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Date:   Sat, 23 Apr 2022 11:24:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH net] net: Use this_cpu_inc() to increment net->core_stats

On Thu, Apr 21, 2022 at 06:51:31PM +0200, Sebastian Andrzej Siewior wrote:
> On 2022-04-21 09:06:05 [-0700], Eric Dumazet wrote:
> > On Thu, Apr 21, 2022 at 7:00 AM Sebastian Andrzej Siewior
> > <bigeasy@...utronix.de> wrote:
> > >
> > 
> > >                 for_each_possible_cpu(i) {
> > >                         core_stats = per_cpu_ptr(p, i);
> > > -                       storage->rx_dropped += local_read(&core_stats->rx_dropped);
> > > -                       storage->tx_dropped += local_read(&core_stats->tx_dropped);
> > > -                       storage->rx_nohandler += local_read(&core_stats->rx_nohandler);
> > > +                       storage->rx_dropped += core_stats->rx_dropped;
> > > +                       storage->tx_dropped += core_stats->tx_dropped;
> > > +                       storage->rx_nohandler += core_stats->rx_nohandler;
> > 
> > I think that one of the reasons for me to use  local_read() was that
> > it provided what was needed to avoid future syzbot reports.
> 
> syzbot report due a plain read of a per-CPU variable which might be
> modified?
> 
> > Perhaps use READ_ONCE() here ?
> > 
> > Yes, we have many similar folding loops that are  simply assuming
> > compiler won't do stupid things.
> 
> I wasn't sure about that and added PeterZ to do some yelling here just
> in case. And yes, we have other sites doing exactly that. In
>    Documentation/core-api/this_cpu_ops.rst
> there is nothing about remote-READ-access (only that there should be no
> writes (due to parallel this_cpu_inc() on the local CPU)). I know that a
> 32bit write can be optimized in two 16bit writes in certain cases but a
> read is a read.
> PeterZ? :)

Eric is right. READ_ONCE() is 'required' to ensure the compiler doesn't
split the load and KCSAN konws about these things.

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