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

From: Joel Fernandes
Date: Thu Feb 21 2019 - 10:11:03 EST


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