Re: [PATCH v3 8/9] x86/resctrl: Ensure domain fully initialized before placed on RCU list

[Chen, Yu C]

On 5/29/2026 12:11 AM, Reinette Chatre wrote:
> On 5/22/26 12:15 PM, Reinette Chatre wrote:
> >   static void l3_mon_domain_setup(int cpu, int id, struct rdt_resource *r, struct list_head *add_pos)
> > @@ -556,14 +554,12 @@ static void l3_mon_domain_setup(int cpu, int id, struct rdt_resource *r, struct
> >   		return;
> >   	}
> >   
> > -	list_add_tail_rcu(&d->hdr.list, add_pos);
> > -
> >   	err = resctrl_online_mon_domain(r, &d->hdr);
> >   	if (err) {
> > -		list_del_rcu(&d->hdr.list);
> > -		synchronize_rcu();
> >   		l3_mon_domain_free(hw_dom);
> > +		return;
> >   	}
> > +	list_add_tail_rcu(&d->hdr.list, add_pos);
> >   }
> >   
> >   static void domain_add_cpu_mon(int cpu, struct rdt_resource *r)
>
> I resubmitted the last three patches of series to obtain Sashiko review [1] and
> respond to that feedback here:
>
> a) Sashiko: "Does this reordering expose the monitor directories to userspace before the
>     domain is actually added to the RCU list?"
>
>     Yes. As pointed out by Sashiko there is a short time where the monitoring data files
>     may be exposed to user space before the monitoring domain is added to the RCU list.
>     Also pointed out by Sashiko, if user attempts to read from such file it will return
>     -ENOENT.
>
>     This behavior looks correct and acceptable to me.

It might not return -ENOENT because the read will be blocked by 
cpus_read_lock()
and after the reader has grabed the cpus_read_lock(), the reader will 
see the newly
added domain on the list:

CPU 0 (writer)                            CPU 1 (reader)
==========================                ==========================
cpus_write_lock held                      cpus_read_lock() **BLOCKED**
  mkdir mondata files
  list_add_tail_rcu()
cpus_write_unlock                         cpus_read_lock() returns
                                            lock(rdtgroup_mutex)
                                            resctrl_find_domain() **see 
a new complete domain**
                                            unlock(rdtgroup_mutex)
                                          cpus_read_unlock()


Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>

thanks,
Chenyu