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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Mon, 28 Jan 2019 09:33:58 +0100
Message-ID: <CACT4Y+Zi7+6aAHf5mzjuSr4NcT+bowC8cU_DqFKwCKLh=m-tAQ@mail.gmail.com>
Subject: Re: [PATCH] kcov: convert kcov.refcount to refcount_t
To:     "Reshetova, Elena" <elena.reshetova@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <andrea.parri@amarulasolutions.com>,
        Kees Cook <keescook@chromium.org>,
        "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>
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On Sun, Jan 27, 2019 at 7:41 PM Reshetova, Elena
<elena.reshetova@intel.com> wrote:
>
> > On Mon, Jan 21, 2019 at 11:05:03AM -0500, Alan Stern wrote:
> > > On Mon, 21 Jan 2019, Peter Zijlstra wrote:
> >
> > > > 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.
> > >
> > > How about adding smp_rmb() into refcount_dec_and_test()?  That would
> > > give acq+rel semantics, which seems to be what people will expect.  And
> > >it wouldn't be nearly as expensive as a full smp_mb().
> >
> > Yes, that's a very good suggestion.
> >
> > I suppose we can add smp_acquire__after_ctrl_dep() on the true branch.
> > Then it reall does become rel_acq.
> >
> > A wee something like so (I couldn't find an arm64 refcount, even though
> > I have distinct memories of talk about it).
> >
> > This isn't compiled, and obviously needs comment/documentation updates
> > to go along with it.
> >
> > ---
> >  arch/x86/include/asm/refcount.h | 9 ++++++++-
> >  lib/refcount.c                  | 7 ++++++-
> >  2 files changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
> > index dbaed55c1c24..6f7a1eb345b4 100644
> > --- a/arch/x86/include/asm/refcount.h
> > +++ b/arch/x86/include/asm/refcount.h
> > @@ -74,9 +74,16 @@ bool refcount_sub_and_test(unsigned int i, refcount_t *r)
> >
> >  static __always_inline __must_check bool refcount_dec_and_test(refcount_t *r)
> >  {
> > -     return GEN_UNARY_SUFFIXED_RMWcc(LOCK_PREFIX "decl",
> > +     bool ret = GEN_UNARY_SUFFIXED_RMWcc(LOCK_PREFIX "decl",
> >
> >       REFCOUNT_CHECK_LT_ZERO,
> >                                       r-
> > >refs.counter, e, "cx");
> > +
> > +     if (ret) {
> > +             smp_acquire__after_ctrl_dep();
> > +             return true;
> > +     }
> > +
> > +     return false;
> >  }
>
> Actually as I started to do this, any reason why the change here only for dec_and_test and not
> for sub_and _test also? Should not the arch. specific logic follow the generic?

I would say these should be exactly the same wrt semantics.
dec_and_test is just syntactic sugar for 1 decrement. If we change
dec_and_test, we should change sub_and_test the same way.