linux-kernel - Re: Confused about hlist_unhashed

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Date:   Mon, 3 Feb 2020 16:02:28 +0000
From:   Will Deacon <will@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     David Laight <David.Laight@...LAB.COM>,
        'Eric Dumazet' <edumazet@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Marco Elver <elver@...gle.com>
Subject: Re: Confused about hlist_unhashed_lockless()

On Mon, Feb 03, 2020 at 07:58:39AM -0800, Paul E. McKenney wrote:
> On Mon, Feb 03, 2020 at 03:45:54PM +0000, David Laight 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() ??
> 
> No, because you use data_race() when there really are data races, but you
> want KCSAN to ignore them.  For example, diagnostic code that doesn't
> participate in the actual concurrency design and that doesn't run all
> that often might use data_race().  For another example, if a developer
> knew that data races existed, but that the compiler could not reasonably
> do anything untoward with those data races, that developer might well
> choose to use data_race() instead of READ_ONCE().  Especially if the
> access in question was on a fastpath where helpful compiler optimizations
> would be prohibited by use of READ_ONCE().

Yes, and in this particular case I think we can remove some WRITE_ONCE()s
from the non-RCU hlist code too (similarly for hlist_nulls).

Will