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

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Message-ID: <20221221121118.GA35081@lothringen>
Date:   Wed, 21 Dec 2022 13:11:18 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.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 10:43:25PM -0500, Mathieu Desnoyers wrote:
> On 2022-12-20 19:58, Frederic Weisbecker wrote:
> > 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).
> 
> Considering that you can have pre-existing readers from arbitrary index
> appearing anywhere in the grace period (because a reader can fetch the
> index and be preempted for an arbitrary amount of time before incrementing
> the lock count), the grace period algorithm needs to deal with the fact that
> a newcoming reader can appear in a given index either before or after the
> flip.

True but the number of preempted tasks is bound and there is a forward progress guarantee.

> I don't see how flipping the index before or after loading the unlock/lock
> values would break anything (except for unlikely counter overflow situations
> as previously discussed).

Forward progress guarantee.

Thanks.

> 
> Thanks,
> 
> Mathieu
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>