英文:
Golang: writing to a variable has no effect in a goroutine if immediately followed by a `for{}` loop
问题
package main
import (
"fmt"
"time"
)
func main() {
var x = 0
go func() {
x = 1
for {
}
}()
time.Sleep(5000 * time.Millisecond)
fmt.Printf("%d", x)
}
为什么上面的代码会打印0？
如果在for{}之前或之中添加任何内容，例如runtime.Gosched()，print(x)，它将按预期打印1。
这与写屏障/先行发生关系吗？我完全困惑了。
我让另一个goroutine在打印值之前休眠了5秒钟。这不足以让写操作同步/缓存刷新/...吗？
英文:
package main
import (
"fmt"
"time"
)
func main() {
var x = 0
go func() {
x = 1
for {
}
}()
time.Sleep(5000 * time.Millisecond)
fmt.Printf("%d", x)
}
Why does the above code print 0?
If you add anything before or in the for{}, for example, runtime.Gosched(), print(x), it will print 1 as expected.
Is it related to write barrier / happened-before, or? I'm totally confused.
I made the other goroutine sleep for 5 seconds before printing the value. Isn't it enough time for the write to sync / cache to flush / ...?
答案1
得分: 3
感谢你的评论，Cerise。
~ go tool compile -S main.go
...
"".main.func1 STEXT nosplit size=3 args=0x0 locals=0x0 funcid=0x0
0x0000 00000 (main.go:10) TEXT "".main.func1(SB), NOSPLIT|NEEDCTXT|ABIInternal, $0-0
0x0000 00000 (main.go:10) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0000 00000 (main.go:10) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0000 00000 (main.go:12) XCHGL AX, AX
0x0001 00001 (main.go:1) JMP 0
0x0000 90 eb fd
看起来，Golang编译器确实在汇编代码中删除了x = 1语句（应该是main.go:11）。
在编程时，我们确实需要牢记《Go内存模型》和《Happens Before》的原则。
英文:
Thanks for the comment, Cerise.
~ go tool compile -S main.go
...
"".main.func1 STEXT nosplit size=3 args=0x0 locals=0x0 funcid=0x0
0x0000 00000 (main.go:10) TEXT "".main.func1(SB), NOSPLIT|NEEDCTXT|ABIInternal, $0-0
0x0000 00000 (main.go:10) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0000 00000 (main.go:10) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
0x0000 00000 (main.go:12) XCHGL AX, AX
0x0001 00001 (main.go:1) JMP 0
0x0000 90 eb fd
Seems like golang compiler indeed deleted the x = 1 statement in the assembly (should be main.go:11).
We really need to keep The Go Memory Model and Happens Before in mind when programming.
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