Re: [PATCH] Documentation: atomic_t.txt: Explain ordering provided by smp_mb__{before,after}_atomic()

[Paul E. McKenney]

On Fri, Apr 19, 2019 at 08:00:17PM +0200, Peter Zijlstra wrote:
> On Fri, Apr 19, 2019 at 01:21:45PM -0400, Alan Stern wrote:
> > Index: usb-devel/Documentation/atomic_t.txt
> > ===================================================================
> > --- usb-devel.orig/Documentation/atomic_t.txt
> > +++ usb-devel/Documentation/atomic_t.txt
> > @@ -171,7 +171,10 @@ The barriers:
> >    smp_mb__{before,after}_atomic()
> >  
> >  only apply to the RMW ops and can be used to augment/upgrade the ordering
> > -inherent to the used atomic op. These barriers provide a full smp_mb().
> > +inherent to the used atomic op. Unlike normal smp_mb() barriers, they order
> > +only the RMW op itself against the instructions preceding the
> > +smp_mb__before_atomic() or following the smp_mb__after_atomic(); they do
> > +not order instructions on the other side of the RMW op at all.
> 
> Now it is I who is confused; what?
> 
> 	x = 1;
> 	smp_mb__before_atomic();
> 	atomic_add(1, &a);
> 	y = 1;
> 
> the stores to both x and y will be ordered as if an smp_mb() where
> there. There is no order between a and y otoh.

Let's look at x86.  And a slightly different example:

	x = 1;
	smp_mb__before_atomic();
	atomic_add(1, &a);
	r1 = y;

The atomic_add() asm does not have the "memory" constraint, which is
completely legitimate because atomic_add() does not return a value,
and thus guarantees no ordering.  The compiler is therefore within
its rights to transform the code into the following:

	x = 1;
	smp_mb__before_atomic();
	r1 = y;
	atomic_add(1, &a);

But x86's smp_mb__before_atomic() is just a compiler barrier, and
x86 is further allowed to reorder prior stores with later loads.
The CPU can therefore execute this code as follows:

	r1 = y;
	x = 1;
	smp_mb__before_atomic();
	atomic_add(1, &a);

So in general, the ordering is guaranteed only to the atomic itself,
not to accesses on the other side of the atomic.

							Thanx, Paul