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

Konstantin Ananyev
2025-05-21T12:35:00Z

> > 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.

> 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).
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.