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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Reinette Chatre <reinette.chatre@intel.com>
Cc:     tglx@linutronix.de, fenghua.yu@intel.com, bp@alien8.de,
        tony.luck@intel.com, kuo-lang.tseng@intel.com, shakeelb@google.com,
        mingo@redhat.com, babu.moger@amd.com, james.morse@arm.com,
        hpa@zytor.com, x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] x86/resctrl: Fix a few issues in moving a task to a resource group
In-reply-to: <cover.1607036601.git.reinette.chatre@intel.com>
Date:   Fri, 11 Dec 2020 20:46:27 +0000
Message-ID: <jhjk0tot6jg.mognet@arm.com>
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Hi Reinette,

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.
>

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().

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.

> [1]: https://lore.kernel.org/lkml/CALvZod7E9zzHwenzf7objzGKsdBmVwTgEJ0nPgs0LUFU3SN5Pw@mail.gmail.com/
> [2]: https://lore.kernel.org/lkml/20201123022433.17905-1-valentin.schneider@arm.com
>
> Fenghua Yu (3):
>   x86/resctrl: Move setting task's active CPU in a mask into helpers
>   x86/resctrl: Update PQR_ASSOC MSR synchronously when moving task to
>     resource group
>   x86/resctrl: Don't move a task to the same resource group
>
>  arch/x86/kernel/cpu/resctrl/rdtgroup.c | 159 +++++++++++++------------
>  1 file changed, 82 insertions(+), 77 deletions(-)
>
>
> base-commit: b65054597872ce3aefbc6a666385eabdf9e288da