# Read/Write Mutex Review

Maps are safe for concurrent *read* access, just not concurrent read/write or write/write access. A read/write mutex allows all the readers to access the map at the same time, but a writer will still lock out all other readers and writers.

```go
package main

import (
	"fmt"
	"sync"
)

func main() {
	m := map[int]int{}

	mux := &sync.RWMutex{}

	go writeLoop(m, mux)
	go readLoop(m, mux)
	go readLoop(m, mux)
	go readLoop(m, mux)
	go readLoop(m, mux)

	// stop program from exiting, must be killed
	block := make(chan struct{})
	<-block
}

func writeLoop(m map[int]int, mux *sync.RWMutex) {
	for {
		for i := 0; i < 100; i++ {
			mux.Lock()
			m[i] = i
			mux.Unlock()
		}
	}
}

func readLoop(m map[int]int, mux *sync.RWMutex) {
	for {
		mux.RLock()
		for k, v := range m {
			fmt.Println(k, "-", v)
		}
		mux.RUnlock()
	}
}
```

By using a `sync.RWMutex`, our program becomes *more efficient*. We can have as many `readLoop()` threads as we want, while still ensuring that the writers have exclusive access.