linux-kernel - Re: [PATCH V5 1/2] base/drivers/arch_topology: Replace mutex with READ_ONCE / WRITE

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Date:   Thu, 29 Nov 2018 10:58:03 +0100
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        viresh.kumar@...aro.org, Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH V5 1/2] base/drivers/arch_topology: Replace mutex with
 READ_ONCE / WRITE_ONCE

On 29/11/18 10:18, Daniel Lezcano wrote:
> On 29/11/2018 08:04, Juri Lelli wrote:
> 
> [ ... ]
> 
> >> With or without this patch, it is the case:
> >>
> >>                 task1                      task2
> >>                   |                          |
> >>   read("/sys/.../cpu1/cpu_capacity)          |
> >>                   |                  write("/sys/.../cpu1/cpu_capacity")
> >>   read("/sys/.../cpu2/cpu_capacity)          |
> >>
> >>
> >> There is no guarantee userspace can have a consistent view of the
> >> capacity. As soon as it reads a capacity, it can be changed in its back.
> > 
> > True, but w/o the mutex task1 could read different cpu_capacity values
> > for a cluster (it actually can also with current implementation, we
> > should grab the mutex in the read path as well if we want to avoid
> > this). 
> 
> Even if the mutex is on the read path, the userspace can see different
> capacities because it will read the cpu_capacity per cpu directory.
> 
> The mutex will be take when reading cpu0/cpu_capacity, not for
> cpu[0-9]/cpu_capacity. Between two reads, a write can happen because the
> lock is released in between.
> 
> Do you agree with the patch ? Or do you want me to drop it ?

I don't actually have cases at hand that are showing regression with it,
I was just trying to understand if we might potentially hit problems in
the future. So, I'm not against this patch. :-)