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loopvar 改动不同版本的影响-并发

news 来源：原创 2024/5/17 14:10:24

看一个关于并发的例子

package mainimport ("fmt""runtime""sync"
)func main() {fmt.Println("golang version:", runtime.Version())concurrencyDemo()
}func concurrencyDemo() {isGold := func(num uint64) bool {return num%65536 == 0}var c = make(chan uint64)var m sync.Mutexfor n, i := 0, uint64(0); n < 5; n++ {go func() {for {m.Lock()i++v := im.Unlock()if isGold(v) {c <- v}}}()}for n := range c {fmt.Println("Found gold:", n)}
}

这个代码定义了一个isGold 的func，可以被65536整除的就是gold，然后定义了一个uint64的channel，接着开了5个go routine,每个go routine都是一个不会停止的协程，加锁i进行自增，并把自增后的i复制给v，如果v可以被65536整除，那么把v放到uint64的channel中，最后打印出channel的值。
如果代码写的不好，并发的时候是很容易出现data race的,运行的时候加上-race这个参数

golang 1.21 运行结果

 go run -race demo/concurrency.go
golang version: go1.21.5
Found gold: 65536
Found gold: 131072
Found gold: 196608
Found gold: 262144
Found gold: 327680
Found gold: 393216
Found gold: 458752
Found gold: 524288
Found gold: 589824
Found gold: 655360
^Csignal: interrupt

golang 1.22运行结果

go run -race demo/concurrency.go
golang version: go1.22.1
==================
WARNING: DATA RACE
Write at 0x00c000014148 by goroutine 7:main.concurrencyDemo.func2()/home/lfeng/GolandProjects/golang101/demo/concurrency.go:24 +0x65Previous read at 0x00c000014148 by main goroutine:main.concurrencyDemo()/home/lfeng/GolandProjects/golang101/demo/concurrency.go:20 +0x87main.main()/home/lfeng/GolandProjects/golang101/demo/concurrency.go:11 +0xc4Goroutine 7 (running) created at:main.concurrencyDemo()/home/lfeng/GolandProjects/golang101/demo/concurrency.go:21 +0x7amain.main()/home/lfeng/GolandProjects/golang101/demo/concurrency.go:11 +0xc4
==================
Found gold: 65536
Found gold: 65536
Found gold: 65536
Found gold: 65536
Found gold: 131072
Found gold: 131072
Found gold: 131072
Found gold: 65536
^Csignal: interrupt

可以看到1.21是没有data race的，但是1.22存在了data race

解决方法

把i的定义移到循环外面，修改后的代码

package mainimport ("fmt""runtime""sync"
)func main() {fmt.Println("golang version:", runtime.Version())concurrencyDemo()
}func concurrencyDemo() {isGold := func(num uint64) bool {return num%65536 == 0}var c = make(chan uint64)var m sync.Mutexi := uint64(0)for n := 0; n < 5; n++ {go func() {for {m.Lock()i++v := im.Unlock()if isGold(v) {c <- v}}}()}for n := range c {fmt.Println("Found gold:", n)}
}

修改后的代码不会存在data race了

再看个例子

package mainimport ("fmt""runtime""sync"
)func main() {fmt.Println("golang version:", runtime.Version())concurrencyDemo()
}func concurrencyDemo() {var wg sync.WaitGroupwg.Add(5)for i := 0; i < 5; i++ {go func() {defer wg.Done()fmt.Println(i)}()}
}

这个例子其实很简单，使用waitGroup来控制协程的数量和协程的结束。协程中只是打印循环变量i

golang 1.21的运行结果

go run -race demo/concurrency2.go
golang version: go1.21.5
5
5
==================
WARNING: DATA RACE
Read at 0x00c000120028 by goroutine 9:main.concurrencyDemo.func1()/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:20 +0xafPrevious write at 0x00c000120028 by main goroutine:main.concurrencyDemo()/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:17 +0xa4main.main()/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:11 +0xc4Goroutine 9 (running) created at:main.concurrencyDemo()/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:18 +0x85main.main()/home/lfeng/GolandProjects/golang101/demo/concurrency2.go:11 +0xc4
==================
5
5
5
Found 1 data race(s)
exit status 66

golang 1.22运行结果

go run -race demo/concurrency2.go
golang version: go1.22.1
2
1
4
3
0

发现1.21存在data race,但是1.22是不存在data race的

解决方法

go func增加参数传递循环的i
修改后的代码

package mainimport ("fmt""runtime""sync"
)func main() {fmt.Println("golang version:", runtime.Version())concurrencyDemo()
}func concurrencyDemo() {var wg sync.WaitGroupwg.Add(5)for i := 0; i < 5; i++ {go func(i int) {defer wg.Done()fmt.Println(i)}(i)}
}