linux-kernel - Re: [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group

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Date:   Mon, 14 Dec 2020 10:38:58 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     tglx@...utronix.de, fenghua.yu@...el.com, bp@...en8.de,
        tony.luck@...el.com, kuo-lang.tseng@...el.com, shakeelb@...gle.com,
        mingo@...hat.com, babu.moger@....com, james.morse@....com,
        hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a
 resource group

Hi Valentin,

On 12/11/2020 12:46 PM, Valentin Schneider wrote:
> On 03/12/20 23:25, Reinette Chatre wrote:
>> Valentin's series in [2] ends by adding memory barriers to support the
>> updating of the task_struct from one CPU and the usage of the task_struct data
>> from another CPU. This work is still needed and as discussed with Valentin in
>> that thread the work would be re-evaluated by him after seeing how this series
>> turns out.
>>

Thank you very much for taking a look.

> 
> So the "problematic" pattern is still there: a context switch can happen
> concurrently with a write to the switching-to-tasks's {closid, rmid}.
> Accesses to these fields would thus need to be wrapped by READ_ONCE() &
> WRITE_ONCE().

ok.

> 
> Thinking a bit more (too much?) about it, we could limit ourselves to
> wrapping only reads not protected by the rdtgroup_mutex: the only two
> task_struct {closid, rmid} writers are
> - rdtgroup_move_task()
> - rdt_move_group_tasks()
> and they are both invoked while holding said mutex. Thus, a reader holding
> the mutex cannot race with a write, so load tearing ought to be safe.

The reads that are not protected by the rdtgroup_mutex can be found in 
__resctrl_sched_in(). It thus sounds to me like your proposed changes to 
this function found in your patch [1] is what is needed? It is not clear 
to me how the pairing would work in this case though. If I understand 
correctly the goal is for the  write to the closid/rmid in the functions 
you mention above to be paired with the reads in resctrl_sched_in() and 
it is not clear how adding a single READ_ONCE would accomplish this 
pairing by itself.

It is also not entirely clear to me what the problematic scenario could 
be. If I understand correctly, the risk is (as you explained in your 
commit message), that a CPU could have its {closid, rmid} fields read 
locally (resctrl_sched_in()) while they are concurrently being written 
to from another CPU (in rdtgroup_move_task() and rdt_move_group_tasks() 
as you state above). If this happens then a task being moved may be 
scheduled in with its old closid/rmid. The update of closid/rmid in 
rdtgroup_move_task()/rdt_move_group_tasks() is followed by 
smp_call_function_xx() where the registers are updated with preemption 
disabled and thus protected against __switch_to. If a task was thus 
incorrectly scheduled in with old closid/rmid, would it not be corrected 
at this point?

Thank you

Reinette

[1] 
https://lore.kernel.org/lkml/20201123022433.17905-4-valentin.schneider@arm.com/