func main() {
 var m = make(map[int]string)

 go func() {
  for {
   m[1] = "xx"
  }
 }()

 go func() {
  for {
   _ = m[1]
  }
 }()
 time.Sleep(time.Second * 3)
}

fatal error: concurrent map read and map write
// ...

const (
 // flags
 iterator     = 1 // there may be an iterator using buckets
 oldIterator  = 2 // there may be an iterator using oldbuckets
 hashWriting  = 4 // a goroutine is writing to the map
 sameSizeGrow = 8 // the current map growth is to a new map of the same size
)
// A header for a Go map.
type hmap struct {
 count     int // # live cells == size of map.  Must be first (used by len() builtin)
 flags     uint8
 B         uint8  // log_2 of # of buckets (can hold up to loadFactor * 2^B items)
 noverflow uint16 // approximate number of overflow buckets; see incrnoverflow for details
 hash0     uint32 // hash seed

 buckets    unsafe.Pointer // array of 2^B Buckets. may be nil if count==0.
 oldbuckets unsafe.Pointer // previous bucket array of half the size, non-nil only when growing
 nevacuate  uintptr        // progress counter for evacuation (buckets less than this have been evacuated)

 extra *mapextra // optional fields
}

// Like mapaccess, but allocates a slot for the key if it is not present in the map.
func mapassign(t *maptype, h *hmap, key unsafe.Pointer) unsafe.Pointer {
 // ...

done:
 if h.flags&hashWriting == 0 {
  fatal("concurrent map writes")
 }
 h.flags &^= hashWriting
 if t.IndirectElem() {
  elem = *((*unsafe.Pointer)(elem))
 }
 return elem
}