Golang競爭狀態

看一段代碼：

package mainimport ("fmt""runtime""sync")var (counter intwg sync.WaitGroup)func main() {wg.Add(2)go incCounter(1)go incCounter(2)wg.Wait()fmt.Println("Final Counter:", counter)}func incCounter(id int) {defer wg.Done()for count := 0; count < 2; count++ {value := idruntime.Gosched()value++counter = value}}

goroutine執行的是副本值，然後將副本值寫入counter，所以在切換goroutine時，goroutine中的值會覆蓋counter。其中Gosched函數是runtime包中用於將goroutine從當前線程退出，給其它goroutine啟動並執行機會。這段代碼執行下來理論上應該是存在競爭狀態的，對於counter這個變數，在兩個goroutine的切換下，一共加了4次，但是由於每次切換後進入隊列的並不是真的這個值，而是一個副本，結果輸出應該為2。

事實貌似是這樣。。。貌似有點小問題。。。

檢測競爭狀態，再把這個gosched函數注釋，然後重新檢測競爭狀態，先後編譯執行得到的是：

為什麼會出現這個情況呢？Final Counter: 3

==================WARNING: DATA RACEWrite at 0x0000005b73c0 by goroutine 6:  main.incCounter()      /home/qmq/gopath/src/github.com/goinaction/code/test.go:49 +0x74Previous write at 0x0000005b73c0 by goroutine 7:  main.incCounter()      /home/qmq/gopath/src/github.com/goinaction/code/test.go:49 +0x74Goroutine 6 (running) created at:  main.main()      /home/qmq/gopath/src/github.com/goinaction/code/test.go:25 +0x68Goroutine 7 (running) created at:  main.main()      /home/qmq/gopath/src/github.com/goinaction/code/test.go:26 +0x89==================Final Counter: 2Found 1 data race(s)

==================WARNING: DATA RACEWrite at 0x0000005b73c0 by goroutine 7:  main.incCounter()      /home/qmq/gopath/src/github.com/goinaction/code/test.go:49 +0x74Previous write at 0x0000005b73c0 by goroutine 6:  main.incCounter()      /home/qmq/gopath/src/github.com/goinaction/code/test.go:49 +0x74Goroutine 7 (running) created at:  main.main()      /home/qmq/gopath/src/github.com/goinaction/code/test.go:26 +0x89Goroutine 6 (finished) created at:  main.main()      /home/qmq/gopath/src/github.com/goinaction/code/test.go:25 +0x68==================Final Counter: 3Found 1 data race(s)================

輸出小機率有3的情況，可能是goroutine沒有退出，所以發生了新goroutine中的值與上一次goroutine副本值相加的情況。對於這樣存在多個goroutine對一個共用資源進行操作的功能還是需要對其加鎖，或使用簡單的atomic，以及使用Go的特色通道 。

1.互斥鎖

package mainimport ("fmt""runtime""sync")var (counter intwg sync.WaitGroup        //互斥鎖    mutex sync.Mutex)func main() {wg.Add(2)go incCounter(1)go incCounter(2)wg.Wait()fmt.Printf("Final Counter: %d\n", counter)}func incCounter(id int) {defer wg.Done()for count := 0; count < 2; count++ {               //互斥鎖鎖定的臨界代碼塊，只允許同一時刻只有一個goroutine訪問mutex.Lock(){　　//習慣地加上大括弧更清晰// Capture the value of counter.value := counter// Yield the thread and be placed back in queue.runtime.Gosched()// Increment our local value of counter.value++// Store the value back into counter.counter = value}mutex.Unlock()//解鎖}}

2.atomic包

package mainimport ("fmt""sync""sync/atomic""time")var (shutdown int64wg sync.WaitGroup)func main() {wg.Add(2)go doWork("A")go doWork("B")        //設定goroutine執行的時間time.Sleep(1 * time.Second)fmt.Println("Shutdown Now")        //安全標誌 判斷是否可以停止goroutine工作atomic.StoreInt64(&shutdown, 1)wg.Wait()}func doWork(name string) {defer wg.Done()for {fmt.Printf("Doing %s Work\n", name)time.Sleep(250 * time.Millisecond)if atomic.LoadInt64(&shutdown) == 1 {fmt.Printf("Shutting %s Down\n", name)break}}}

3.通道

package mainimport ("fmt""math/rand""sync""time")var wg sync.WaitGroupfunc init() {rand.Seed(time.Now().UnixNano())}func main() {court := make(chan int)wg.Add(2)go player("Ding", court)go player("Sha", court)court <- 1wg.Wait()}func player(name string, court chan int) {defer wg.Done()for {ball, ok := <-courtif !ok {fmt.Printf("Player %s Won\n", name)return}n := rand.Intn(100)if n%13 == 0 {fmt.Printf("Player %s Missed\n", name)close(court)return}fmt.Printf("Player %s Hit %d\n", name, ball)ball++court <- ball}}