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

[Babu Moger]

Hi Reinette,

On 5/28/26 11:11, 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.

Yes. I agree,

> b) Sashiko: "This is a pre-existing issue, but does resctrl_find_domain() safely traverse
>     the RCU list?
>     Yes, this is safe because resctrl_find_domain() is run with cpus_read_lock() that ensures
>     the list can be traversed safely because the list can only be modified with
>     CPU write lock held.
>     One improvement to resctrl that would help support this is to replace the "list_for_each()"
>     domain list traversals with something like:
>     	list_for_each_entry_rcu(pos, head, member, lockdep_is_cpus_held())
>     Doing something like above would help document why such list traversal outside of
>     RCU read-side critical section is safe.

Are you planning to make this change at this time? It appears to be a 
corner case, and we haven’t observed it in real-world scenarios.

Thanks
Babu