When do you use sync.Mutex versus sync.RWMutex, and what are the critical usage rules?

sync.Mutex provides mutual exclusion — at most one goroutine holds the lock at any time. sync.RWMutex distinguishes readers from writers, allowing multiple concurrent readers OR one exclusive writer — more efficient for read-heavy workloads.

// sync.Mutex — mixed or write-heavy access
type BankAccount struct {
    mu      sync.Mutex
    balance float64
}

func (a *BankAccount) Deposit(amount float64) {
    a.mu.Lock()
    defer a.mu.Unlock() // ALWAYS defer — fires even on panic or early return
    a.balance += amount
}

func (a *BankAccount) Balance() float64 {
    a.mu.Lock()
    defer a.mu.Unlock()
    return a.balance
}

// sync.RWMutex — read-heavy (reads >> writes)
type SafeConfig struct {
    mu   sync.RWMutex
    data map[string]string
}

func (c *SafeConfig) Get(key string) string {
    c.mu.RLock()          // multiple goroutines can RLock at the same time
    defer c.mu.RUnlock()
    return c.data[key]
}

func (c *SafeConfig) Set(key, val string) {
    c.mu.Lock()           // exclusive: no readers OR writers allowed
    defer c.mu.Unlock()
    c.data[key] = val
}

// CRITICAL MISTAKES:
// 1. NEVER copy a Mutex after first use
//    Copying duplicates internal state → broken synchronisation
//    Always use *BankAccount, never BankAccount by value

// 2. Mutex is NOT reentrant
//    A goroutine that calls Lock() while already holding it → DEADLOCK

// 3. Writer starvation protection in RWMutex:
//    When a writer is waiting, new readers are blocked
//    even if existing readers still hold RLock

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