linux-kernel - Re: [PATCH v3 5/5] psi: introduce psi monitor

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Message-ID: <CAJuCfpHdtY7cRBfVP-+e9hQrTigStj2XK_qpk8z91HSKW+Y5WA@mail.gmail.com>
Date:   Tue, 29 Jan 2019 10:31:40 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>, lizefan@...wei.com,
        Johannes Weiner <hannes@...xchg.org>, axboe@...nel.dk,
        dennis@...nel.org, Dennis Zhou <dennisszhou@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, cgroups@...r.kernel.org,
        linux-mm <linux-mm@...ck.org>, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...roid.com
Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor

On Tue, Jan 29, 2019 at 10:18 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Tue, Jan 29, 2019 at 4:38 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Thu, Jan 24, 2019 at 01:15:18PM -0800, Suren Baghdasaryan wrote:
> > > +                     atomic_set(&group->polling, polling);
> > > +                     /*
> > > +                      * Memory barrier is needed to order group->polling
> > > +                      * write before times[] read in collect_percpu_times()
> > > +                      */
> > > +                     smp_mb__after_atomic();
> >
> > That's broken, smp_mb__{before,after}_atomic() can only be used on
> > atomic RmW operations, something atomic_set() is _not_.
>
> Oh, I didn't realize that. After reading the following example from
> atomic_ops.txt I was under impression that smp_mb__after_atomic()
> would make changes done by atomic_set() visible:
>
> /* All memory operations before this call will
> * be globally visible before the clear_bit().
> */
> smp_mb__before_atomic();
> clear_bit( ... );
> /* The clear_bit() will be visible before all
> * subsequent memory operations.
> */
> smp_mb__after_atomic();
>
> but I'm probably missing something. Is there a more detailed
> description of these rules anywhere else?

I was referred to memory-barriers.txt that explains this clearly
stating that "These functions do not imply memory barriers.". Thanks
for noticing! Will change to smp_mb().

> Meanwhile I'll change smp_mb__after_atomic() into smp_mb(). Would that
> fix the ordering?
>
> > --
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