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

[Peter Zijlstra]

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).