Re: [PATCH v2 1/2] watchdog: Introduce arch_watchdog_nmi_enable and arch_watchdog_nmi_disable
From: Andrew Morton
Date: Thu Oct 20 2016 - 23:25:33 EST
On Thu, 20 Oct 2016 12:14:14 -0400 Don Zickus <dzickus@xxxxxxxxxx> wrote:
> > > -static int watchdog_nmi_enable(unsigned int cpu) { return 0; }
> > > -static void watchdog_nmi_disable(unsigned int cpu) { return; }
> > > +/*
> > > + * These two functions are mostly architecture specific
> > > + * defining them as weak here.
> > > + */
> > > +int __weak arch_watchdog_nmi_enable(unsigned int cpu) { return 0; }
> > > +void __weak arch_watchdog_nmi_disable(unsigned int cpu) { return; }
> > > +
> > > #endif /* CONFIG_HARDLOCKUP_DETECTOR */
> >
> > This is a strange way of using __weak.
> >
> > Take a look at (one of many examples) kernel/module.c:module_alloc().
> > We simply provide a default implementation and some other compilation
> > unit can override (actually replace) that at link time. No strange
> > ifdeffing needed.
>
> Yeah, this is mostly because of how we enable the hardlockup detector.
>
> Some arches use the perf hw and enable CONFIG_HARDLOCKUP_DETECTOR. Other
> arches just use their own variant of nmi and set CONFIG_HAVE_NMI_WATCHDOG and
> the rest of the arches do not use this.
>
> So the thought was if CONFIG_HARDLOCKUP_DETECTOR use that implementation,
> everyone else use the __weak version. Then the arches like sparc can override
> the weak version with their own nmi enablement.
>
> I don't know how to represent those 3 states correctly and the above is what
> we end up with.
<head spins>
Is there a suitable site where we could capture these considerations in
a code comment?