go - 何时使用互斥量

Question

Go 真的需要它们吗？我读过 https://gobyexample.com/mutexes并多次运行示例代码，当我删除 mutex.Lock() 和 mutex.Unlock() 时，它的工作原理完全相同。

Answer 1

I've read https://gobyexample.com/mutexes and run example code multiple times, when I remove mutex.Lock() and mutex.Unlock() it works exactly the same.

---

正如预期的那样，当我删除互斥量时，我遇到了数据争用和 panic 。

---

输出:

$ go run -race racer.go

fatal error: concurrent map read and map write

==================
WARNING: DATA RACE
Write at 0x00c00009a690 by goroutine 113:
  runtime.mapassign_fast64()
      /home/peter/go/src/runtime/map_fast64.go:92 +0x0
  main.main.func2()
      /home/peter/gopath/src/so/racer.go:56 +0xba

Previous read at 0x00c00009a690 by goroutine 64:
  runtime.mapaccess1_fast64()
      /home/peter/go/src/runtime/map_fast64.go:12 +0x0
  main.main.func1()
      /home/peter/gopath/src/so/racer.go:37 +0x72

Goroutine 113 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:51 +0x124

Goroutine 64 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:29 +0xe0
==================
==================
WARNING: DATA RACE
Write at 0x00c00009a690 by goroutine 115:
  runtime.mapassign_fast64()
      /home/peter/go/src/runtime/map_fast64.go:92 +0x0
  main.main.func2()
      /home/peter/gopath/src/so/racer.go:56 +0xba

Previous read at 0x00c00009a690 by goroutine 39:
  runtime.mapaccess1_fast64()
      /home/peter/go/src/runtime/map_fast64.go:12 +0x0
  main.main.func1()
      /home/peter/gopath/src/so/racer.go:37 +0x72

Goroutine 115 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:51 +0x124

Goroutine 39 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:29 +0xe0
==================
==================
WARNING: DATA RACE
Read at 0x00c0001f0048 by goroutine 79:
  main.main.func1()
      /home/peter/gopath/src/so/racer.go:37 +0x80

Previous write at 0x00c0001f0048 by goroutine 113:
  main.main.func2()
      /home/peter/gopath/src/so/racer.go:56 +0xcf

Goroutine 79 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:29 +0xe0

Goroutine 113 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:51 +0x124
==================
==================
WARNING: DATA RACE
Read at 0x00c0001f0050 by goroutine 81:
  main.main.func1()
      /home/peter/gopath/src/so/racer.go:37 +0x80

Previous write at 0x00c0001f0050 by goroutine 115:
  main.main.func2()
      /home/peter/gopath/src/so/racer.go:56 +0xcf

Goroutine 81 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:29 +0xe0

Goroutine 115 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:51 +0x124
==================
==================
WARNING: DATA RACE
Read at 0x00c0001f0050 by goroutine 106:
  main.main.func1()
      /home/peter/gopath/src/so/racer.go:37 +0x80

Previous write at 0x00c0001f0050 by goroutine 115:
  main.main.func2()
      /home/peter/gopath/src/so/racer.go:56 +0xcf

Goroutine 106 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:29 +0xe0

Goroutine 115 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:51 +0x124
==================
==================
WARNING: DATA RACE
Read at 0x00c0001f0050 by goroutine 94:
  main.main.func1()
      /home/peter/gopath/src/so/racer.go:37 +0x80

Previous write at 0x00c0001f0050 by goroutine 115:
  main.main.func2()
      /home/peter/gopath/src/so/racer.go:56 +0xcf

Goroutine 94 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:29 +0xe0

Goroutine 115 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:51 +0x124
==================
==================
WARNING: DATA RACE
Write at 0x00c0001f0050 by goroutine 114:
  main.main.func2()
      /home/peter/gopath/src/so/racer.go:56 +0xcf

Previous write at 0x00c0001f0050 by goroutine 115:
  main.main.func2()
      /home/peter/gopath/src/so/racer.go:56 +0xcf

Goroutine 114 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:51 +0x124

Goroutine 115 (running) created at:
  main.main()
      /home/peter/gopath/src/so/racer.go:51 +0x124
==================
exit status 2
$ 

---

racer.go:

package main

import (
    "fmt"
    "math/rand"

    //*"sync"
    "sync/atomic"
    "time"
)

func main() {

    // For our example the state will be a map.

    var state = make(map[int]int)

    // This mutex will synchronize access to state.

    //*var mutex = &sync.Mutex{}

    // We’ll keep track of how many read and write operations we do.

    var readOps uint64
    var writeOps uint64

    // Here we start 100 goroutines to execute repeated reads against the state, once per millisecond in each goroutine.

    for r := 0; r < 100; r++ {
        go func() {
            total := 0
            for {

                // For each read we pick a key to access, Lock() the mutex to ensure exclusive access to the state, read the value at the chosen key, Unlock() the mutex, and increment the readOps count.

                key := rand.Intn(5)
                //*mutex.Lock()
                total += state[key]
                //*mutex.Unlock()
                atomic.AddUint64(&readOps, 1)

                // Wait a bit between reads.

                time.Sleep(time.Millisecond)
            }
        }()
    }

    // We’ll also start 10 goroutines to simulate writes, using the same pattern we did for reads.

    for w := 0; w < 10; w++ {
        go func() {
            for {
                key := rand.Intn(5)
                val := rand.Intn(100)
                //*mutex.Lock()
                state[key] = val
                //*mutex.Unlock()
                atomic.AddUint64(&writeOps, 1)
                time.Sleep(time.Millisecond)
            }
        }()
    }

    // Let the 10 goroutines work on the state and mutex for a second.

    time.Sleep(time.Second)

    // Take and report final operation counts.

    readOpsFinal := atomic.LoadUint64(&readOps)
    fmt.Println("readOps:", readOpsFinal)
    writeOpsFinal := atomic.LoadUint64(&writeOps)
    fmt.Println("writeOps:", writeOpsFinal)

    // With a final lock of state, show how it ended up.

    //*mutex.Lock()
    fmt.Println("state:", state)
    //*mutex.Unlock()
}