Bug in non-power-of-2 rings?

[Morten Brørup]

+CC Olivier, referring to your review of the original patch.

> From: Konstantin Ananyev [mailto:konstantin.ananyev at huawei.com]
> Sent: Monday, 21 August 2023 11.29
> 
> Hi everyone,
> 
> > On Sun, Aug 20, 2023 at 11:07:33AM +0200, Morten Brørup wrote:
> > > Bruce, Honnappa, Konstantin,
> > >
> > > Back in 2017, Bruce added support for non-power-of-2 rings with this
> patch [1].
> > >
> > > [1]:
> https://git.dpdk.org/dpdk/commit/lib/librte_ring/rte_ring.h?id=b74461155
> 543430f5253e96ad6d413ebcad36693
> > >
> > > I think that the calculation of "entries" in
> __rte_ring_move_cons_head() [2][3] is incorrect when the ring capacity
> is not power-of-2,
> > because it is missing the capacity comparison you added to
> rte_ring_count() [4]. Please review if I'm mistaken.
> > >
> > > [2]:
> https://elixir.bootlin.com/dpdk/v23.07/source/lib/ring/rte_ring_c11_pvt.
> h#L159
> > > [3]:
> https://elixir.bootlin.com/dpdk/v23.07/source/lib/ring/rte_ring_generic_
> pvt.h#L150
> > > [4]:
> https://elixir.bootlin.com/dpdk/v23.07/source/lib/ring/rte_ring.h#L502
> 
> Just to confirm you suggest something like that:
> -	*entries = (r->prod.tail - *old_head);
> +	count = (r->prod.tail - *old_head);
> + 	entries = (count > r->capacity) ? r->capacity : count;
>  right?

Yes, since rte_ring_count() does it, it might be required here too.

> 
> > >
> > thanks for flagging this inconsistency, but I think we are ok.
> >
> > For consumer, I think this is correct, because we are only ever
> reducing
> > the number of entries in the ring, and the calculation of the number
> of
> > entries is made in the usual way using modulo arithmetic. We should
> never
> > have more than capacity entries in the ring so the check in ring count
> I
> > believe is superflous. [The exception would be if someone bypassed the
> > inline functions and accessed the ring directly themselves - at which
> point
> > "all bets are off", to use the English phrase]

I have now found the comments to the original patch [5]. It seems that Olivier flagged this as a risk for rte_ring_free_count(), which reads r->prod.tail and r->cons.tail without synchronization.

However, since __rte_ring_move_cons_head() uses synchronization, I guess that such a risk is not present here.

[5]: https://patchwork.dpdk.org/project/dpdk/patch/20170607133620.275801-2-bruce.richardson@intel.com/

> >
> > The producer code (__rte_ring_move_prod_head) does do a capacity
> check,
> > which is where one is required to ensure we never exceed capacity.

Agreed. Thanks for double checking.

> 
> I also can't come up with the case, when current code will cause an
> issue..
> In properly operating ring, I think we should never  have more then r-
> >capacity
> entries populated, so this extra check can be skipped.
> Unless you do have some particular case in mind?

No special case in mind. Just stumbled over this code looking different than similar code in rte_ring_free_count().