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Date:   Wed, 16 Dec 2020 17:41:22 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Reinette Chatre <reinette.chatre@...el.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


On 14/12/20 18:38, Reinette Chatre wrote:
>> 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?

Right.

> 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.
>

So all the writes would need WRITE_ONCE(), but not all reads would require
a READ_ONCE() (those that can't race with writes shouldn't need them).

I'll go and update that patch so that you can bundle it with v2 of this
series.

> 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.

Worse, it may be scheduled with a mangled closid/rmid if the read in
resctrl_sched_in() is torn (i.e. compiled as a sequence of multiple
smaller-sized loads). This one of the things READ_ONCE() / WRITE_ONCE()
try to address.

> 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?
>

Excluding load/store tearing, then yes, the above works fine.

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

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