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

Peter Zijlstra
2019-01-22T09:05:06Z

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;
}

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);