Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 21 Feb 2019 10:21:39 -0500
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     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, "Paul E. McKenney" <paulmck@linux.ibm.com>,
        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
Message-ID: <20190221152139.GB19213@google.com>
References: <20190221054942.132388-1-joel@joelfernandes.org>
 <20190221054942.132388-4-joel@joelfernandes.org>
 <20190221091805.GX32477@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190221091805.GX32477@hirez.programming.kicks-ass.net>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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);
> 
> This doesn't make any kind of sense to me.
> 

As per the rcu_assign_pointer() line, I inferred that
cpufreq_update_util_data is expected to be RCU protected. Reading the pointer
value of RCU pointers generally needs to be done from RCU read section, and
using rcu_dereference() (or using rcu_access()).

In this patch, I changed cpufreq_update_util_data to be __rcu annotated to
avoid the sparse error thrown by rcu_assign_pointer().

Instead of doing that, If your intention here is RELEASE barrier, should I
just replace in this function:
	rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
with:
	smp_store_release(per_cpu(cpufreq_update_util_data, cpu), data))
?

It would be nice IMO to be explicit about the intention of release/publish
semantics by using smp_store_release().

thanks,

 - Joel