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Date: Wed, 25 May 2016 10:51:14 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: David Carrillo-Cisneros <davidcc@google.com>
cc: Peter Zijlstra <peterz@infradead.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
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        Tony Luck <tony.luck@intel.com>, Stephane Eranian <eranian@google.com>,
        Paul Turner <pjt@google.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 10/32] perf/x86/intel/cqm: introduce (I)state and
 limbo prmids
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On Tue, 24 May 2016, David Carrillo-Cisneros wrote:
> >> +static inline bool __pmonr__in_instate(struct pmonr *pmonr)
> >> +{
> >> +     lockdep_assert_held(&__pkg_data(pmonr, pkg_data_lock));
> >> +     return __pmonr__in_istate(pmonr) && !__pmonr__in_ilstate(pmonr);
> >>  }
> >
> > This state tracking sucks. It's completely non obvious which combinations of
> > members are denoting a certain state.
> >
> > What's wrong with having:
> >
> >        pmonr->state
> >
> > and a enum
> >
> > enum pmonr_state {
> >      PMONR_UNUSED,
> >      PMONR_ACTIVE,
> >      PMONR_LIMBO,
> >      PMONR_INHERITED,
> > };
> >
> > That would make all this horror readable and understandable. I bet you can't
> > remember the meaning of all this state stuff 3 month from now. That's going to
> > be the hell of a ride to track down a problem in this code.
> 
> In the pmonr, the state can be inferred by the values of:
>   - pmonr->ancestor_pmonr
>   - pmonr->prmid
>   - pmonr->limbo_prmid

And exaclty that stuff drives me nuts. You update stuff here and there and
then you infer the state from this.

> Redundantly storing the state in an extra variable opens the door to
> bugs that updates pmonr::state inconsistently with the member above.

Well, your 'infer' state from three other variables is error prone as well and
you can simply add a debug feature which makes sure that the variables are
consistent.

validate_state(p)
{
	switch (p->state) {
	case PMONR_UNUSED:
	     WARN_ON(p->xxxx .....);

	case PMONR_ACTIVE:
	     WARN_ON(p->xxxx .....);
	}
}

That's way better than relying on three variables which are updated here and
there to reflect the proper state.

It's not only the 3 variables which are involved there. You also have lists
and whatever which depend on this. So having a proper 'state' variable as the
central anchor gives you the ability to verify the dependent contents of your
other variables, lists etc.

Thanks,

	tglx