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From:   Marco Elver <elver@google.com>
Date:   Mon, 7 Dec 2020 19:19:51 +0100
Message-ID: <CANpmjNNQiTbnkkj+ZHS5xxQuQfnWN_JGwSnN-_xqfa=raVrXHQ@mail.gmail.com>
Subject: Re: [patch 3/3] tick: Annotate tick_do_timer_cpu data races
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Will Deacon <will@kernel.org>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        syzbot+23a256029191772c2f02@syzkaller.appspotmail.com,
        syzbot+56078ac0b9071335a745@syzkaller.appspotmail.com,
        syzbot+867130cb240c41f15164@syzkaller.appspotmail.com
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On Mon, 7 Dec 2020 at 18:46, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Mon, Dec 07 2020 at 13:09, Peter Zijlstra wrote:
> > On Sun, Dec 06, 2020 at 10:12:56PM +0100, Thomas Gleixner wrote:
> >> +            if (data_race(tick_do_timer_cpu) == TICK_DO_TIMER_BOOT) {
> >
> > I prefer the form:
> >
> >       if (data_race(tick_do_timer_cpu == TICK_DO_TIMER_BOOT)) {
> >
> > But there doesn't yet seem to be sufficient data_race() usage in the
> > kernel to see which of the forms is preferred. Do we want to bike-shed
> > this now and document the outcome somewhere?
>
> Yes please before we get a gazillion of patches changing half of them
> half a year from now.

That rule should be as simple as possible. The simplest would be:
"Only enclose the smallest required expression in data_race(); keep
the number of required data_race() expressions to a minimum." (=> want
least amount of code inside data_race() with the least number of
data_race()s).

In the case here, that'd be the "if (data_race(tick_do_timer_cpu) ==
..." variant.

Otherwise there's the possibility that we'll end up with accesses
inside data_race() that we hadn't planned for. For example, somebody
refactors some code replacing constants with variables.

I currently don't know what the rule for Peter's preferred variant
would be, without running the risk of some accidentally data_race()'d
accesses.

Thoughts?

Thanks,
-- Marco