linux-kernel - Re: [PATCH] refcount: provide same memory ordering guarantees as in atomic

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Date:   Thu, 16 Nov 2017 09:58:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "ishkamiel@...il.com" <ishkamiel@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        boqun.feng@...il.com, dhowells@...hat.com, david@...morbit.com
Subject: Re: [PATCH] refcount: provide same memory ordering guarantees as in
 atomic_t

On Wed, Nov 15, 2017 at 10:01:11PM +0100, Andrea Parri wrote:

> > And in specific things like:
> > 
> >   135e8c9250dd5
> >   ecf7d01c229d1
> > 
> > which use the release of rq->lock paired with the next acquire of the
> > same rq->lock to match with an smp_rmb().
> 
> Those cycles are currently forbidden by LKMM _when_ you consider the
> smp_mb__after_spinlock() from schedule().  See rfi-rel-acq-is-not-mb
> from my previous email and Alan's remarks about cumul-fence.

I'm not sure I get your point; and you all seem to forget I do not in
fact speak the ordering lingo. So I have no idea what
rfi-blah-blah or cumul-fence mean.

I know rel-acq isn't smp_mb() and I don't think any of the above patches
need it to be. They just need it do be a local ordering, no?

Even without smp_mb__after_spinlock() we get that:

	spin_lock(&x)
	x = 1
	spin_unlock(&x)
	spin_lock(&x)
	y = 1
	spin_unlock(&x)

guarantees that x happens-before y, right?

And that should be sufficient to then order something else against, like
for example:

	r2 = y
	smp_rmb()
	r1 = x

no?