An example of racy map access pattern

Output

$ go build --race ./main.go
$ ./main                   
5
==================
WARNING: DATA RACE
Read by goroutine 7:
  runtime.mapaccess1_fast64()
      /usr/local/Cellar/go/1.6/libexec/src/runtime/hashmap_fast.go:103 +0x0
  main.main.func2()
      /Users/ash2k/sources/gotest2/main.go:18 +0x4e

Previous write by goroutine 6:
  runtime.mapassign1()
      /usr/local/Cellar/go/1.6/libexec/src/runtime/hashmap.go:429 +0x0
  main.main.func1()
      /Users/ash2k/sources/gotest2/main.go:14 +0xa6

Goroutine 7 (running) created at:
  main.main()
      /Users/ash2k/sources/gotest2/main.go:20 +0xe0

Goroutine 6 (finished) created at:
  main.main()
      /Users/ash2k/sources/gotest2/main.go:16 +0xb5
==================
0
Found 1 data race(s)

racy reads.go

package main

import (
	"fmt"
	"sync"
)

func main()  {
	var l sync.Mutex
	x := make(map[int]int)
	go func(z int) {
		l.Lock()
		defer l.Unlock()
		x[z] = z
		fmt.Println(z)
	}(5)
	go func(z int) {
		v := x[z] // Read without proper locking
		fmt.Println(v)
	}(0)
}