Go: What is the difference between mutex and RWMutex?

Mutex — exclusive lock. RWMutex — read-write lock with concurrent reads.

sync.Mutex:

• Lock()/Unlock() — exclusive access.
• One goroutine at a time.
• Lower overhead (~20ns).

sync.RWMutex:

• RLock()/RUnlock() — shared reads.
• Lock()/Unlock() — exclusive writes.
• Higher overhead (~30-40ns).

1// Mutex for simple counter
2var mu sync.Mutex
3var count int
4
5func increment() {
6    mu.Lock()
7    defer mu.Unlock()
8    count++
9}
10
11// RWMutex for cache
12var rwmu sync.RWMutex
13var cache map[string]string
14
15func get(key string) (string, bool) {
16    rwmu.RLock()
17    defer rwmu.RUnlock()
18    val, ok := cache[key]
19    return val, ok
20}
21
22func set(key, value string) {
23    rwmu.Lock()
24    defer rwmu.Unlock()
25    cache[key] = value
26}

When to use RWMutex:

• Read-heavy workloads.
• Caches, config stores.

When to use Mutex:

• Write-heavy workloads.
• Simple cases.

Common mistakes:

• Using RWMutex for write-heavy work.
• Copying mutex after first use.
• Forgetting defer Unlock().
• Not benchmarking.
• Using RWMutex for single-goroutine.