Re: [PATCH v3 1/2] nmi_backtrace: Allow excluding an arbitrary CPU

[Michal Hocko <mhocko@suse.com>]

On Fri 04-08-23 09:06:07, Doug Anderson wrote:
> Hi,
> 
> On Fri, Aug 4, 2023 at 8:02 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > > > It would have been slightly safer to modify arch_trigger_cpumask_backtrace
> > > > by switching arguments so that some leftovers are captured easier.
> > >
> > > I'm not sure I understand. Oh, you're saying make the prototype of
> > > arch_trigger_cpumask_backtrace() incompatible so that if someone is
> > > directly calling it then it'll be a compile-time error?
> >
> > exactly. bool to int promotion would be too easy to miss while the
> > pointer to int would complain loudly.
> >
> > > I guess the
> > > hope is that nobody is calling that directly and they're calling
> > > through the trigger_...() functions.
> >
> > Hope is one thing, being preventive another.
> >
> > > For now I'm going to leave this alone.
> >
> > If you are going to send another version then please consider this. Not
> > a hard requirement but better.
> 
> If I do send another version, do you have any suggestions for how to
> change this to make it incompatible?

I would swap parameters as this seems simplest.

> I guess swapping the order of the
> parameters would be best? I considered doing that for v4 but I felt
> like long term the current order of the parameters was better.

Yes the current ordering is better but having it other way around is not
really horrendous either.

> I also
> considered a rename, but that different problems. ;-) If I rename both
> the #define and the function then if someone has an out-of-tree patch
> adding arch_trigger_cpumask_backtrace() for another architecture, like
> say arm64, then there would be no compile-time failure indicating that
> the out-of-tree patch needs updating. I could rename the functions but
> _not_ the #define, I guess?

I think that swapping would be simplest as the type mismatch should
catch also pending out-of-tree potential implementations.

-- 
Michal Hocko
SUSE Labs