Mutex Review

The principle problem that mutexes help us avoid is the concurrent read/write problem. This problem arises when one thread is writing to a variable while another thread is reading from that same variable at the same time.

When this happens, a Go program will panic because the reader could be reading bad data while it's being mutated in place.

Mutex example

package main

import (
	"fmt"
)

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

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

func writeLoop(m map[int]int) {
	for {
		for i := 0; i < 100; i++ {
			m[i] = i
		}
	}
}

func readLoop(m map[int]int) {
	for {
		for k, v := range m {
			fmt.Println(k, "-", v)
		}
	}
}

The example above creates a map, then starts two goroutines which each have access to the map. One goroutine continuously mutates the values stored in the map, while the other prints the values it finds in the map.

If we run the program on a multi-core machine, we get the following output: fatal error: concurrent map iteration and map write

In Go, it isn’t safe to read from and write to a map at the same time.

Mutexes to the rescue

package main

import (
	"fmt"
	"sync"
)

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

	mux := &sync.Mutex{}

	go writeLoop(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.Mutex) {
	for {
		for i := 0; i < 100; i++ {
			mux.Lock()
			m[i] = i
			mux.Unlock()
		}
	}
}

func readLoop(m map[int]int, mux *sync.Mutex) {
	for {
		mux.Lock()
		for k, v := range m {
			fmt.Println(k, "-", v)
		}
		mux.Unlock()
	}
}

In this example, we added a sync.Mutex{} and named it mux. In the write loop, the Lock() method is called before writing, and then the Unlock() is called when we're done. This Lock/Unlock sequence ensures that no other threads can Lock() the mutex while we have it locked – any other threads attempting to Lock() will block and wait until we Unlock().

In the reader, we Lock() before iterating over the map, and likewise Unlock() when we're done. Now the threads share the memory safely!