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Date:   Mon, 21 Jan 2019 12:45:05 +0100
From:   Andrea Parri <andrea.parri@amarulasolutions.com>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Elena Reshetova <elena.reshetova@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Anders Roxell <anders.roxell@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] kcov: convert kcov.refcount to refcount_t
Message-ID: <20190121114505.GA7307@andrea>
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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
> > >