Golang 并发读写文件问题啊啊啊啊

想着用 Go 写个 mmap + 并发读写文件的小程序,但是加了锁后输出顺序很有问题,俩文件都不一样.

求教怎么并发按顺序写出...

var mmFile, err = mmap.Open(filename)
var size, n = mmFile.Len() / 10, 0
var merr error
var Mutex sync.Mutex
var Wg sync.WaitGroup
var buf = make([]byte, size)
for i := 0; i < mmFile.Len() && io.EOF != merr; i += len(buf) {
	Wg.Add(1)
	go func(off int64) {
		n, merr = mmFile.ReadAt(buf, off)
		Mutex.Lock()
		defer Mutex.Unlock()
		if merr != nil {
			log.Println(merr)
		}
		wFile.Write(buf[:n])
		buf = make([]byte, size)
		Wg.Done()
	}(int64(i))
}

Wg.Wait()