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

[Reshetova, Elena]

> 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?

Best Regards,
Elena.