linux-kernel - Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

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Message-ID: <20221221005858.GA29316@lothringen>
Date:   Wed, 21 Dec 2022 01:58:58 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

On Wed, Dec 21, 2022 at 01:49:57AM +0100, Frederic Weisbecker wrote:
> 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@...nel.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.

Or rather, look at it the other way, if there is no barrier between the lock
scan and the index flip (E), then the index flip can appear to be written before the
lock is read. Which means you may start activating the index before you finish
reading it (at least it appears that way from the readers pont of view).