From: "Paul E. McKenney" <paulmck@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Eric Dumazet <edumazet@google.com>, Will Deacon <will@kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: Confused about hlist_unhashed_lockless()
Date: Fri, 31 Jan 2020 12:52:04 -0800 [thread overview]
Message-ID: <20200131205204.GT2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <87blqj4ddg.fsf@nanos.tec.linutronix.de>
On Fri, Jan 31, 2020 at 08:47:23PM +0100, Thomas Gleixner wrote:
> Eric Dumazet <edumazet@google.com> writes:
> > On Fri, Jan 31, 2020 at 9:21 AM Will Deacon <will@kernel.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
next prev parent reply other threads:[~2020-01-31 20:52 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-31 16:43 Will Deacon
2020-01-31 16:48 ` Eric Dumazet
2020-01-31 16:57 ` Will Deacon
2020-01-31 17:06 ` Eric Dumazet
2020-01-31 17:20 ` Will Deacon
2020-01-31 17:33 ` Eric Dumazet
2020-01-31 18:32 ` Will Deacon
2020-01-31 19:47 ` Thomas Gleixner
2020-01-31 20:52 ` Paul E. McKenney [this message]
2020-01-31 18:43 ` Peter Zijlstra
2020-01-31 18:48 ` Eric Dumazet
2020-01-31 18:52 ` Eric Dumazet
2020-01-31 20:53 ` Paul E. McKenney
2020-01-31 21:04 ` Eric Dumazet
2020-01-31 21:19 ` Paul E. McKenney
2020-02-03 15:45 ` David Laight
2020-02-03 15:54 ` Marco Elver
2020-02-03 16:05 ` David Laight
2020-02-03 17:25 ` Paul E. McKenney
2020-02-03 15:58 ` Paul E. McKenney
2020-02-03 16:02 ` Will Deacon
2020-02-03 17:29 ` Paul E. McKenney
2020-02-03 18:27 ` Will Deacon
2020-02-03 21:16 ` Paul E. McKenney
2020-01-31 19:20 ` Marco Elver
2020-01-31 18:52 ` Paul E. McKenney
2020-01-31 18:55 ` Eric Dumazet
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