Re: [PATCH] kcov: convert kcov.refcount to refcount_t

[Andrea Parri]

On Mon, Jan 21, 2019 at 01:29:11PM +0100, Dmitry Vyukov wrote:
>  On Mon, Jan 21, 2019 at 12:45 PM Andrea Parri
> <andrea.parri@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Jan 21, 2019 at 10:52:37AM +0100, Dmitry Vyukov wrote:
> >
> > [...]
> >
> > > > Am I missing something or refcount_dec_and_test does not in fact
> > > > provide ACQUIRE ordering?
> > > >
> > > > +case 5) - decrement-based RMW ops that return a value
> > > > +-----------------------------------------------------
> > > > +
> > > > +Function changes:
> > > > +                atomic_dec_and_test() --> refcount_dec_and_test()
> > > > +                atomic_sub_and_test() --> refcount_sub_and_test()
> > > > +                no atomic counterpart --> refcount_dec_if_one()
> > > > +                atomic_add_unless(&var, -1, 1) --> refcount_dec_not_one(&var)
> > > > +
> > > > +Memory ordering guarantees changes:
> > > > +                fully ordered --> RELEASE ordering + control dependency
> > > >
> > > > I think that's against the expected refcount guarantees. When I
> > > > privatize an  atomic_dec_and_test I would expect that not only stores,
> > > > but also loads act on a quiescent object. But loads can hoist outside
> > > > of the control dependency.
> > > >
> > > > Consider the following example, is it the case that the BUG_ON can still fire?
> >
> > Can't it fire in an SC world? (wrong example, or a Monday morning? ;D)
> 
> I don't see how. Maybe there is a stupid off-by-one, but overall
> that's the example I wanted to show. refcount is 2, each thread sets
> own done flag, drops refcount, last thread checks done flag of the
> other thread.

You're right: looking at the example again, I think that the BUG_ON()
in your example can indeed trigger with a CTRL+RELEASE semantics (but
_not with the fully-ordered semantics).

I apologize for the confusion (it must have been _my Monday...  ;-/).

  Andrea


> 
> 
> 
> > > > struct X {
> > > >   refcount_t rc; // == 2
> > > >   int done1, done2; // == 0
> > > > };
> > > >
> > > > // thread 1:
> > > > x->done1 = 1;
> > > > if (refcount_dec_and_test(&x->rc))
> > > >   BUG_ON(!x->done2);
> > > >
> > > > // thread 2:
> > > > x->done2 = 1;
> > > > if (refcount_dec_and_test(&x->rc))
> > > >   BUG_ON(!x->done1);