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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 29 Jan 2019 10:25:58 -0800
Message-ID: <CAJuCfpHyuY89Aw3YpQpaor2hKvdqJOH5sMoXNZ-yNAnAb+tY6A@mail.gmail.com>
Subject: Re: [PATCH v3 5/5] psi: introduce psi monitor
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>, lizefan@huawei.com,
        Johannes Weiner <hannes@cmpxchg.org>, axboe@kernel.dk,
        dennis@kernel.org, Dennis Zhou <dennisszhou@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>, cgroups@vger.kernel.org,
        linux-mm <linux-mm@kvack.org>, linux-doc@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>, kernel-team@android.com
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On Tue, Jan 29, 2019 at 7:16 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Jan 29, 2019 at 01:38:43PM +0100, Peter Zijlstra 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_.
>
> Also; the comment should explain _why_ not only what.

Got it. Will change the comment to something like:

Order group->polling=0 before reading times[] in
collect_percpu_times() to detect possible race with hotpath that
modifies times[] before it sets group->polling=1 (see Race #1 in the
comments at the top).

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