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

[Nicholas Piggin]

Paul E. McKenney's on April 20, 2019 4:26 am:
> 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.

That's interesting. I don't think that's what all our code expects.
I had the same idea as Peter.

IIRC the primitive was originally introduced exactly so x86 would not
need to have the unnecessary hardware barrier with sequences like

  smp_mb();
  ...
  atomic_inc(&v);

The "new" semantics are a bit subtle. One option might be just to
replace it entirely with atomic_xxx_mb() ?

Thanks,
Nick