Re: [PATCH v2 0/3] x86/intel_rdt: task_work vs task_struct rmid/closid write race

[Valentin Schneider]

On 25/11/20 19:06, Reinette Chatre wrote:
> Hi Valentin,
>
> On 11/25/2020 10:39 AM, Valentin Schneider wrote:
>> The (default) TWA_RESUME ensures the targeted (userspace) task gets kicked
>> if it is currently running, and doesn't perturb any CPU otherwise;
>> see set_notify_resume() + exit_to_user_mode_loop() (or do_notify_resume()
>> on arm64)
>
> I missed that, thanks. The first issue is thus not a problem. Thank you
> very much for clearing this up. Queueing work for tasks that are not
> running remains unnecessary and simplifying this with a targeted
> smp_call_function addresses that (while also taking care of the other
> issues with using the queued work).
>

Right.

>>> In the new solution, after updating closid/rmid in the task_struct, the
>>> CPU register is updated via smp_call_function_single() on a CPU the task
>>> is running. Nothing is done for tasks not running, next time they are
>>> scheduled in the CPU's register will be updated to reflect the task's
>>> closid/rmid. Moving to the smp_call_function_xxx() API would also bring
>>> this update in line with how other register updates are already done in
>>> resctrl.
>>>
>>>> Kernel threads however are a prickly matter because they quite explicitly
>>>> don't have this return to userspace - they only run their task_work
>>>> callbacks on exit. So we currently have to wait for those kthreads to go
>>>> through a context switch to update the relevant register, but I don't
>>>> see any other alternative that wouldn't involve interrupting every other
>>>> CPU (the kthread could move between us triggering some remote work and its
>>>> previous CPU receiving the IPI).
>>>
>>> This seems ok? In the new solution the closid/rmid would be updated in
>>> task_struct and a smp_call_function_single() attempted on the CPU where
>>> the kthread is running. If the kthread is no longer running at the time
>>> the function is called the CPU register will not be changed.
>>
>> Right, if the update happens before triggering the remote work then that
>> should all work. I was stuck thinking about keeping the update contained
>> within the remote work itself to prevent any other races (i.e. patch 3).
>
> Are you saying that the task_struct update as well as register update
> should both be done in the remote work? I think I may be
> misunderstanding though.

It would simplify the concurrency aspect - if the {closid, rmid} update is
always done on the targeted task' context, then there can be no races
between an update (write) and a context switch (read). Sadly I don't see a
nice way to do this for kthreads, so I think it'll have to be update +
smp_call.


> Currently, with your entire series applied, the
> update to task_struct is done before the remote work is queued that only
> changes the register. The new solution would also first update the
> task_struct and then the remote work (this time with smp_call_function)
> will just update the register.
>
>  From what I understand your work in patch 3 would continue to be
> welcome with the new solution that will also update the task_struct and
> then trigger the remote work to just update the register.
>

That's how I see it as well ATM.

>> Anywho, that's enough speculation from me, I'll just sit tight and see what
>> comes next!
>>
>
> Reinette
>
>>> I assume
>>> the kthread move would include a context switch that would result in the
>>> register change (__switch_to()->resctrl_sched_in()) for the kthread to
>>> run with its new closid/rmid after the move.
>>>
>
>
> Reinette