sync: Data race happen when using sync.WaitGroup

[DrmagicE]

I run the below code with race detector

package main

import (
	"sync"
)

func main() {
	var m *sync.WaitGroup
	m = &sync.WaitGroup{}
	for i := 0; i < 100; i++ {
		go func(m *sync.WaitGroup) {
			m.Add(1)
		}(m)
	}
	m.Wait()
}

and got a data race warning:

WARNING: DATA RACE
Write at 0x00c04200e03c by main goroutine:
  internal/race.Write()
      D:/Go/src/internal/race/race.go:41 +0x3f
  sync.(*WaitGroup).Wait()
      D:/Go/src/sync/waitgroup.go:129 +0xfb
  main.main()
      C:/Users/Lifang/go/src/ago.go:15 +0xbb

Previous read at 0x00c04200e03c by goroutine 5:
  internal/race.Read()
      D:/Go/src/internal/race/race.go:37 +0x3f
  sync.(*WaitGroup).Add()
      D:/Go/src/sync/waitgroup.go:71 +0x176
  main.main.func1()
      C:/Users/Lifang/go/src/ago.go:12 +0x48

I am confused.. Is it the Add() function should not call in other goroutine?

[davecheney]

This is not a bug, because Add is being called in a separate goroutine, it is executing concurrently with the call to Wait.

Don’t do that, call add outside the go statement.

https://golang.org/pkg/sync/#WaitGroup.Add

[DrmagicE]

@davecheney Thanks!