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

[Peter Zijlstra]

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.