Re: [PATCH v7 02/24] x86/resctrl: kfree() rmid_ptrs from rdtgroup_exit()

[James Morse]

Hi Reinette,

On 13/12/2023 23:27, Reinette Chatre wrote:
> Hi James,
> 
> On 12/13/2023 10:03 AM, James Morse wrote:
>> On 09/11/2023 17:39, Reinette Chatre wrote:
>>> On 10/25/2023 11:03 AM, James Morse wrote:
> 
> ...
> 
>>>> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
>>>> index 19e0681f0435..0056c9962a44 100644
>>>> --- a/arch/x86/kernel/cpu/resctrl/core.c
>>>> +++ b/arch/x86/kernel/cpu/resctrl/core.c
>>>> @@ -992,7 +992,13 @@ late_initcall(resctrl_late_init);
>>>>  
>>>>  static void __exit resctrl_exit(void)
>>>>  {
>>>> +	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_L3].r_resctrl;
>>>> +
>>>>  	cpuhp_remove_state(rdt_online);
>>>> +
>>>> +	if (r->mon_capable)
>>>> +		rdt_put_mon_l3_config(r);
>>>> +
>>>>  	rdtgroup_exit();
>>>>  }
>>>
>>> I expect cleanup to do the inverse of init. I do not know what was the
>>> motivation for the rdtgroup_exit() to follow cpuhp_remove_state()
>>
>> This will invoke the hotplug callbacks, making it look to resctrl like all CPUs are
>> offline. This means it is then impossible for rdtgroup_exit() to race with the hotplug
>> notifiers. (if you could run this code...)

> hmmm ... if there is a risk of such a race would the init code not also be
> vulnerable to that with the notifiers up before rdtgroup_init()?

Nope, because this array is allocated behind rdt_get_mon_l3_config(), which ultimately
comes from get_rdt_resources() in resctrl_late_init() - which calls cpuhp_setup_state()
after all this init work has been done.

(cpu hp always gives me a headache1)


> The races you mention
> are not obvious to me. I see the filesystem and hotplug code protected against races via
> the mutex and static keys. Could you please elaborate on the flows of concern?

Functions like __check_limbo() (calling __rmid_entry()) are called under the
rdtgroup_mutex, but they don't consider that rmid_ptrs[] may be NULL.

But this could only happen if the limbo work ran after cpuhp_remove_state() - this can't
happen because the hotplug callbacks cancel the limbo work, and won't reschedule it if the
domain is going offline.


The only other path is via free_rmid(), I've not thought too much about this as
resctrl_exit() can't actually be invoked - this code is discarded by the linker.

It could be run on MPAM, but only in response to an 'error interrupt' (which is optional)
- and all the MPAM error interrupts indicate a software bug.

I've only invoked this path once, and rdtgroup_exit()s unregister_filesystem() didn't
remove all the files. I anticipate digging into this teardown code more once the bulk of
the MPAM driver is upstream.


> I am not advocating for cpuhp_remove_state() to be called later. I understand that
> it simplifies the flows to consider.
> 
>>> but I
>>> was expecting this new cleanup to be done after rdtgroup_exit() to be inverse
>>> of init. This cleanup is inserted in middle of two existing cleanup - could
>>> you please elaborate how this location was chosen?
>>
>> rdtgroup_exit() does nothing with the resctrl structures, it removes sysfs and debugfs
>> entries, and unregisters the filesystem.
>>
>> Hypothetically, you can't observe any effect of the rmid_ptrs array being freed as
>> all the CPUs are offline and the overflow/limbo threads should have been cancelled.
>> Once cpuhp_remove_state() has been called, this really doesn't matter.

> Sounds like nothing prevents this code from following the custom of cleanup to be
> inverse of init (yet keep cpuhp_remove_state() first).

I'll put the the rdt_put_mon_l3_config() call after rdtgroup_exit()...


Thanks,

James