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

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Message-ID: <1e013fee-e218-c489-9cd6-a384950d304f@linaro.org>
Date:   Fri, 23 Nov 2018 17:54:22 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Sudeep Holla <sudeep.holla@....com>
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>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH 2/4] base/drivers/arch_topology: Replace mutex with
 READ_ONCE / WRITE_ONCE

On 23/11/2018 17:20, Sudeep Holla wrote:
> 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.

It's ok [added Juri Lelli]

I've been through the history:

commit be8f185d8af4dbd450023a42a48c6faa8cbcdfe6
Author: Juri Lelli <juri.lelli@....com>
Date:   Thu Nov 3 05:40:18 2016 +0000

    arm64: add sysfs cpu_capacity attribute

    Add a sysfs cpu_capacity attribute with which it is possible to read and
    write (thus over-writing default values) CPUs capacity. This might be
    useful in situations where values needs changing after boot.

    The new attribute shows up as:

     /sys/devices/system/cpu/cpu*/cpu_capacity

    Cc: Will Deacon <will.deacon@....com>
    Cc: Mark Brown <broonie@...nel.org>
    Cc: Sudeep Holla <sudeep.holla@....com>
    Signed-off-by: Juri Lelli <juri.lelli@....com>
    Signed-off-by: Catalin Marinas <catalin.marinas@....com>

Juri do you have a use case where we want to override the capacity?

Shall we switch the sysfs attribute to read-only?


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