Can multiple goroutines read from a single map?

read-race.go

 package main

// "go run -race read-race.go"

 import (
     "fmt"
     "sync"
 )

 func main() {
     fmt.Println("starting goroutines")
     var wg sync.WaitGroup
     m := map[string]int{"hola": 42}
     for i := 0; i < 10; i++ {
         wg.Add(1)
         go func() {                                                                                                                                                                                                                                                                                                        defer wg.Done()
             for i := 0; i < 10000000; i++ {
                 x := m["hola"]
                 if x != 42 {
                     panic("read something crazy")
                 }
                 if i == 973 {
                     // uncomment to show that writes cause races
                     //m["elsewhere"] = 7
                 }
             }
         }()
     }

     wg.Wait()
     fmt.Println("all done")
 }

If goroutines only read from a map and nobody is writing to it, concurrent access is OK. As soon as someone modifies the map, concurrency causes trouble and needs synchronization.