Re: x86/smpboot: Question regarding native_play_dead() __noreturn warning

[Waiman Long]

On 10/27/25 8:50 AM, Peter Zijlstra wrote:
> On Mon, Oct 27, 2025 at 01:23:02PM +0100, Thorsten Blum wrote:
> > Hi,
> >
> > I just came across this comment in arch/x86/kernel/smpboot.c:
> >
> > /*
> > * native_play_dead() is essentially a __noreturn function, but it can't
> > * be marked as such as the compiler may complain about it.
> > */
> > void native_play_dead(void) {
> > 	...
> > }
> >
> > and when I mark native_play_dead() as __noreturn, neither gcc nor clang
> > complain about it.
> >
> > The commit message 2743fe89d4d4 ("x86/idle: Disable IBRS when CPU is
> > offline to improve single-threaded performance") says:
> >
> > "Add a comment to say that native_play_dead() is a __noreturn function,
> > but it can't be marked as such to avoid confusion about the missing
> > MSR restoration code."
> >
> > Unfortunately, that doesn't really help me either. Can someone explain
> > what the issue was and if the comment is still valid? Otherwise, I'd
> > like to submit a patch adding __noreturn and removing the comment.
> I'm not sure either, it wasn't there in v2 but appeared in v3.
>
> v2: 20230620140625.1001886-3-longman@xxxxxxxxxx
> v3: 20230622003603.1188364-2-longman@xxxxxxxxxx
>
> The difference is that v2 tried to restore the msr after 'play_dead'
> which is silly, since it would never reach that code. v3 removed that
> dead restore code and added the confusing comment.
>
> There is a clue here though:
>
>    20230622054053.uy577qezu5a65buc@treble
>
> Josh suggests play_dead() should be marked noreturn (which it is in
> current kernels).
>
> Waiman then replies:
>
>    921e1b98-af36-1f51-5abe-dea36425b706@xxxxxxxxxx
>
> which is utterly confused again.

I don't remember exactly how I got the warning when __noreturn is added 
to native_play_dead(). It may be a limitation of the objtool or gcc that 
I was using at that time. If Thorsten doesn't have problem adding 
__noreturn, I won't mind him doing that and taking out the the comment. 
We can see if there is other issue coming up in the future.

Cheers,
Longman