From: "Paul E. McKenney" <paulmck@kernel.org>
To: Marco Elver <elver@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
kasan-dev <kasan-dev@googlegroups.com>,
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
Subject: Re: [patch 3/3] tick: Annotate tick_do_timer_cpu data races
Date: Mon, 7 Dec 2020 11:44:06 -0800 [thread overview]
Message-ID: <20201207194406.GK2657@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <CANpmjNNQiTbnkkj+ZHS5xxQuQfnWN_JGwSnN-_xqfa=raVrXHQ@mail.gmail.com>
On Mon, Dec 07, 2020 at 07:19:51PM +0100, Marco Elver wrote:
> 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?
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
next prev parent reply other threads:[~2020-12-07 19:44 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-06 21:12 [patch 0/3] tick: Annotate and document the intentionaly racy tick_do_timer_cpu Thomas Gleixner
2020-12-06 21:12 ` [patch 1/3] tick: Remove pointless cpu valid check in hotplug code Thomas Gleixner
2020-12-07 11:59 ` Peter Zijlstra
2020-12-07 17:44 ` Thomas Gleixner
2020-12-11 22:21 ` Frederic Weisbecker
2020-12-12 0:16 ` Thomas Gleixner
2020-12-12 1:20 ` Frederic Weisbecker
2020-12-11 22:31 ` Frederic Weisbecker
2020-12-16 10:50 ` [tip: timers/urgent] " tip-bot2 for Thomas Gleixner
2020-12-06 21:12 ` [patch 2/3] tick/sched: Remove bogus boot "safety" check Thomas Gleixner
2020-12-11 22:41 ` Frederic Weisbecker
2020-12-16 10:50 ` [tip: timers/urgent] " tip-bot2 for Thomas Gleixner
2020-12-06 21:12 ` [patch 3/3] tick: Annotate tick_do_timer_cpu data races Thomas Gleixner
2020-12-07 12:09 ` Peter Zijlstra
2020-12-07 17:46 ` Thomas Gleixner
2020-12-07 18:19 ` Marco Elver
2020-12-07 19:43 ` Thomas Gleixner
2020-12-07 19:44 ` Paul E. McKenney [this message]
2020-12-07 21:46 ` Thomas Gleixner
2020-12-07 22:38 ` Paul E. McKenney
2020-12-07 22:46 ` Thomas Gleixner
2020-12-07 22:55 ` Paul E. McKenney
2020-12-08 8:11 ` Peter Zijlstra
2020-12-08 15:03 ` Paul E. McKenney
2020-12-16 0:27 ` Thomas Gleixner
2020-12-16 21:19 ` Paul E. McKenney
2020-12-16 21:23 ` Thomas Gleixner
2020-12-16 21:32 ` Paul E. McKenney
2020-12-17 10:48 ` Peter Zijlstra
2020-12-17 14:59 ` Paul E. McKenney
2020-12-08 8:01 ` Peter Zijlstra
2020-12-07 11:05 ` [patch 0/3] tick: Annotate and document the intentionaly racy tick_do_timer_cpu Marco Elver
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