On Thu, Feb 21, 2019 at 04:29:44PM +0100, Peter Zijlstra wrote: > 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(). May be then, all those pointers should be made __rcu as well. Then we can use rcu_assign_pointer() here. I will look more into it and study these functions as you are suggesting. thanks, - Joel
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