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

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Date:   Wed, 21 Dec 2022 01:49:57 +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 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.