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

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Message-ID: <20201208080108.GP2414@hirez.programming.kicks-ass.net>
Date:   Tue, 8 Dec 2020 09:01:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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.

So I was worried that data_race(var) == const, would not allow the
compiler to emit

	cmpq $CONST, ();

but would instead force a separate load. But I checked and it does
generate the right code.