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

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Date:   Thu, 16 Nov 2017 11:00:58 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 Thu, Nov 16, 2017 at 09:58:04AM +0100, Peter Zijlstra wrote:
> 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 expand on my comment. Consider the following test:

C T1

{}

P0(int *x, int *y, spinlock_t *s)
{
	spin_lock(s);
        WRITE_ONCE(*x, 1);
	spin_unlock(s);
	spin_lock(s);
        WRITE_ONCE(*y, 1);
	spin_unlock(s);
}

P1(int *x, int *y)
{
        int r0;
        int r1;

        r0 = READ_ONCE(*y);
        smp_rmb();
        r1 = READ_ONCE(*x);
}

exists (1:r0=1 /\ 1:r1=0)

According to LKMM, the store to x happens before the store to y but there
is no guarantee that the former store propagate (to P1) before the latter
(which is what we need to forbid that state).  As a result, that state in
the "exists" clause is _allowed_ by LKMM.

The LKMM encodes happens-before (or execution) ordering with a relation
named "hb", while it encodes "propagation ordering" with "cumul-fence".

  Andrea


> 
> 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?
> 
>