为什么结果与标志“ -race”不符？

Question

Why the result is not as expected with flag "-race" ? It expected the same result: 1000000 - with flag "-race" and without this

https://gist.github.com/romanitalian/f403ceb6e492eaf6ba953cf67d5a22ff

package main

import (
    "fmt"
    "runtime"
    "sync/atomic"
    "time"
)

//$ go run -race main_atomic.go
//954203
//
//$ go run main_atomic.go
//1000000

type atomicCounter struct {
    val int64
}

func (c *atomicCounter) Add(x int64) {
    atomic.AddInt64(&c.val, x)
    runtime.Gosched()
}

func (c *atomicCounter) Value() int64 {
    return atomic.LoadInt64(&c.val)
}

func main() {
    counter := atomicCounter{}

    for i := 0; i < 100; i++ {
        go func(no int) {
            for i := 0; i < 10000; i++ {
                counter.Add(1)
            }
        }(i)
    }

    time.Sleep(time.Second)
    fmt.Println(counter.Value())
}

Answer 1

The reason why the result is not the same is because time.Sleep(time.Second) does not guarantee that all of your goroutines are going to be executed in the timespan of one second. Even if you execute go run main.go, it's not guaranteed that you will get the same result every time. You can test this out if you put time.Milisecond instead of time.Second, you will see much more inconsistent results.

Whatever value you put in the time.Sleep method, it does not guarantee that all of your goroutines will be executed, it just means that it's less likely that all of your goroutines won't finish in time.

For consistent results, you would want to synchronise your goroutines a bit. You can use WaitGroup or channels.

With WaitGroup:

//rest of the code above is the same
func main() {
    counter := atomicCounter{}
    var wg sync.WaitGroup
    for i := 0; i < 100; i++ {
        wg.Add(1)
        go func(no int) {
            for i := 0; i < 10000; i++ {
                counter.Add(1)
            }
            wg.Done()
        }(i)
    }

    wg.Wait()
    fmt.Println(counter.Value())
}

With channels:

func main() {
    valStream := make(chan int)
    doneStream := make(chan int)
    result := 0
    for i := 0; i < 100; i++ {
        go func() {
            for i := 0; i < 10000; i++ {
                valStream <- 1
            }
            doneStream <- 1
        }()
    }

    go func() {
        counter := 0
        for count := range doneStream {
            counter += count
            if counter == 100 {
                close(doneStream)
            }
        }
        close(valStream)
    }()

    for val := range valStream {
        result += val
    }
    fmt.Println(result)
}