netdev - Re: [PATCH net] net: Use this_cpu_inc() to increment net->core

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Date:   Thu, 21 Apr 2022 09:06:05 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH net] net: Use this_cpu_inc() to increment net->core_stats

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.

Perhaps use READ_ONCE() here ?

Yes, we have many similar folding loops that are  simply assuming
compiler won't do stupid things.