Re: [PATCH v4] power: introduce library for device-specific OPPs

[Nishanth Menon]

Paul E. McKenney had written, on 09/25/2010 07:56 PM, the following:
> On Sat, Sep 25, 2010 at 10:55:20PM +0200, Rafael J. Wysocki wrote:
> > On Friday, September 24, 2010, Paul E. McKenney wrote:
> > > On Fri, Sep 24, 2010 at 07:50:40AM -0500, Nishanth Menon wrote:
> > ...
> > > Looks like a good start!!!  Some questions and suggestions about RCU
> > > usage interspersed below.
> > ...
> > > > + * Locking: RCU reader.
> > > > + */
> > > > +int opp_get_opp_count(struct device *dev)
> > > > +{
> > > > +	struct device_opp *dev_opp;
> > > > +	struct opp *temp_opp;
> > > > +	int count = 0;
> > > > +
> > > > +	dev_opp = find_device_opp(dev);
> > > > +	if (IS_ERR(dev_opp))
> > > > +		return PTR_ERR(dev_opp);
> > > > +
> > > > +	rcu_read_lock();
> > > > +	list_for_each_entry_rcu(temp_opp, &dev_opp->opp_list, node) {
> > > > +		if (temp_opp->available)
> > > > +			count++;
> > > > +	}
> > > > +	rcu_read_unlock();
> > > This one is OK as well.  You are returning a count, so if all of the
> > > counted structures are freed at this point, no problem.  The count was
> > > valid when it was accumulated, and the fact that it might now be obsolete
> > > is (usually) not a problem.
> > However, it looks like it should run rcu_read_lock() before calling
> > find_device_opp(dev), shouldn't it?
>
> Indeed it does appear that you are right -- good catch!!!
>
> 							Thanx, Paul
dev_opp as discussed before is safe as it is never freed 
(find_device_opp uses it's own rcu_read_lock, the rcu_read_lock in this 
context is for the opp list. what am I missing?

ack on Paul's comments w.r.t risk on opp structures itself.. will look 
to fix that in v5.

--
Regards,
Nishanth Menon
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