What is causing this data race? - go

Question

Why does this code cause data race?
I have already used atomic add.
package main
import (
"sync/atomic"
"time"
)
var a int64
func main() {
for {
if a < 100 {
atomic.AddInt64(&a, 1)
go run()
}
}
}
func run() {
<-time.After(5 * time.Second)
atomic.AddInt64(&a, -1)
}
I run command go run --race with this code and get:
==================
WARNING: DATA RACE
Write at 0x000001150f30 by goroutine 8:
sync/atomic.AddInt64()
/usr/local/Cellar/go/1.11.2/libexec/src/runtime/race_amd64.s:276 +0xb
main.run()
/Users/flask/test.go:22 +0x6d
Previous read at 0x000001150f30 by main goroutine:
main.main()
/Users/flask/test.go:12 +0x3a
Goroutine 8 (running) created at:
main.main()
/Users/flask/test.go:15 +0x75
==================
Could you help me explain this?
And how to fix this warning?
Thanks!

Answer 1

You didn't use the atomic package at all places where you accessed the variable. All access must be synchronized to variables that are accessed from multiple goroutines concurrently, including reads:
for {
if value := atomic.LoadInt64(&a); value < 100 {
atomic.AddInt64(&a, 1)
go run()
}
}
With that change, the race condition goes away.
If you just want to inspect the value, you don't even need to store it in a variable, so you may simply do:
for {
if atomic.LoadInt64(&a) < 100 {
atomic.AddInt64(&a, 1)
go run()
}
}