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

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Message-ID: <20201207225542.GM2657@paulmck-ThinkPad-P72>
Date:   Mon, 7 Dec 2020 14:55:42 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Marco Elver <elver@...gle.com>,
        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 11:46:48PM +0100, Thomas Gleixner wrote:
> On Mon, Dec 07 2020 at 14:38, Paul E. McKenney wrote:
> 
> > On Mon, Dec 07, 2020 at 10:46:33PM +0100, Thomas Gleixner wrote:
> >> On Mon, Dec 07 2020 at 11:44, Paul E. McKenney wrote:
> >> > 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:
> >> >> 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()?
> >> 
> >> That was your suggestion a week or so ago :)
> >
> > You expected my suggestion to change?  ;-)
> 
> Your suggestion was data_race() IIRC but I might have lost track in that
> conversation.

OK, I am inconsistent after all.  I would have suggested READ_ONCE() given
no difference between them, so it is probably best to assume that there is
(or at least was) a good reason for data_race() instead of READ_ONCE().
Couldn't tell you what it might be, though.  :-/

							Thanx, Paul