That's a really useful workaround! It makes much more sense now.
Conclusion: when `sigtramp` is returned, nothing is called before entering
`ayncPreempt`.
Thank you so much for releasing me from confusing :)
sigtramp is returned
asyncPreempt is called
OK
CALL runtime·sigtrampgo(SB)
CALL runtime·printmsg(SB)
TEXT ·asyncPreempt(SB),NOSPLIT|NOFRAME,$0-0
CALL runtime·printmsg2(SB)
var msg1 = []byte("sigtramp is returned\n")
var msg2 = []byte("asyncPreempt is called\n")
//go:nosplit
func printmsg() {
write(2, unsafe.Pointer(&msg1[0]), int32(len(msg1)))
}
//go:nosplit
func printmsg2() {
write(2, unsafe.Pointer(&msg2[0]), int32(len(msg2)))
}
On Monday, December 9, 2019 at 12:42:42 AM UTC+1, Ian Lance Taylor wrote:
>
> On Sun, Dec 8, 2019 at 1:10 PM changkun <euryu...@gmail.com <javascript:>>
> wrote:
> >
> > Thank you so much for your hint, I think I've figured it out.
> > The root cause seems similar to the "uncertainty principle".
> >
> > As an observer, by adding a `println` call in `asyncPreempt`
> > as well as `asyncPreempt2` influences the actual behavior
> > after signal handling. The `println` involves stack split check,
> > which calls the `morestack`.
>
> Ah, of course. One workaround is to use write, as in
>
> // Package-scope variable
> var myMsg = []byte("my message")
>
> // In function.
> write(2, unsafe.Pointer(&myMsg[0]), int32(len(myMsg)))
>
> This works because the various implementations of write are marked
> nosplit.
>
> Ian
>
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