problem description
Recently I learned the option -race to check the exist of race condition in go. The full command is go run -race xxx.go It really helped me a lot. But as with the code below, I think the check result is wrong, and tried a lot of method (My try to get a panic below) to get a REAL panic but failed. So I want to know whether the code is correct and the race check is wrong，or can you revise my code so that I can SEE a real panic. Thanks a lot.
The code
package main
import "fmt"
type myType struct {
A int
}
func main(){
c:=make(chan bool)
x := new(myType)
go func(){
x = new(myType) // write to x
c <- true
}()
_ = *x // read from x
<-c
fmt.Println("end")
}
The race check result
go run -race test.go
==================
WARNING: DATA RACE
Write at 0x00c00009c010 by goroutine 6:
main.main.func1()
/Users/yaodongen/test.go:12 +0x56
Previous read at 0x00c00009c010 by main goroutine:
main.main()
/Users/yaodongen/test.go:15 +0xe2
Goroutine 6 (running) created at:
main.main()
/Users/yaodongen/test.go:11 +0xd4
==================
end
Found 1 data race(s)
exit status 66
My point
I tried to find the reason for the race condition report.In a post(Chinese), it mentions that the operation a = in64(0) is not atomic. For example in one 32 bit Machine and the data like int64 may be 64 bit length, CPU could copy half of the data and be interruptted by others. In the following code (Prove the golang copy is not atomic), I write a code to prove its true. But in my case, the code x = new(myType) is to copy a pointer value, and I think it can be done in one CPU copy. In other word, the operation is atomic and will never reach race condition.
Prove the golang copy is not atomic
package main
import "fmt"
import "time"
func main(){
var x = [...]int{1,1,1,1,1,1}
c := make(chan int, 100)
go func(){
for i:=0;;i++{
if i&1 == 0 {
x = [...]int{2,2,2,2,2,2} // write to x
}else{
x = [...]int{1,1,1,1,1,1} // write to x
}
c<-0 // let other goroutine see the change of x
<-c
}
}()
go func(){
for {
d := x // read from x
if d[0] != d[5] {
fmt.Println(d)
panic("error") // proved the copy operation is not atomic
}
c<-0
<-c
}
}()
time.Sleep(time.Millisecond * 10000)
fmt.Println("end")
}
My try to get a panic
But it failed, the code will panic if there exists a race condition (wrong memory address).
package main
import "fmt"
import "time"
type myType struct {
A int
}
func main(){
x := new(myType)
c := make(chan int, 100)
go func(){
for {
x = new(myType) // write to x
c<-0
<-c
}
}()
for i:=0; i<4; i++{
go func(){
for {
_ = *x // if exists a race condition, `*x` will visit a wrong memory address, and will panic
c<-0
<-c
}
}()
}
time.Sleep(time.Second * 10)
fmt.Println("end")
}
