Re: [PATCH RFC 3/5] sched/cpufreq: Fix incorrect RCU API usage

[Paul E. McKenney]

On Thu, Feb 21, 2019 at 12:13:11PM -0500, Joel Fernandes wrote:
> On Thu, Feb 21, 2019 at 05:11:44PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 21, 2019 at 07:52:18AM -0800, Paul E. McKenney wrote:
> > > On Thu, Feb 21, 2019 at 04:31:17PM +0100, Peter Zijlstra wrote:
> > > > On Thu, Feb 21, 2019 at 10:21:39AM -0500, Joel Fernandes wrote:
> > > > > On Thu, Feb 21, 2019 at 10:18:05AM +0100, Peter Zijlstra wrote:
> > > > > > On Thu, Feb 21, 2019 at 12:49:40AM -0500, Joel Fernandes (Google) wrote:
> > > > > > > @@ -34,8 +34,12 @@ void cpufreq_add_update_util_hook(int cpu, struct update_util_data *data,
> > > > > > >  	if (WARN_ON(!data || !func))
> > > > > > >  		return;
> > > > > > >  
> > > > > > > -	if (WARN_ON(per_cpu(cpufreq_update_util_data, cpu)))
> > > > > > > +	rcu_read_lock();
> > > > > > > +	if (WARN_ON(rcu_dereference(per_cpu(cpufreq_update_util_data, cpu)))) {
> > > > > > > +		rcu_read_unlock();
> > > > > > >  		return;
> > > > > > > +	}
> > > > > > > +	rcu_read_unlock();
> > > > > > >  
> > > > > > >  	data->func = func;
> > > > > > >  	rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
> > 
> > > For whatever it is worth, in that case it could use rcu_access_pointer().
> > > And this primitive does not do the lockdep check for being within an RCU
> > > read-side critical section.  As Peter says, if there is no dereferencing,
> > > there can be no use-after-free bug, so the RCU read-side critical is
> > > not needed.
> > 
> > On top of that, I suspect this is under the write-side lock (we're doing
> > assignment after all).
> 
> Yes it is under a policy->rwsem, just confirmed that.
> 
> And indeed rcu_read_lock() is not needed here in this patch, thanks for
> pointing that out. Sometimes the word "dereference" plays some visual tricks
> and in this case tempted me to add an RCU reader section ;-) Assuming you
> guys are in agreement with usage of rcu_access_pointer() here to keep sparse
> happy, I'll rework the patch accordingly and resubmit with that.

Works for me!

							Thanx, Paul