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Date:   Mon, 28 Jan 2019 10:28:00 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Reshetova, Elena" <elena.reshetova@intel.com>
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Dmitry Vyukov <dvyukov@google.com>,
        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>
Subject: Re: [PATCH] kcov: convert kcov.refcount to refcount_t
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On Sun, Jan 27, 2019 at 06:41:38PM +0000, Reshetova, Elena wrote:
> > On Mon, Jan 21, 2019 at 11:05:03AM -0500, Alan Stern wrote:
> > > On Mon, 21 Jan 2019, Peter Zijlstra wrote:

> > 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?

Yes, sub_and_test also needs it; I simply overlooked it.