Messages in this thread Patch in this message | | | Date | Tue, 22 Jan 2019 10:04:56 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] kcov: convert kcov.refcount to refcount_t |
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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);
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