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

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Date:   Sat, 23 Apr 2022 06:45:08 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 Sat, Apr 23, 2022 at 2:24 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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.

More details can be found in https://lwn.net/Articles/793253/

Thanks !