substack-concurrency-6.go

var count int
var lock sync.Mutex

increment := func() {
    lock.Lock() // <--- locking section
    defer lock.Unlock() // <--- unlocking
    count++
}

decrement := func() {
    lock.Lock() 
    defer lock.Unlock()
    count--
}

var arithmetic sync.WaitGroup
for i := 0; i<=5; i++ {
    arithmetic.Add(1)
    go func() {
        defer arithmetic.Done()
        increment()
    }
}

for i := 0; i<=5; i++ {
    arithmetic.Add(1)
    go func() {
        defer arithmetic.Done()
        decrement()
    }
}