Re: [PATCH net-next v8 3/4] ptr_ring: move free-space check into separate helper

[Eric Dumazet <edumazet@google.com>]

On Thu, Mar 12, 2026 at 2:06 PM Simon Schippers
<simon.schippers@tu-dortmund.de> wrote:
>
> This patch moves the check for available free space for a new entry into
> a separate function. As a result, __ptr_ring_produce() remains logically
> unchanged, while the new helper allows callers to determine in advance
> whether subsequent __ptr_ring_produce() calls will succeed. This
> information can, for example, be used to temporarily stop producing until
> __ptr_ring_peek() indicates that space is available again.
>
> Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
> ---
>  include/linux/ptr_ring.h | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index 534531807d95..a5a3fa4916d3 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -96,6 +96,14 @@ static inline bool ptr_ring_full_bh(struct ptr_ring *r)
>         return ret;
>  }
>
> +static inline int __ptr_ring_produce_peek(struct ptr_ring *r)
> +{
> +       if (unlikely(!r->size) || r->queue[r->producer])

I think this should be

       if (unlikely(!r->size) || READ_ONCE(r->queue[r->producer]))

And of course:

@@ -194,7 +194,7 @@ static inline void *__ptr_ring_peek(struct ptr_ring *r)
 static inline bool __ptr_ring_empty(struct ptr_ring *r)
 {
        if (likely(r->size))
-               return !r->queue[READ_ONCE(r->consumer_head)];
+               return !READ_ONCE(r->queue[READ_ONCE(r->consumer_head)]);
        return true;
 }

@@ -256,7 +256,7 @@ static inline void __ptr_ring_zero_tail(struct
ptr_ring *r, int consumer_head)
         * besides the first one until we write out all entries.
         */
        while (likely(head > r->consumer_tail))
-               r->queue[--head] = NULL;
+               WRITE_ONCE(r->queue[--head], NULL);

        r->consumer_tail = consumer_head;
 }


Presumably we should fix this in net tree first.