Such operations are not atomic, and is a common misuse of the sync/atomic API. To make them atomic, one need to remove the assignment to use a direct call: atomic.AddUint64(&x, 1). In that case, the value of x will be updated atomically by address.

Example:

import (
 "sync/atomic"
)

type Counter uint64

func AtomicTests() {
 x := uint64(1)
 x = atomic.AddUint64(&x, 1)         // ERROR "direct assignment to atomic value"
 _, x = 10, atomic.AddUint64(&x, 1)  // ERROR "direct assignment to atomic value"
 x, _ = atomic.AddUint64(&x, 1), 10  // ERROR "direct assignment to atomic value"
}