Settings and accessing a pointer from concurrent goroutines

Question

I have a map which is used by goroutine A and replaced once in a time in goroutine B. By replacement I mean:

var a map[T]N

// uses the map
func goroutineA() {
    for (...) {
        tempA = a
        ..uses tempA in some way...
    }
}

//refreshes the map
func gorountineB() {
    for (...) {
        time.Sleep(10 * time.Seconds)
        otherTempA = make(map[T]N)
        ...initializes other tempA....
        a = otherTempA 
    }
}

Do you see any problem in this pseudo code? (in terms of concurrecy)

Answer 1

The code isn't safe, since assignments and reads to a pointer value are not guaranteed to be atomic. This can mean that as one goroutine writes the new pointer value, the other may see a mix of bytes from the old and new value, which will cause your program to die in a nasty way. Another thing that may happen is that since there's no synchronisation in your code, the compiler may notice that nothing can change a in goroutineA, and lift the tempA := a statement out of the loop. This will mean that you'll never see new map assignments as the other goroutine updates them.

You can use go test -race to find these sorts of problems automatically.

One solution is to lock all access to the map with a mutex.

You may wish to read the Go Memory Model document, which explains clearly when changes to variables are visible inside goroutines.