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

["Morten Brørup" <mb@smartsharesystems.com>]

> 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;
+		}

> 
> > 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.