linux-kernel - Re: [PATCH V2 2/8] PM / OPP: Don't use OPP structure outside of rcu protected section

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Date:   Tue, 25 Oct 2016 09:07:36 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Stephen Boyd <sboyd@...eaurora.org>
Cc:     Rafael Wysocki <rjw@...ysocki.net>, nm@...com,
        Viresh Kumar <vireshk@...nel.org>,
        linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>, robh@...nel.org,
        d-gerlach@...com, broonie@...nel.org
Subject: Re: [PATCH V2 2/8] PM / OPP: Don't use OPP structure outside of rcu
 protected section

On 24-10-16, 15:52, Stephen Boyd wrote:
> On 10/20, Viresh Kumar wrote:
> > The OPP structure must not be used out of the rcu protected section.
> > Cache the values to be used in separate variables instead.
> > 
> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> 
> Was this found by visual inspection or through some static
> checker? Just curious.

Visual inspection :)

> > @@ -633,6 +634,14 @@ int dev_pm_opp_set_rate(struct device *dev, unsigned long target_freq)
> >  		return ret;
> >  	}
> >  
> > +	if (IS_ERR(old_opp)) {
> > +		old_u_volt = 0;
> > +	} else {
> > +		old_u_volt = old_opp->u_volt;
> > +		old_u_volt_min = old_opp->u_volt_min;
> > +		old_u_volt_max = old_opp->u_volt_max;
> > +	}
> > +
> >  	u_volt = opp->u_volt;
> >  	u_volt_min = opp->u_volt_min;
> >  	u_volt_max = opp->u_volt_max;
> > @@ -677,9 +686,10 @@ int dev_pm_opp_set_rate(struct device *dev, unsigned long target_freq)
> >  			__func__, old_freq);
> >  restore_voltage:
> >  	/* This shouldn't harm even if the voltages weren't updated earlier */
> > -	if (!IS_ERR(old_opp))
> > -		_set_opp_voltage(dev, reg, old_opp->u_volt,
> > -				 old_opp->u_volt_min, old_opp->u_volt_max);
> > +	if (old_u_volt) {
> 
> What if old_u_volt == 0 is valid?

How can that be valid ?

> We could have another variable
> like 'valid' or something that we use to figure out if we should
> set values instead. Then this isn't a potential pitfall.

I can do that but just wanted to know if we need that or not.

-- 
viresh