Bug in non-power-of-2 rings?

[Bruce Richardson]

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=b74461155543430f5253e96ad6d413ebcad36693
> 
> 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
> 
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]

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.

/Bruce