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runtime: why a variable in closure can not be read in time? #33591

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chenqinghe opened this issue Aug 12, 2019 · 4 comments
Closed

runtime: why a variable in closure can not be read in time? #33591

chenqinghe opened this issue Aug 12, 2019 · 4 comments
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@chenqinghe
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What version of Go are you using (go version)?

$ go version
go version go1.11 windows/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\username\AppData\Local\go-build
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOOS=windows
set GOPATH=E:\libs;E:\projects
set GOPROXY=
set GORACE=
set GOROOT=C:\Go
set GOTMPDIR=
set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
set GCCGO=gccgo
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\username\AppData\Local\Temp\go-build715944026=/tmp/go-build -gno-record-gcc-switches

What did you do?

here is the code:

package main

import (
	"fmt"
	"sync"
)

func main() {
	var (
		i int
		wg sync.WaitGroup
	)

	wg.Add(2)

	go func() {
		for {
			i++
		}
	}()

	go func() {
		for {
			fmt.Println(i)
		}
	}()

	wg.Wait()
}

my computer has 4 logic cpu, so these two goroutines may both have opportunity to run, but unfortunately, what was printed is always 0.

What did you expect to see?

print different value of i

What did you see instead?

always print 0
@bserdar
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bserdar commented Aug 12, 2019

The two goroutines do not communicate, so there is no guarantee that something happens in one is visible to the other.

https://golang.org/ref/mem

@av86743
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av86743 commented Aug 12, 2019

Dup of #10958.

@golang golang locked and limited conversation to collaborators Aug 11, 2020
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@av86743 @bserdar @gopherbot @chenqinghe and others