actor)

Common Mistakes

Mistake 1: Using Lock for Async Coordination

swift

// ❌ Locks don't work with async
let mutex = Mutex<Bool>(false)
Task {
    await someWork()
    mutex.withLock { $0 = true }  // Race condition still possible
}

// ✅ Use actor or async state
actor AsyncState {
    var isComplete = false
    func complete() { isComplete = true }
}

Mistake 2: Recursive Locking Attempt

swift

// ❌ Deadlock — OSAllocatedUnfairLock is non-recursive
lock.withLock {
    doWork()  // If doWork() also calls withLock → deadlock
}

// ✅ Refactor to avoid nested locking
let data = lock.withLock { $0.copy() }
doWork(with: data)

Mistake 3: Mixing Lock Styles

swift

// ❌ Don't mix lock/unlock with withLock
lock.lock()
lock.withLock { /* ... */ }  // Deadlock!
lock.unlock()

// ✅ Pick one style
lock.withLock { /* all work here */ }

Memory Ordering Quick Reference

Ordering	Read	Write	Use Case
`.relaxed`	Yes	Yes	Counters, no dependencies
`.acquiring`	Yes	-	Load before dependent ops
`.releasing`	-	Yes	Store after dependent ops
`.acquiringAndReleasing`	Yes	Yes	Read-modify-write
`.sequentiallyConsistent`	Yes	Yes	Strongest guarantee

Default choice: .relaxed for counters, .acquiringAndReleasing for read-modify-write.

Resources

Docs: /synchronization, /synchronization/mutex, /os/osallocatedunfairlock

Swift Evolution: SE-0433

Skills: axiom-swift-concurrency, axiom-swift-performance