linux-kernel - Re: [patch 3/3] tick: Annotate tick_do_timer

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Date:   Mon, 7 Dec 2020 11:44:06 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Ingo Molnar <mingo@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Will Deacon <will@...nel.org>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        syzbot+23a256029191772c2f02@...kaller.appspotmail.com,
        syzbot+56078ac0b9071335a745@...kaller.appspotmail.com,
        syzbot+867130cb240c41f15164@...kaller.appspotmail.com
Subject: Re: [patch 3/3] tick: Annotate tick_do_timer_cpu data races

On Mon, Dec 07, 2020 at 07:19:51PM +0100, Marco Elver wrote:
> On Mon, 7 Dec 2020 at 18:46, Thomas Gleixner <tglx@...utronix.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?

I am also concerned about inadvertently covering code with data_race().

Also, in this particular case, why data_race() rather than READ_ONCE()?
Do we really expect the compiler to be able to optimize this case
significantly without READ_ONCE()?

							Thanx, Paul