From: Frederic Weisbecker <frederic@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [RFC 0/2] srcu: Remove pre-flip memory barrier
Date: Wed, 21 Dec 2022 01:49:57 +0100 [thread overview]
Message-ID: <20221221004957.GA29021@lothringen> (raw)
In-Reply-To: <CAEXW_YSAv+MzZoW9RK9H3E6DAyZH+wB=4bMy5nXTOExSwgSwAg@mail.gmail.com>
On Tue, Dec 20, 2022 at 07:15:00PM -0500, Joel Fernandes wrote:
> On Tue, Dec 20, 2022 at 5:45 PM Frederic Weisbecker <frederic@kernel.org> wrote:
> Agreed about (1).
>
> > _ In (2), E pairs with the address-dependency between idx and lock_count.
>
> But that is not the only reason. If that was the only reason for (2),
> then there is an smp_mb() just before the next-scan post-flip before
> the lock counts are read.
The post-flip barrier makes sure the new idx is visible on the next READER's
turn, but it doesn't protect against the fact that "READ idx then WRITE lock[idx]"
may appear unordered from the update side POV if there is no barrier between the
scan and the flip.
If you remove the smp_mb() from the litmus test I sent, things explode.
next prev parent reply other threads:[~2022-12-21 0:50 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-18 19:13 [RFC 0/2] srcu: Remove pre-flip memory barrier Joel Fernandes (Google)
2022-12-18 19:13 ` [RFC 1/2] srcu: Remove comment about prior read lock counts Joel Fernandes (Google)
2022-12-18 21:08 ` Mathieu Desnoyers
2022-12-18 21:19 ` Joel Fernandes
2022-12-18 19:13 ` [RFC 2/2] srcu: Remove memory barrier "E" as it is not required Joel Fernandes (Google)
2022-12-18 21:42 ` Frederic Weisbecker
2022-12-18 23:26 ` Joel Fernandes
2022-12-19 0:30 ` Joel Fernandes
2022-12-18 20:57 ` [RFC 0/2] srcu: Remove pre-flip memory barrier Mathieu Desnoyers
2022-12-18 21:30 ` Joel Fernandes
2022-12-18 23:26 ` Paul E. McKenney
2022-12-18 23:38 ` Mathieu Desnoyers
2022-12-19 0:04 ` Joel Fernandes
2022-12-19 0:24 ` Joel Fernandes
2022-12-19 1:50 ` Mathieu Desnoyers
2022-12-20 0:55 ` Joel Fernandes
2022-12-20 1:04 ` Joel Fernandes
2022-12-20 17:00 ` Mathieu Desnoyers
2022-12-20 18:05 ` Joel Fernandes
2022-12-20 18:14 ` Mathieu Desnoyers
2022-12-20 18:29 ` Joel Fernandes
2022-12-20 19:01 ` Mathieu Desnoyers
2022-12-20 19:06 ` Joel Fernandes
2022-12-20 23:05 ` Frederic Weisbecker
2022-12-20 23:46 ` Joel Fernandes
2022-12-21 0:27 ` Frederic Weisbecker
2022-12-20 22:57 ` Frederic Weisbecker
2022-12-21 3:34 ` Mathieu Desnoyers
2022-12-21 11:59 ` Frederic Weisbecker
2022-12-21 17:11 ` Mathieu Desnoyers
2022-12-22 12:40 ` Frederic Weisbecker
2022-12-22 13:19 ` Joel Fernandes
2022-12-22 16:43 ` Paul E. McKenney
2022-12-22 18:19 ` Joel Fernandes
2022-12-22 18:53 ` Paul E. McKenney
2022-12-22 18:56 ` Joel Fernandes
2022-12-22 19:45 ` Paul E. McKenney
2022-12-23 4:43 ` Joel Fernandes
2022-12-23 16:12 ` Joel Fernandes
2022-12-23 18:15 ` Paul E. McKenney
2022-12-23 20:10 ` Joel Fernandes
2022-12-23 20:52 ` Paul E. McKenney
2022-12-20 20:55 ` Joel Fernandes
2022-12-21 3:52 ` Mathieu Desnoyers
2022-12-21 5:02 ` Joel Fernandes
2022-12-21 0:07 ` Frederic Weisbecker
2022-12-21 3:47 ` Mathieu Desnoyers
2022-12-20 4:07 ` Joel Fernandes
2022-12-20 12:34 ` Frederic Weisbecker
2022-12-20 12:40 ` Frederic Weisbecker
2022-12-20 13:44 ` Joel Fernandes
2022-12-20 14:07 ` Frederic Weisbecker
2022-12-20 14:20 ` Joel Fernandes
2022-12-20 22:44 ` Frederic Weisbecker
2022-12-21 0:15 ` Joel Fernandes
2022-12-21 0:49 ` Frederic Weisbecker [this message]
2022-12-21 0:58 ` Frederic Weisbecker
2022-12-21 3:43 ` Mathieu Desnoyers
2022-12-21 4:26 ` Joel Fernandes
2022-12-21 14:04 ` Frederic Weisbecker
2022-12-21 16:30 ` Mathieu Desnoyers
2022-12-21 12:11 ` Frederic Weisbecker
2022-12-21 17:20 ` Mathieu Desnoyers
2022-12-21 18:18 ` Joel Fernandes
2022-12-21 2:41 ` Joel Fernandes
2022-12-21 11:26 ` Frederic Weisbecker
2022-12-21 16:02 ` Boqun Feng
2022-12-21 17:30 ` Frederic Weisbecker
2022-12-21 19:33 ` Joel Fernandes
2022-12-21 19:57 ` Joel Fernandes
2022-12-21 20:19 ` Boqun Feng
2022-12-22 12:16 ` Frederic Weisbecker
2022-12-22 12:24 ` Frederic Weisbecker
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