linux-kernel - Re: Confused about hlist_unhashed

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Message-ID: <20200203172554.GL2935@paulmck-ThinkPad-P72>
Date:   Mon, 3 Feb 2020 09:25:54 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Marco Elver' <elver@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Confused about hlist_unhashed_lockless()

On Mon, Feb 03, 2020 at 04:05:25PM +0000, David Laight wrote:
> From: Marco Elver
> > Sent: 03 February 2020 15:55
> > 
> > On Mon, 3 Feb 2020 at 16:45, David Laight <David.Laight@...lab.com> wrote:
> > >
> > > From: Eric Dumazet
> > > > Sent: 31 January 2020 18:53
> > > >
> > > > On Fri, Jan 31, 2020 at 10:48 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > > > >
> > > >
> > > > > This is nice, now with have data_race()
> > > > >
> > > > > Remember these patches were sent 2 months ago, at a time we were
> > > > > trying to sort out things.
> > > > >
> > > > > data_race() was merged a few days ago.
> > > >
> > > > Well, actually data_race() is not there yet anyway.
> > >
> > > Shouldn't it be NO_DATA_RACE() ??
> > 
> > Various options were considered, and based on feedback from Linus,
> > decided 'data_race(..)' is the best option:
> >   https://lore.kernel.org/linux-fsdevel/CAHk-
> > =wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@...l.gmail.com/
> > 
> > It's meant to be as unobtrusive as possible, and an all-caps macro was
> > ruled out.
> 
> Except that it then looks like something that actually does something.
> 
> > Second, the "NO_" prefix would be incorrect, since it'd be the
> > opposite of what it is. The macro is meant to document and mark a
> > deliberate data race.
> 
> It should be IGNORE_DATA_RACE() then.

People will get used to the name more quickly than they will get used
to typing the extra seven characters.  Here is the current comment header:

/*
 * data_race(): macro to document that accesses in an expression may conflict with
 * other concurrent accesses resulting in data races, but the resulting
 * behaviour is deemed safe regardless.
 *
 * This macro *does not* affect normal code generation, but is a hint to tooling
 * that data races here should be ignored.
 */

I will be converting this to docbook form.

In addition, in the KCSAN documentation:

* KCSAN understands the ``data_race(expr)`` annotation, which tells KCSAN that
  any data races due to accesses in ``expr`` should be ignored and resulting
  behaviour when encountering a data race is deemed safe.

Fair enough?

								Thanx, Paul