linux-kernel - Re: [PATCH 2/4] base/drivers/arch_topology: Replace mutex with READ_ONCE / WRITE

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Date:   Fri, 23 Nov 2018 16:20:40 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     rjw@...ysocki.net, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH 2/4] base/drivers/arch_topology: Replace mutex with
 READ_ONCE / WRITE_ONCE

On Fri, Nov 23, 2018 at 05:04:18PM +0100, Daniel Lezcano wrote:
> On 23/11/2018 14:58, Sudeep Holla wrote:
> > On Mon, Oct 29, 2018 at 05:23:18PM +0100, Daniel Lezcano wrote:
> >> The mutex protects a per_cpu variable access. The potential race can
> >> happen only when the cpufreq governor module is loaded and at the same
> >> time the cpu capacity is changed in the sysfs.
> >>
> >
> > I wonder if we really need that sysfs entry to be writable. For some
> > reason, I had assumed it's read only, obviously it's not. I prefer to
> > make it RO if there's no strong reason other than debug purposes.
>
> Are you suggesting to remove the READ_ONCE/WRITE_ONCE patch and set the
> sysfs file read-only ?
>

Just to be sure, if we retain RW capability we still need to fix the
race you are pointing out.

However I just don't see the need for RW cpu_capacity sysfs and hence
asking the reason here. IIRC I had pointed this out long back(not sure
internally or externally) but seemed to have missed the version that got
merged. So I am just asking if we really need write capability given that
it has known issues.

If user-space starts writing the value to influence the scheduler, then
it makes it difficult for kernel to change the way it uses the
cpu_capacity in future.

Sorry if there's valid usecase and I am just making noise here.

--
Regards,
Sudeep