Re: [PATCH 1/1] ring: fix off by 1 mistake

[Olivier Matz <olivier.matz@6wind.com>]

Hi Andrzej,

On Mon, Jan 03, 2022 at 03:22:01PM +0100, Andrzej Ostruszka wrote:
> ring: fix off by 1 mistake

I suggest something less scary for the title:

  ring: optimize corner case for enqueue/dequeue

> When enqueueing/dequeueing to/from the ring we try to optimize by manual
> loop unrolling.  The check for this optimization looks like:
> 
> 	if (likely(idx + n < size)) {
> 
> where 'idx' points to the first usable element (empty slot for enqueue,
> data for dequeue).  The correct comparison here should be '<=' instead
> of '<'.
> 
> This is not a functional error since we fall back to the loop with
> correct checks on indexes.  Just a minor suboptimal behaviour for the
> case when we want to enqueue/dequeue exactly the number of elements that
> we have in the ring before wrapping to its beginning.
> 
> Signed-off-by: Andrzej Ostruszka <amo@semihalf.com>

Reviewed-by: Olivier Matz <olivier.matz@6wind.com>

I'll tend to add:
Fixes: cc4b218790f6 ("ring: support configurable element size")

But the same error was in the ENQUEUE_PTRS()/DEQUEUE_PTRS() macros
since the beginning, so we may also add:
Fixes: 286bd05bf70d ("ring: optimisations")

This macro was removed in commit 2d6ed071a8b9 ("ring: use custom element
for fixed size API")

Thanks!