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

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Date:   Wed, 21 Dec 2022 01:27:30 +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 06:46:10PM -0500, Joel Fernandes wrote:
> On Tue, Dec 20, 2022 at 6:05 PM Frederic Weisbecker <frederic@...nel.org> wrote:
> >
> > 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
> 
> But see the comments also in srcu_readers_active_idx_check()
> 
> * Needs to be a smp_mb() as the read side may
> * contain a read from a variable that is written to before the
> * synchronize_srcu() in the write side
> 
> So that appears to be already covered. Or is your point that the scans
> are not happening on the same CPU as the pre-GP writer, as scans are
> happening from workqueue ?

Nah I think you're right. Although I guess we still need the barrier between
updater's pre-gp accesses and srcu_unlock scans...


> 
> Perhaps that comment misled me.
> 
> Confused,
> 
>  - Joel