[TOC]
Dave Cheney post a quiz on his blog several years ago,
package main
import (
"fmt"
"time"
)
type RPC struct {
result int
done chan struct{}
}
func (rpc *RPC) compute() {
time.Sleep(time.Second) // strenuous computation intensifies
rpc.result = 42
close(rpc.done)
}
func (RPC) version() int {
return 1 // never going to need to change this
}
func main() {
rpc := &RPC{done: make(chan struct{})}
go rpc.compute() // kick off computation in the background
version := rpc.version() // grab some other information while we're waiting
<-rpc.done // wait for computation to finish
result := rpc.result
fmt.Printf("RPC computation complete, result: %d, version: %d\n", result, version)
}
When you use race tool to run it, it will give you warning as following:
~/W/g/c/src ❯❯❯ go run -race race.go
rpc addr = 0xc0000961c0
version = 2
==================
WARNING: DATA RACE
Write at 0x00c0000961c0 by goroutine 6:
main.(*RPC).compute()
/Users/andrewhoo/Work/go/codeschool/src/race.go:15 +0x3a
Previous read at 0x00c0000961c0 by main goroutine:
main.main()
/Users/andrewhoo/Work/go/codeschool/src/race.go:28 +0x163
Goroutine 6 (running) created at:
main.main()
/Users/andrewhoo/Work/go/codeschool/src/race.go:27 +0x14c
==================
RPC computation complete, result: 42, version: 1
Found 1 data race(s)
exit status 66
This code share the variable rpc on 2 goroutines, race tool give its detection result. But is it really a race-condition in the code?
After change the Line 32 code into:
func (rpc *RPC) version() int {
return 1 // never going to need to change this
}
Race tool will not complain the race-condition. But when you compile the code without race tools into assemble language, it show nothing different on this line of code as this line of code was optimized away by the compiler.
But why the golang race-detection tool do such complaints?
Let’s show what race-detection tool do:
version := rpc.version() // grab some other information while we're waiting
0x10f712d 488b442440 MOVQ 0x40(SP), AX
0x10f7132 48890424 MOVQ AX, 0(SP)
0x10f7136 48c744240810000000 MOVQ $0x10, 0x8(SP)
0x10f713f e8cc44f9ff CALL runtime.racereadrange(SB)
Golang use ThreadSanitizer to detect the possible race-conditions. Obviously, the ASM show the tools take the whole RPC struct as one protected unit if you use the struct RPC as the operation object, This explain the reason when the line 28 code change the member of this global variable, it will violate the State Machine Code code made by the Sanitizer. But actually it will not happen such race-condition case.
Anyway this code need to be refactor, try not to use global variable between goroutines except you have enough reasons to do that.