package main import ( "fmt" "runtime" "sync" "time" ) type value struct { memAccess sync.Mutex value int } func main() { runtime.GOMAXPROCS(3) var wg sync.WaitGroup sum := func(v1, v2 *value) { defer wg.Done() v1.memAccess.Lock() time.Sleep(2 * time.Second) v2.memAccess.Lock() fmt.Printf("sum = %d\n", v1.value+v2.value) v2.memAccess.Unlock() v1.memAccess.Unlock() } product := func(v1, v2 *value) { defer wg.Done() v2.memAccess.Lock() time.Sleep(2 * time.Second) v1.memAccess.Lock() fmt.Printf("product = %d\n", v1.value*v2.value) v1.memAccess.Unlock() v2.memAccess.Unlock() } var v1, v2 value v1.value = 1 v2.value = 1 wg.Add(2) go sum(&v1, &v2) go product(&v1, &v2) wg.Wait() }运行上面的代码,可能会看到:
fatal error: all goroutines are asleep - deadlock!
为什么呢?如果仔细观察,就可以在此代码中看到时机问题,以下是运行时的图形表示。
图 :一个因时间问题导致死锁的演示
package main import ( "bytes" "fmt" "runtime" "sync" "sync/atomic" "time" ) func main() { runtime.GOMAXPROCS(3) cv := sync.NewCond(&sync.Mutex{}) go func() { for range time.Tick(1 * time.Second) { // 通过tick控制两个人的步调 cv.Broadcast() } }() takeStep := func() { cv.L.Lock() cv.Wait() cv.L.Unlock() } tryDir := func(dirName string, dir *int32, out *bytes.Buffer) bool { fmt.Fprintf(out, " %+v", dirName) atomic.AddInt32(dir, 1) takeStep() //走上一步 if atomic.LoadInt32(dir) == 1 { //走成功就返回 fmt.Fprint(out, ". Success!") return true } takeStep() // 没走成功,再走回来 atomic.AddInt32(dir, -1) return false } var left, right int32 tryLeft := func(out *bytes.Buffer) bool { return tryDir("向左走", &left, out) } tryRight := func(out *bytes.Buffer) bool { return tryDir("向右走", &right, out) } walk := func(walking *sync.WaitGroup, name string) { var out bytes.Buffer defer walking.Done() defer func() { fmt.Println(out.String()) }() fmt.Fprintf(&out, "%v is trying to scoot:", name) for i := 0; i < 5; i++ { if tryLeft(&out) || tryRight(&out) { return } } fmt.Fprintf(&out, "\n%v is tried!", name) } var trail sync.WaitGroup trail.Add(2) go walk(&trail, "男人") // 男人在路上走 go walk(&trail, "女人") // 女人在路上走 trail.Wait() }输出结果如下:
go run main.go
女人 is trying to scoot: 向左走 向右走 向左走 向右走 向左走 向右走 向左走 向右走 向左走 向右走
女人 is tried!
男人 is trying to scoot: 向左走 向右走 向左走 向右走 向左走 向右走 向左走 向右走 向左走 向右走
男人 is tried!
package main import ( "fmt" "runtime" "sync" "time" ) func main() { runtime.GOMAXPROCS(3) var wg sync.WaitGroup const runtime = 1 * time.Second var sharedLock sync.Mutex greedyWorker := func() { defer wg.Done() var count int for begin := time.Now(); time.Since(begin) <= runtime; { sharedLock.Lock() time.Sleep(3 * time.Nanosecond) sharedLock.Unlock() count++ } fmt.Printf("Greedy worker was able to execute %v work loops\n", count) } politeWorker := func() { defer wg.Done() var count int for begin := time.Now(); time.Since(begin) <= runtime; { sharedLock.Lock() time.Sleep(1 * time.Nanosecond) sharedLock.Unlock() sharedLock.Lock() time.Sleep(1 * time.Nanosecond) sharedLock.Unlock() sharedLock.Lock() time.Sleep(1 * time.Nanosecond) sharedLock.Unlock() count++ } fmt.Printf("Polite worker was able to execute %v work loops\n", count) } wg.Add(2) go greedyWorker() go politeWorker() wg.Wait() }输出如下:
Greedy worker was able to execute 276 work loops
Polite worker was able to execute 92 work loops
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