从两个互斥体报告的数据竞争

Question

An interesting issue has arisen today where I have code that contains more than one Mutex, each covering locking for distinct Maps.

Here is a similar in struct of my source code that I am using:

type MyStruct struct {
    dogMutex sync.RWMutex
    dogMap   map[int]Dog // keyed by PID
    catMutex sync.RWMutex
    catMap   map[int]Cat // keyed by (localAddress + localPort)
}

A more detailed example of the issue is here: https://play.golang.org/p/eic8q2VrNq

After building the executable with 'go build -race ..." the generated executable reports the following race

As the code is way more complex than the example above, it is interesting to notice that the data race is reported on the areas indicated in the code.

The following stack is from the real application.

1) The write operation reported on wwww.go:95 is equivalent to my WRITE comment in the code (MethodOne)

2) The previous read operattion reported on wwww.go:218 is equivalent to my READ comment in the code (MethodTwo)

=================
WARNING: DATA RACE
Write at 0x00c420017890 by goroutine 97:
  runtime.mapassign1()
      /usr/local/go/src/runtime/hashmap.go:442 +0x0
  main.(*NetworkManager).MethodOne()
      /opt/doppler/src/xxx/yyy/wwww.go:95 +0x745

Previous read at 0x00c420017890 by goroutine 70:
  runtime.mapiterinit()
      /usr/local/go/src/runtime/hashmap.go:620 +0x0
  main.NetworkManager.MethodTwo()
      /opt/xxx/src/xxx/yyy/wwww.go:218 +0x1e9
  main.(*NetworkManager).SomethingELse()
      /opt/xxx/src/xxx/yyy/wwww.go:174 +0x99d
  main.(*NetworkManager).SomethingFurther()
      /opt/xxx/src/xxx/yyy/wwww.go:102 +0x3c

I am wondering if this is a proper way of using mutexes. My code has much concurrency, but I am focusing this question on the fact that the race detector is reporting based on Apples vs Bananas (two completely distinct mutexes)

Answer 1

Just going to take a guess, but one common cause of this issue is accidentally passing the struct which contains the pointers by value instead of by reference. For example:

type MyStruct struct {
    dogMutex sync.RWMutex
    dogMap   map[int]Dog // keyed by PID
    catMutex sync.RWMutex
    catMap   map[int]Cat // keyed by (localAddress + localPort)
}

func (s MyStruct) Example() {
    // this lock doesn't actually work, but the map access does because its a 
    // reference type
    s.catMutex.Lock()
    s.catMap[1] = Cat{}
    s.catMutex.Unlock()
}

Another possibility is that perhaps you are passing the maps into your init:

func (s *MyStruct) Init(cats map[int]Cat) {
    s.catMap = cats
}

And then you are modifying the map somewhere outside of the struct. If that's the case you need to create a new map and copy all the values.

Incidentally go vet can detect many of these issues, try running it on your code: https://golang.org/cmd/vet/. (Also worth a try is the metalinter)