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

Peter Zijlstra
2019-01-21T13:18:35Z

On Mon, Jan 21, 2019 at 10:52:37AM +0100, Dmitry Vyukov wrote:
> On Wed, Jan 16, 2019 at 1:51 PM Dmitry Vyukov <dvyukov@google.com> wrote:

> > KCOV uses refcounts in a very simple canonical way, so no hidden
> > ordering implied.
> >
> > 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?
> >
> > 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);

I'm the one responsible for that refcount_t ordering.

The rationale for REL+CTRL is that for the final put we want to ensure
all prior load/store are complete, because any later access could be a
UAF; consider:

P0()
{
x->foo=0;
if (refcount_dec_and_test(&x->rc))
free(x);
}

P1()
{
x->bar=1;
if (refcount_dec_and_test(&->rc))
free(x);
}

without release, if would be possible for either (foo,bar) store to
happen after the free().

Additionally we also need the CTRL to ensure that the actual free()
happens _after_ the dec_and_test, freeing early would be bad.

But after these two requirements, the basic refcounting works.

> The refcount_dec_and_test guarantees look too weak to me, see the
> example above. Shouldn't refcount_dec_and_test returning true give the
> object in fully quiescent state? Why only control dependency? Loads
> can hoist across control dependency, no?

Yes, loads can escape like you say.

Any additional ordering; like the one you have above; are not strictly
required for the proper functioning of the refcount. Rather, you rely on
additional ordering and will need to provide this explicitly:

if (refcount_dec_and_text(&x->rc)) {
/*
* Comment that explains what we order against....
*/
smp_mb__after_atomic();
BUG_ON(!x->done*);
free(x);
}

Also; these patches explicitly mention that refcount_t is weaker,
specifically to make people aware of this difference.

A full smp_mb() (or two) would also be much more expensive on a number
of platforms and in the vast majority of the cases it is not required.