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

From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	linux-kernel@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
	Christian Brauner <christian@brauner.io>,
	Daniel Borkmann <daniel@iogearbox.net>,
	David Ahern <dsahern@gmail.com>,
	"David S. Miller" <davem@davemloft.net>,
	Ido Schimmel <idosch@mellanox.com>,
	Ingo Molnar <mingo@redhat.com>,
	"moderated list:INTEL ETHERNET DRIVERS" 
	<intel-wired-lan@lists.osuosl.org>,
	Jakub Kicinski <jakub.kicinski@netronome.com>,
	Jeff Kirsher <jeffrey.t.kirsher@intel.com>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	Josh Triplett <josh@joshtriplett.org>,
	keescook@chromium.org, Lai Jiangshan <jiangshanlai@gmail.com>,
	Martin KaFai Lau <kafai@fb.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	netdev@vger.kernel.org, rcu@vger.kernel.org,
	Song Liu <songliubraving@fb.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	xdp-newbies@vger.kernel.org, Yonghong Song <yhs@fb.com>
Subject: Re: [PATCH RFC 3/5] sched/cpufreq: Fix incorrect RCU API usage
Date: Thu, 21 Feb 2019 09:29:48 -0800	[thread overview]
Message-ID: <20190221172948.GA11787@linux.ibm.com> (raw)
In-Reply-To: <20190221171311.GA118415@google.com>

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