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From:   Eric Dumazet <edumazet@google.com>
Date:   Thu, 12 May 2022 16:43:20 -0700
Message-ID: <CANn89iKiTiGwMvV6K+Zbr_9+knaR-x1N3tOeMX+2No2=4zn6pA@mail.gmail.com>
Subject: Re: [PATCH net] tcp: Add READ_ONCE() to read tcp_orphan_count
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Marco Elver <elver@google.com>, Liu Jian <liujian56@huawei.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        David Miller <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Neal Cardwell <ncardwell@google.com>,
        netdev <netdev@vger.kernel.org>
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On Thu, May 12, 2022 at 4:10 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Thu, May 12, 2022 at 02:31:48PM -0700, Eric Dumazet wrote:
> > On Thu, May 12, 2022 at 2:18 PM Marco Elver <elver@google.com> wrote:
> >
> > >
> > > I guess the question is, is it the norm that per_cpu() retrieves data
> > > that can legally be modified concurrently, or not. If not, and in most
> > > cases it's a bug, the annotations should be here.
> > >
> > > Paul, was there any guidance/documentation on this, but I fail to find
> > > it right now? (access-marking.txt doesn't say much about per-CPU
> > > data.)
> >
> > Normally, whenever we add a READ_ONCE(), we are supposed to add a comment.
>
> I am starting to think that comments are even more necessary for unmarked
> accesses to shared variables, with the comments setting out why the
> compiler cannot mess things up.  ;-)
>
> > We could make an exception for per_cpu_once(), because the comment
> > would be centralized
> > at per_cpu_once() definition.
>
> This makes a lot of sense to me.
>
> > We will be stuck with READ_ONCE() in places we are using
> > per_cpu_ptr(), for example
> > in dev_fetch_sw_netstats()
>
> If this is strictly statistics, data_race() is another possibility.
> But it does not constrain the compiler at all.

Statistics are supposed to be monotonically increasing ;)

Some SNMP agents would be very confused if they could observe 'garbage' there.

I sense that we are going to add thousands of READ_ONCE() soon :/