linux-kernel - Re: Confused about hlist_unhashed

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Message-ID: <20200131205204.GT2935@paulmck-ThinkPad-P72>
Date:   Fri, 31 Jan 2020 12:52:04 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Eric Dumazet <edumazet@...gle.com>, Will Deacon <will@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: Confused about hlist_unhashed_lockless()

On Fri, Jan 31, 2020 at 08:47:23PM +0100, Thomas Gleixner wrote:
> Eric Dumazet <edumazet@...gle.com> writes:
> > On Fri, Jan 31, 2020 at 9:21 AM Will Deacon <will@...nel.org> wrote:
> >> Without serialisation, timer_pending() as currently implemented does
> >> not reliably tell you whether the timer is in the hlist. Is that not a
> >> problem?
> >
> > No it is not a problem.
> 
> Even if we would take the base lock then this is just a snapshot, which
> can be wrong at the moment the lock is dropped. So why bother?

The risk of leaving it as-is or of using data_race() is that if it is
checked multiple times, the compiler might use the old value.  Yes,
we could say that things like barrier() should be used in those cases,
but READ_ONCE() has the advantage of making it so that no one has to
worry about those cases.

							Thanx, Paul