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Date: Thu, 2 Nov 2017 16:28:45 -0400 (EDT)
From: Alan Stern <stern@rowland.harvard.edu>
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To: Andrea Parri <parri.andrea@gmail.com>
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        Paul McKenney <paulmck@linux.vnet.ibm.com>, <boqun.feng@gmail.com>,
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Subject: Re: [PATCH] refcount: provide same memory ordering guarantees as in
 atomic_t
In-Reply-To: <20171102174504.GA19833@andrea>
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On Thu, 2 Nov 2017, Andrea Parri wrote:

> > This is forbidden.  It would remain forbidden even if the smp_mb in P1 
> > were replaced by a similar release/acquire pair for the same memory 
> > location.
> 
> Hopefully, the LKMM does not agree with this assessment... ;-)

No, it doesn't.

> Here's a two-threads example showing that "(w)mb is _not_ rfi-rel-acq":
> 
> C rfi-rel-acq-is-not-mb
> 
> {}
> 
> P0(int *x, int *y, int *a)
> {
> 	WRITE_ONCE(*x, 1);
> 	smp_store_release(a, 1);
> 	r1 = smp_load_acquire(a);
> 	WRITE_ONCE(*y, 1);
> }
> 
> 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)

Right.  There is a happens-before edge between the two WRITE_ONCE calls
in P0 but no cumul-fence edge, and therefore the test is allowed.  
Here is an example where a happens-before edge suffices to provide
ordering:

P0(int *x, int *y)
{
	WRITE_ONCE(*x, 1);
	smp_wmb();
	WRITE_ONCE(*y, 1);
}

P1(int *x, int *y, int *a)
{
	int rx, ry, ra;

	ry = READ_ONCE(*y);
	smp_store_release(a, 1);
	ra = smp_load_acquire(a);
	rx = READ_ONCE(*x);
}

exists (1:rx=0 /\ 1:ry=1)

This test is forbidden, but it would be allowed if the release and 
acquire accessed different locations.

Alan Stern