linux-kernel - RE: [PATCH] kcov: convert kcov.refcount to refcount

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Date:   Fri, 25 Jan 2019 09:02:42 +0000
From:   "Reshetova, Elena" <elena.reshetova@...el.com>
To:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Alan Stern <stern@...land.harvard.edu>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Andrey Ryabinin" <aryabinin@...tuozzo.com>,
        Anders Roxell <anders.roxell@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] kcov: convert kcov.refcount to refcount_t

> > 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).
> 
> In the end, arm and arm64 chose to use REFCOUNT_FULL unconditionally,
> so there's no arch-specific implementation.
> 
> > This isn't compiled, and obviously needs comment/documentation updates
> > to go along with it.
> 
> Elena, can you do the doc updates?

Well, doc updates should go with below patch + and additional testing...

Peter, should I create a full version from below with doc/comments
updates, run the whole thing via zero day and send to you if it all looks ok?

Best Regards,
Elena.

> >
> > ---
> >  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;
> >  }
> >
> >  static __always_inline __must_check
> > diff --git a/lib/refcount.c b/lib/refcount.c
> > index ebcf8cd49e05..8276ad349d48 100644
> > --- a/lib/refcount.c
> > +++ b/lib/refcount.c
> > @@ -190,7 +190,12 @@ bool refcount_sub_and_test_checked(unsigned int i,
> refcount_t *r)
> >
> >         } while (!atomic_try_cmpxchg_release(&r->refs, &val, new));
> >
> > -       return !new;
> > +       if (!new) {
> > +               smp_acquire__after_ctrl_dep();
> > +               return true;
> > +       }
> > +
> > +       return false;
> >  }
> >  EXPORT_SYMBOL(refcount_sub_and_test_checked);
> >
> 
> Thanks for this!
> 
> --
> Kees Cook