linux-kernel - Re: [RFC PATCH v4 03/12] PM: Introduce an Energy Model management framework

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Message-ID: <20180705150908.GF32579@e108498-lin.cambridge.arm.com>
Date:   Thu, 5 Jul 2018 16:09:08 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, gregkh@...uxfoundation.org,
        mingo@...hat.com, dietmar.eggemann@....com,
        morten.rasmussen@....com, chris.redpath@....com,
        patrick.bellasi@....com, valentin.schneider@....com,
        vincent.guittot@...aro.org, thara.gopinath@...aro.org,
        viresh.kumar@...aro.org, tkjos@...gle.com, joel@...lfernandes.org,
        smuckle@...gle.com, adharmap@...cinc.com, skannan@...cinc.com,
        pkondeti@...eaurora.org, juri.lelli@...hat.com,
        edubezval@...il.com, srinivas.pandruvada@...ux.intel.com,
        currojerez@...eup.net, javi.merino@...nel.org
Subject: Re: [RFC PATCH v4 03/12] PM: Introduce an Energy Model management
 framework

On Thursday 05 Jul 2018 at 16:39:01 (+0200), Peter Zijlstra wrote:
> On Thu, Jun 28, 2018 at 12:40:34PM +0100, Quentin Perret wrote:
> > +/**
> > + * em_fd_nr_cap_states() - Get the number of capacity states of a freq. domain
> > + * @fd		: frequency domain for which want to do this
> > + *
> > + * Return: the number of capacity state in the frequency domain table
> > + */
> > +static inline int em_fd_nr_cap_states(struct em_freq_domain *fd)
> > +{
> > +	struct em_cs_table *table;
> > +	int nr_states;
> > +
> > +	rcu_read_lock();
> > +	table = rcu_dereference(fd->cs_table);
> > +	nr_states = table->nr_cap_states;
> > +	rcu_read_unlock();
> 
> So right here, we can continue to free @table...
> 
> > +
> > +	return nr_states;
> > +}
> 
> and then what does the value we return mean?

Hmm that's a good point. In practice the number of entries in the table
never changes, so that makes no sense to keep that value in the
RCU-protected struct. I can just move nr_cap_states in struct
freq_domain directly and access it without RCU protection, it is simply
not needed.

I'll change that for the next version.

Thanks,
Quentin