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

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Chenyu,

On 5/31/26 1:37 AM, Chen, Yu C wrote:
> 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()

Indeed. Thank you very much for the correction.

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

Thank you very much.

Reinette