linux-kernel - Re: [PATCH RFC 4/5] sched/topology: Annonate RCU pointers properly

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Date:   Thu, 21 Feb 2019 16:29:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Ahern <dsahern@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Ido Schimmel <idosch@...lanox.com>,
        Ingo Molnar <mingo@...hat.com>,
        "moderated list:INTEL ETHERNET DRIVERS" 
        <intel-wired-lan@...ts.osuosl.org>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Josh Triplett <josh@...htriplett.org>, keescook@...omium.org,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Martin KaFai Lau <kafai@...com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        netdev@...r.kernel.org, "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        rcu@...r.kernel.org, Song Liu <songliubraving@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        xdp-newbies@...r.kernel.org, Yonghong Song <yhs@...com>
Subject: Re: [PATCH RFC 4/5] sched/topology: Annonate RCU pointers properly

On Thu, Feb 21, 2019 at 10:10:57AM -0500, Joel Fernandes wrote:
> Hi Peter,
> 
> Thanks for taking a look.
> 
> On Thu, Feb 21, 2019 at 10:19:44AM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 21, 2019 at 12:49:41AM -0500, Joel Fernandes (Google) wrote:
> > 
> > > Also replace rcu_assign_pointer call on rq->sd with WRITE_ONCE. This
> > > should be sufficient for the rq->sd initialization.
> > 
> > > @@ -668,7 +668,7 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
> > >  
> > >  	rq_attach_root(rq, rd);
> > >  	tmp = rq->sd;
> > > -	rcu_assign_pointer(rq->sd, sd);
> > > +	WRITE_ONCE(rq->sd, sd);
> > >  	dirty_sched_domain_sysctl(cpu);
> > >  	destroy_sched_domains(tmp);
> > 
> > Where did the RELEASE barrier go?
> > 
> > That was a publish operation, now it is not.
> 
> Funny thing is, initially I had written this patch with smp_store_release()
> instead of WRITE_ONCE, but checkpatch complaints with that since it needs a
> comment on top of it, and I wasn't sure if RELEASE barrier was the intent of
> using rcu_assign_pointer (all the more reason to replace it with something
> more explicit).
> 
> I will replace it with the following and resubmit it then:
> 
> /* Release barrier */
> smp_store_release(&rq->sd, sd);
> 
> Or do we want to just drop the "Release barrier" comment and live with the
> checkpatch warning?

How about we keep using rcu_assign_pointer(), the whole sched domain
tree is under rcu; peruse that destroy_sched_domains() function for
instance.

Also check how for_each_domain() uses rcu_dereference().