linux-kernel - Re: [RFD] resctrl: reassigning a running container's CTRL

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Date:   Thu, 27 Oct 2022 09:56:33 +0200
From:   Peter Newman <peternewman@...gle.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
Cc:     James Morse <james.morse@....com>, Tony Luck <tony.luck@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Eranian, Stephane" <eranian@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Babu Moger <Babu.Moger@....com>,
        Gaurang Upasani <gupasani@...gle.com>
Subject: Re: [RFD] resctrl: reassigning a running container's CTRL_MON group

Hi Reinette,

On Wed, Oct 26, 2022 at 11:12 PM Reinette Chatre
<reinette.chatre@...el.com> wrote:
> The original concern is "the stores to t->closid and t->rmid could be
> reordered with the task_curr(t) and task_cpu(t) reads which follow". I can see
> that issue. Have you considered using the compiler barrier, barrier(), instead?
> From what I understand it will prevent the compiler from moving the memory accesses.
> This is what is currently done in __rdtgroup_move_task() and could be done here also?

A memory system (including those on x86) is allowed to reorder a store with a
later load, in addition to the compiler.

Also because the locations in question can be concurrently accessed by another
CPU, a compiler barrier would not be sufficient.

-Peter