Read-Write Mutexes

In this lesson, explore read-write mutexes through one small Go program, and connect its output to the language rules behind it.

sync.RWMutex allows multiple readers to hold RLock together, or one writer to hold Lock exclusively. It is useful only when the workload benefits from shared reads.

Do not attempt to upgrade an existing RLock into Lock; that can deadlock. Always unlock using the corresponding method.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"fmt"
	"sync"
)

type Counter struct {
	mu    sync.RWMutex
	value int
}

func (c *Counter) Get() int { c.mu.RLock(); defer c.mu.RUnlock(); return c.value }
func (c *Counter) Add()     { c.mu.Lock(); defer c.mu.Unlock(); c.value++ }
func main() {
	var c Counter
	c.Add()
	c.Add()
	fmt.Println("Value:", c.Get())
}

Output

Value: 2

Add acquires the exclusive write lock twice in sequence. Get uses the read lock to safely return 2.

• A read lock must not be used while mutating protected data.
• RWMutex is not universally faster than Mutex.

Spawn three goroutines that call Get while a separate goroutine updates using Add; wait for all.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Read-Write Mutexes in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.