Re: [PATCH net-next v6 2/8] ptr_ring: add helper to check if consume created space

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Simon Schippers wrote:
> Add __ptr_ring_consume_created_space() to check whether the previous
> __ptr_ring_consume() call successfully consumed an element and created
> space in the ring buffer. This enables callers to conditionally notify
> producers when space becomes available.
> 
> The function is only valid immediately after a single consume operation
> and should not be used after calling __ptr_ring_consume_batched().

Please explain why it is only valid in that case.
 
> Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
> Co-developed by: Jon Kohler <jon@nutanix.com>
> Signed-off-by: Jon Kohler <jon@nutanix.com>
> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
> ---
>  include/linux/ptr_ring.h | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index da141cc8b075..76d6840b45a3 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -453,6 +453,23 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
>  	return ret;
>  }
>  
> +/*
> + * Check if the previous consume operation created space
> + *
> + * Returns true if the last call to __ptr_ring_consume() has created
> + * space in the ring buffer (i.e., an element was consumed).
> + *
> + * Note: This function is only valid immediately after a single call to
> + * __ptr_ring_consume(). If multiple calls to ptr_ring_consume*() have
> + * been made, this check must be performed after each call individually.
> + * Likewise, do not use this function after calling
> + * __ptr_ring_consume_batched().
> + */
> +static inline bool __ptr_ring_consume_created_space(struct ptr_ring *r)
> +{
> +	return r->consumer_tail >= r->consumer_head;
> +}
> +
>  /* Cast to structure type and call a function without discarding from FIFO.
>   * Function must return a value.
>   * Callers must take consumer_lock.
> -- 
> 2.43.0
>