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

Andrea Parri
2019-01-21T11:45:17Z

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)

> >
> > 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);
>
> +more people knowledgeable in memory ordering
>
> Unfortunately I can't find a way to reply to the
> Documentation/core-api/refcount-vs-atomic.rst patch review thread.
>
> 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?

As you remarked, the doc. says CTRL+RELEASE (so yes, loads can hoist);
AFAICR, implementations do comply to this requirement.

(FWIW, I sometimes think at this "weird" ordering as a weak "acq_rel",
the latter, acq_rel, being missing from the current APIs.)

Andrea

>
>
>
> > > Suggested-by: Kees Cook <keescook@chromium.org>
> > > Reviewed-by: David Windsor <dwindsor@gmail.com>
> > > Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
> > > Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> > > ---
> > > kernel/kcov.c | 9 +++++----
> > > 1 file changed, 5 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/kernel/kcov.c b/kernel/kcov.c
> > > index c2277db..051e86e 100644
> > > --- a/kernel/kcov.c
> > > +++ b/kernel/kcov.c
> > > @@ -20,6 +20,7 @@
> > > #include <linux/debugfs.h>
> > > #include <linux/uaccess.h>
> > > #include <linux/kcov.h>
> > > +#include <linux/refcount.h>
> > > #include <asm/setup.h>
> > >
> > > /* Number of 64-bit words written per one comparison: */
> > > @@ -44,7 +45,7 @@ struct kcov {
> > > * - opened file descriptor
> > > * - task with enabled coverage (we can't unwire it from another task)
> > > */
> > > - atomic_t refcount;
> > > + refcount_t refcount;
> > > /* The lock protects mode, size, area and t. */
> > > spinlock_t lock;
> > > enum kcov_mode mode;
> > > @@ -228,12 +229,12 @@ EXPORT_SYMBOL(__sanitizer_cov_trace_switch);
> > >
> > > static void kcov_get(struct kcov *kcov)
> > > {
> > > - atomic_inc(&kcov->refcount);
> > > + refcount_inc(&kcov->refcount);
> > > }
> > >
> > > static void kcov_put(struct kcov *kcov)
> > > {
> > > - if (atomic_dec_and_test(&kcov->refcount)) {
> > > + if (refcount_dec_and_test(&kcov->refcount)) {
> > > vfree(kcov->area);
> > > kfree(kcov);
> > > }
> > > @@ -312,7 +313,7 @@ static int kcov_open(struct inode *inode, struct file *filep)
> > > if (!kcov)
> > > return -ENOMEM;
> > > kcov->mode = KCOV_MODE_DISABLED;
> > > - atomic_set(&kcov->refcount, 1);
> > > + refcount_set(&kcov->refcount, 1);
> > > spin_lock_init(&kcov->lock);
> > > filep->private_data = kcov;
> > > return nonseekable_open(inode, filep);
> > > --
> > > 2.7.4
> > >