linux-kernel - Re: [PATCH v2 1/2] watchdog: Introduce arch_watchdog_nmi_enable and arch_watchdog_nmi

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Date:   Thu, 20 Oct 2016 12:14:14 -0400
From:   Don Zickus <dzickus@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Babu Moger <babu.moger@...cle.com>, mingo@...nel.org,
        ak@...ux.intel.com, jkosina@...e.cz,
        baiyaowei@...s.chinamobile.com, atomlin@...hat.com,
        uobergfe@...hat.com, tj@...nel.org, hidehiro.kawai.ez@...achi.com,
        johunt@...mai.com, davem@...emloft.net, sparclinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, sam@...nborg.org
Subject: Re: [PATCH v2 1/2] watchdog: Introduce arch_watchdog_nmi_enable and
 arch_watchdog_nmi_disable

On Wed, Oct 19, 2016 at 05:00:12PM -0700, Andrew Morton wrote:
> On Thu, 13 Oct 2016 13:38:01 -0700 Babu Moger <babu.moger@...cle.com> wrote:
> 
> > Currently we do not have a way to enable/disable arch specific
> > watchdog handlers if it was implemented by any of the architectures.
> > 
> > This patch introduces new functions arch_watchdog_nmi_enable and
> > arch_watchdog_nmi_disable which can be used to enable/disable architecture
> > specific NMI watchdog handlers. These functions are defined as weak as
> > architectures can override their definitions to enable/disable nmi
> > watchdog behaviour.
> > 
> > --- a/kernel/watchdog.c
> > +++ b/kernel/watchdog.c
> > @@ -676,8 +660,13 @@ static void watchdog_nmi_disable(unsigned int cpu)
> >  }
> >  
> >  #else
> > -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.

> 
> And I'm not really understanding the interaction with
> CONFIG_HARDLOCKUP_DETECTOR here.  I haven't really worked out why the
> code is all this way but it seems....  odd?

If the above explaination doesn't help, then can you point to some examples
where things seem odd?

Cheers,
Don