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

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Message-ID: <20221220230521.GC26563@lothringen>
Date:   Wed, 21 Dec 2022 00:05:21 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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 07:06:57PM +0000, Joel Fernandes wrote:
> On Tue, Dec 20, 2022 at 7:01 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
> >
> > On 2022-12-20 13:29, Joel Fernandes wrote:
> > >
> >
> > > I do want to finish my memory barrier studies of SRCU over the holidays since I have been deep in the hole with that already. Back to the post flip memory barrier here since I think now even that might not be needed…
> >
> > I strongly suspect the memory barrier after flip is useless for the same
> > reasons I mentioned explaining why the barrier before the flip is useless.
> >
> > However, we need to double-check that we have memory barriers at the
> > beginning and end of synchronize_srcu, and between load of "unlock"
> > counters and load of "lock" counters.
> >
> > Where is the barrier at the beginning of synchronize_srcu ?
> 
> I believe we don't need another memory barrier at the beginning of
> synchronize_srcu() (but this part of my SRCU study is still pending
> ;)) . The grace period guarantee (read-side critical sections don't
> span the GP) is already enforced by the memory barrier between
> scanning for all unlocks, and scanning for all locks (Paired with
> corresponding memory barriers on the read-side).
> 
> Accordingly, before we scan all locks and match lock == unlock, there
> is an smp_mb(). Why is that not sufficient?

That's not enough, you still need a barrier between the updater's pre-GP
accesses and the scans, so that post-GP read side sees the updater's pre-GP
accesses:


            UPDATER                        READER
            -------                        ------
            WRITE A                        WRITE srcu_read_lock
            smp_mb() //rcu_seq_snap()      smp_mb()
            READ srcu_read_lock //scans    READ A

Thanks.

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