RE: [PATCH v1 1/4] ring: introduce extra run-time checks

Konstantin Ananyev
2025-05-21T19:39:02Z

> > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> > Sent: Wednesday, 21 May 2025 14.35
> >
> > > > From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> > > > Sent: Wednesday, 21 May 2025 13.14
> > > >
> > > > Add RTE_ASSERT() to check that different move_tail() flavors
> > > > return meaningful *entries value.
> > > > It also helps to ensure that inside move_tail(), it uses correct
> > > > head/tail values.
> > > >
> > > > Signed-off-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> > > > ---
> > > > lib/ring/rte_ring_c11_pvt.h | 2 +-
> > > > lib/ring/rte_ring_elem_pvt.h | 8 ++++++--
> > > > lib/ring/rte_ring_hts_elem_pvt.h | 8 ++++++--
> > > > lib/ring/rte_ring_rts_elem_pvt.h | 8 ++++++--
> > > > lib/ring/soring.c | 2 ++
> > > > 5 files changed, 21 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/lib/ring/rte_ring_c11_pvt.h
> > b/lib/ring/rte_ring_c11_pvt.h
> > > > index b9388af0da..0845cd6dcf 100644
> > > > --- a/lib/ring/rte_ring_c11_pvt.h
> > > > +++ b/lib/ring/rte_ring_c11_pvt.h
> > > > @@ -104,10 +104,10 @@ __rte_ring_headtail_move_head(struct
> > > > rte_ring_headtail *d,
> > > > n = (behavior == RTE_RING_QUEUE_FIXED) ?
> > > > 0 : *entries;
> > > >
> > > > + *new_head = *old_head + n;
> > > > if (n == 0)
> > > > return 0;
> > > >
> > > > - *new_head = *old_head + n;
> > > > if (is_st) {
> > > > d->head = *new_head;
> > > > success = 1;
> > >
> > > Is there a need to assign a value to *new_head if n==0?
> >
> > Not really, main reason I just moved this line up - to keep compiler
> > happy.
> > Otherwise it complained that *new_head might be left uninitialized.
>
> Your change might give the impression that *new_head is used by a caller. (Like I asked about.)
> To please the compiler, you could mark new_head __rte_unused, or:
>
> - if (n == 0)
> + if (n == 0) {
> + RTE_SET_USED(new_head);
> return 0;
> + }
>
> >

Makes sense, will re-spin.
Do you have any comments for other patches in the series?
Thanks
Konstantin

> > > I don't think your suggestion is multi-thread safe.
> > > If d->head moves, the value in *new_head will be incorrect.
> >
> > If d->head moves, then *old_head will also be incorrect.
> > For that case we do have CAS() below, it will return zero if (d->head
> > != *old_head)
> > and we shall go to the next iteration (attempt).
>
> Exactly.
> And with my suggestion the same will happen if n==0, and the next attempt will update them both, until they are both correct.
>
> > Basically - if n == 0, your *old_head and *new_head might be invalid
> > and should not be used
> > (and they are not used).
> >
> > > Instead, suggest:
> > >
> > > - if (n == 0)
> > > - return 0;
> > >
> > > *new_head = *old_head + n;
> > > if (is_st) {
> > > d->head = *new_head;
> > > success = 1;
> > > } else
> > > /* on failure, *old_head is updated */
> > > success =
> > rte_atomic_compare_exchange_strong_explicit(
> > > &d->head, old_head, *new_head,
> > > rte_memory_order_relaxed,
> > > rte_memory_order_relaxed);
> > > } while (unlikely(success == 0));
> >
> > That's possible, but if (n ==0) we probably don't want to update the
> > head -
> > as we are not moving head - it is pointless, while still expensive.
>
> Agree. Let's avoid unnecessary cost!
> My suggestion was only relevant if *new_head needed to be updated when n==0.
>