Messages in this thread | | | From | Kees Cook <> | Date | Wed, 23 Jan 2019 12:22:44 +1300 | Subject | Re: [PATCH] kcov: convert kcov.refcount to refcount_t |
| |
On Tue, Jan 22, 2019 at 10:05 PM Peter Zijlstra <peterz@infradead.org> 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).
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?
> > --- > 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
| |