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

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Message-ID: <20201216213218.GM2657@paulmck-ThinkPad-P72>
Date:   Wed, 16 Dec 2020 13:32:18 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>,
        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 Wed, Dec 16, 2020 at 10:23:57PM +0100, Thomas Gleixner wrote:
> On Wed, Dec 16 2020 at 13:19, Paul E. McKenney wrote:
> > On Wed, Dec 16, 2020 at 01:27:43AM +0100, Thomas Gleixner wrote:
> >> So my intent was to document that this code does not care about anything
> >> else than what I'd consider to be plain compiler bugs.
> >> 
> >> My conclusion might be wrong as usual :)
> >
> > Given that there is no optimization potential, then the main reason to use
> > data_race() instead of *_ONCE() is to prevent KCSAN from considering the
> > accesses when looking for data races.  But that is mostly for debugging
> > accesses, in cases when these accesses are not really part of the
> > concurrent algorithm.
> >
> > So if I understand the situation correctly, I would be using *ONCE().
> 
> Could this be spelled out somewhere in Documentation/ please?

Good point!  I will put a patch together.

							Thanx, Paul