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Date:   Thu, 21 Feb 2019 10:10:57 -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>,
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        David Ahern <dsahern@gmail.com>,
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        Ingo Molnar <mingo@redhat.com>,
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        <intel-wired-lan@lists.osuosl.org>,
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        Martin KaFai Lau <kafai@fb.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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        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 4/5] sched/topology: Annonate RCU pointers properly
Message-ID: <20190221151057.GA19213@google.com>
References: <20190221054942.132388-1-joel@joelfernandes.org>
 <20190221054942.132388-5-joel@joelfernandes.org>
 <20190221091944.GY32477@hirez.programming.kicks-ass.net>
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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?

(my same response applies to patch 5/5).

thanks,

 - Joel