On Monday, September 27, 2010, Nishanth Menon wrote:
> 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?
It's simply safer to put the rcu_read_lock() before find_device_opp(), in case
someone will make it possible to remove things from the opp list in the future.
Besides, your entire data structure consists of the opp list and the per-power
domain lists, so you should really tell the writers when you have finished to
traverse it entirely, not in the middle of the operation.
Thanks,
Rafael
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