linux-kernel - Re: [PATCH net] tcp: Add READ_ONCE() to read tcp_orphan

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Date:   Tue, 17 May 2022 13:28:28 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Marco Elver <elver@...gle.com>, Liu Jian <liujian56@...wei.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tcp: Add READ_ONCE() to read tcp_orphan_count

On Thu, May 12, 2022 at 04:43:20PM -0700, Eric Dumazet wrote:
> On Thu, May 12, 2022 at 4:10 PM Paul E. McKenney <paulmck@...nel.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@...gle.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 :/

Indeed, adding READ_ONCE() instances can be annoying.  Then again, it
can also be annoying to have to debug the problems that sometimes arise
from omitting them where they are needed.

							Thanx, Paul