| HN Mirror

It really depends on where you draw the line. This is an obviously-incorrect program (`++` isn't atomic):

  func main() {
        var x, y int64
        doneCh := make(chan struct{})
        inc := func() {
                for i := 0; i < 2<<20; i++ {
                        x++ // line 13 in test/main.go
                        atomic.AddInt64(&y, 1)
                }
                doneCh <- struct{}{}
                return
        }
        go inc()
        go inc()
        <-doneCh
        <-doneCh
        fmt.Printf("x, y = %v, %v\n", x, y)
  }

This prints something like:

  $ go run main.go
  x, y = 3482626, 4194304

If you run it with the race detector the problem is clear:

  $ go run -race main.go
  ==================
  WARNING: DATA RACE
  Read at 0x00c0001bc008 by goroutine 7:
    main.main.func1()
        /home/jrockway/test/main.go:13 +0x50

  Previous write at 0x00c0001bc008 by goroutine 8:
    main.main.func1()
        /home/jrockway/test/main.go:13 +0x64

  Goroutine 7 (running) created at:
    main.main()
        /home/jrockway/test/main.go:19 +0x176

  Goroutine 8 (running) created at:
    main.main()
        /home/jrockway/test/main.go:20 +0x184
  ==================
  x, y = 4189962, 4194304
  Found 1 data race(s)
  exit status 66

This is not strictly a memory safety problem (this can't crash the runtime), but a program that returns the wrong answer is pretty useless, so there is that. Obviously if x were going to be used as a pointer through the `unsafe` package, you could have problems. (Though I think that x will always be less than y, so if you have proved that memory[base+y] is safe to read, then memory[base+x] is safe to read. But it's easy to imagine a scenario where you overcount instead of undercount.