How to detect data races

Data races can happen easily and are hard to debug. Luckily, the Go runtime is often able to help.

Use -race to enable the built-in data race detector.

$ go test -race [packages]
$ go run -race [packages]

Example

Here’s a program with a data race:

package main
import "fmt"

func main() {
    i := 0
    go func() {
        i++ // write
    }()
    fmt.Println(i) // concurrent read
}

Running this program with the -race options tells us that there’s a race between the write at line 7 and the read at line 9:

$ go run -race main.go
0
==================
WARNING: DATA RACE
Write by goroutine 6:
  main.main.func1()
      /tmp/main.go:7 +0x44

Previous read by main goroutine:
  main.main()
      /tmp/main.go:9 +0x7e

Goroutine 6 (running) created at:
  main.main()
      /tmp/main.go:8 +0x70
==================
Found 1 data race(s)
exit status 66

Details

The data race detector does not perform any static analysis. It checks the memory access in runtime and only for the code paths that are actually executed.

It runs on darwin/amd64, freebsd/amd64, linux/amd64 and windows/amd64.

The overhead varies, but typically there’s a 5-10x increase in memory usage, and 2-20x increase in execution time.

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